Atmospheric Environment (v.70, #C)

Volatile organic compounds of biogenic origin (BVOCs) in the atmosphere are a key component of the Earth system, influencing ozone and secondary organic aerosol formation, and the global oxidant budget. Global BVOC emissions are dominated by terrestrial vegetation, in particular the compound isoprene, whose emission rate, in common with many BVOCs, is strongly and non-linearly dependent on temperature and the photosynthetically active radiation (PAR) flux. Detailed models of BVOC emission are now starting to be deployed in global chemistry–transport and chemistry–climate models. By necessity, the spatial resolution of these models is coarse (of the order of a few degrees), and spatial averaging removes information about areas of high temperature and PAR which contribute disproportionately to the isoprene flux. By comparing output from a BVOC emission model driven by both high- and low-resolution meteorological data, we show that this averaging effect does not lead to substantial discrepancies in simulated isoprene emissions (∼2%) when considering fluxes averaged over regional scales, but can lead to large discrepancies of up to ∼150% at much finer scales (e.g., 10 × 10 km). These smaller scale results have implications for highly coupled chemistry–climate simulations. The application of such models for the assessment and prediction of air quality, and subsequent decisions regarding the potential for mitigation or the need for adaptation, should be conducted using climate data with the highest possible spatial resolution. In particular, isoprene emissions calculated for topographically-heterogeneous regions, including coasts, should be treated with increased caution.► Modelled isoprene emissions are affected by the spatial resolution of meteorology. ► Coarse meteorological data has little influence on global isoprene emissions. ► Small scales may show discrepancies in isoprene emission of up to 150%. ► Coastal and topographically varying regions should be treated with caution.
Keywords: Isoprene; BVOC; Biogenic emissions; Spatial resolution;

Like many countries, the Central Pollution Control Board (CPCB), Delhi, in India evaluates exceedences of air pollution levels against the National Ambient Air Quality Standards (NAAQS). One of the mandatory requirements for NAAQS compliance is that the probability of non-exceedence should be at least 0.98, meaning that the formulated framework of NAAQS is essentially statistical. The current practice for assessing the compliance criterion is based on simple computation of the count of number of exceedences in a given year, without giving any consideration to the distribution function followed by different pollutants in the ambient air. This becomes even more important for monitoring stations where continuous monitoring is not done for all 365 days, but assessment is based on a minimum sample of 104 readings recorded in a year. The proper method for evaluating the compliance is the foreknowledge of the population distribution and computation of non-exceedence (or exceedence) probability of NAAQS from the probability density function (pdf). The study proposes an integrated and scientifically robust methodology that is generic in nature and could well be used for assessing the air quality compliance criteria laid out by the NAAQS for India, besides suggesting percent reduction in source emissions to those pollutants that exceed the NAAQS. The usefulness of proposed methodology is exhibited by a case study conducted on four criteria air pollutants – sulphur dioxide (SO2), nitrogen dioxide (NO2), suspended particulate matter (SPM), and particulate matter less 10 micron in size (PM10) – monitored in the ambient air of megacity Delhi at six monitoring stations. The collected data at all these sites underwent statistical analysis for the: (i) identification and estimation of the best-fit distributions, (ii) computation of probability of exceedence of the NAAQS for the non-complying pollutants, (iii) determination of return period of NAAQS violation, and (iv) estimation of percentage source emission reduction to meet the NAAQS criteria for the non-complying pollutants using the statistical rollback theory. It was concluded that the knowledge of pdf is a basic and essential requirement for realistically evaluating the compliance of NAAQS.► Integrated statistical approach has been used to evaluate compliance of air quality standards. ► Approach uses statistical distribution model and exceedence probability of standards. ► Approach allows source reduction estimates for non-compliance criteria pollutants. ► Unlike other criteria pollutants, PM10 and SPM require source emission reductions. ► Use of probability density function needed for realistic estimates of exceedences.
Keywords: Probability density function; Distributional models; Ambient air quality data; Air quality standards; Model identification and estimation; Megacity Delhi;

In vitro evaluation of pulmonary deposition of airborne volcanic ash by Anna Lähde; Sigurbjörg Sæunn Gudmundsdottir; Jorma Joutsensaari; Unto Tapper; Jarno Ruusunen; Mika Ihalainen; Tommi Karhunen; Tiina Torvela; Jorma Jokiniemi; Kristiina Järvinen; Sigurður Reynir Gíslason; Haraldur Briem; Sveinbjörn Gizurarson (18-27).
There has been an increasing interest in the effects of volcanic eruption on the environment, climate, and health following two recent volcanic eruptions in Iceland. Although health issues are mainly focused on subjects living close to the eruption due to the high concentration of airborne ash and gasses in close vicinity to the volcanoes, the ash may also reach high altitude and get distributed thousands of kilometers away from the volcano. Ash particles used in the studies were collected at the Eyjafjallajökull and Grímsvötn eruption sites. The composition, size, density and morphology of the particles were analyzed and the effect of particle properties on the re-dispersion and lung deposition were studied. The aerodynamic size and morphology of the particles were consistent with field measurement results obtained during the eruptions. Due to their size and structure, the ash particles can be re-suspended and transported into the lungs. The total surface area of submicron ash particles deposited into the alveolar and tracheobronchial regions of the lungs were 3–9% and 1–2%, respectively. Although the main fraction of the surface area is deposited in the head airways region, a significant amount of particles can deposit into the alveolar and tracheobronchial regions. The results indicate that a substantial increase in the concentration of respirable airborne ash particles and associated health hazard can take place if the deposited ash particles are re-suspended under dry, windy conditions or by outdoor human activity.► Re-suspension of ash can substantially increase airborne particle concentrations. ► Up to 9% of the ash particle surface area was deposited into the alveolar region. ► Most of the particle surface area was deposited to the head airways.
Keywords: Volcanic ash; Aerosolization; Lung deposition; Health; Particle size;

In the winter of 2011, a field campaign was undertaken in the small township of Nelson, New Zealand to measure the vertical and horizontal distribution of concentrations of airborne particulate matter. The aim of this campaign was to improve our understanding of the causal factors which result in periods of very high concentrations of particulate pollution in small townships during winter where emissions are dominated by the combustion of wood for domestic heating. The results showed that mean hourly surface concentrations of particulates throughout the airshed were characterized by a distinctive diurnal cycle, with two peaks in concentration (one in the late evening and then, unusually, a second mid-morning). Although the timing and magnitude of hourly peak concentrations was variable throughout the valley, there was no evidence to suggest that regional or topographic flows played a significant role in the build-up of pollutants at any given location.Analysis of vertical profiles of black carbon showed that high concentrations of particulates were confined to the lowest 50 m of the boundary layer. Concentrations decreased with increasing height within this polluted surface layer. The atmosphere was very stable during the evening period. After midnight, a period of increased mixing was consistently identified throughout the lowest 100 m of the boundary layer and associated with the sudden cleansing of the surface and lower layers of the boundary layer. Throughout the observational period there was no evidence for the storage of pollutants aloft. Thus the vertical mixing of pollutants to the surface could not account for increased pollutant concentrations during the morning period. However, at this time the boundary layer remained stable and concentrations of black carbon were mixed through a very shallow layer. This suggests that despite lower domestic heating emissions in the morning, the reduced mixing volume is a likely cause of the observed marked peak in morning surface concentrations.► We studied particulate air pollution in a small urban township. ► Domestic burning of biomass caused high concentrations of particulates. ► Spatial and temporal patterns were identified for the particulate pollution. ► Controls on particulate concentrations were complex for the studied small urban township.
Keywords: Domestic wood combustion; Nocturnal boundary layer; PM10; Black carbon; Complex coastal terrain;

Emission inventory of primary pollutants and chemical speciation in 2010 for the Yangtze River Delta region, China by Xiao Fu; Shuxiao Wang; Bin Zhao; Jia Xing; Zhen Cheng; Huan Liu; Jiming Hao (39-50).
We developed a high-resolution emission inventory of primary air pollutants for Yangtze River Delta (YRD) region, which included Shanghai plus 24 cities in the provinces of Jiangsu and Zhejiang. The emissions of SO2, NO X , PM10, PM2.5, NMVOCs and NH3 in the year of 2010 were estimated as 2147 kt, 2776 kt, 1006 kt, 643 kt, 3822 kt and 1439 kt, respectively. Power plants are the largest emission sources for SO2 and NO X , which contributes 44.1% and 37.3% of total SO2 and NO X emissions. Emissions from industrial process accounted for 26.9%, 28.9% and 33.7% of the total PM10, PM2.5 and NMVOCs respectively. Besides, 37.3% of NMVOCs emissions were contributed by solvent use. Livestock and fertilizer application contribute over 90% of NH3 emissions. High emission densities are visible in Shanghai and the area around Tai Lake. This emission inventory includes the speciation of PM2.5 for the YRD region for the first time, which is important to source apportionment and secondary-pollution analysis. In 2010, emissions of three major PM2.5 species, namely OC, EC and sulfate, are 136.9 kt, 75.0 kt and 76.2 kt, respectively. Aromatics and alkanes are the main NMVOC species, accounting for 30.4% and 20.3% of total VOCs. Non-road transportation and biomass burning were main uncertain sources because of a lack of proper activity and emission factor data. Compared with other pollutants, NMVOCs and NH3 have higher uncertainty. From 2000 to 2010, emissions of all pollutants have changed significantly, suggesting that the newly updated and high-resolution emission inventory will be useful for the identification of air pollution sources in YRD.► An updated high-resolution emission inventory of 2010 for YRD is established. ► Facility-based emissions are calculated for large point sources. ► PM2.5 speciation database is established based on Chinese local data. ► Emissions of NMVOC species are estimated based on Chinese local data.
Keywords: Emission inventory; PM2.5 speciation; VOC speciation; Yangtze River Delta;

A numerical study of the effect of different aerosol types on East Asian summer clouds and precipitation by Yiquan Jiang; Xiaohong Liu; Xiu-Qun Yang; Minghuai Wang (51-63).
In this study, the anthropogenic aerosol impact on the summer monsoon clouds and precipitation in East Asia is investigated using the NCAR Community Atmospheric Model version 5 (CAM5), a state-of-the-art climate model considering aerosol direct, semi-direct and indirect effects. The effects of all anthropogenic aerosols, and anthropogenic black carbon (BC), sulfate, and primary organic matter (POM) are decomposed from different sensitivity simulations. Anthropogenic sulfate and POM reduce the solar flux reaching the surface directly by scattering the solar radiation, and indirectly by increasing the cloud droplet number concentration and cloud liquid water path over East China. The surface air temperature over land is reduced, and the precipitation in North China is suppressed. Unlike anthropogenic sulfate and POM, anthropogenic BC does not have a significant effect on the air temperature at the surface, because of the reduction of the cloud liquid water path and the weakening of shortwave cloud forcing by its semi-direct effect. The anthropogenic BC strengthens the southwesterly wind over South China and leads to stronger deep convection at the 25°N–30°N latitudinal band. The effect of all anthropogenic aerosols on air temperature, clouds, and precipitation is not a linear summation of effects from individual anthropogenic sulfate, BC and POM. Overall all anthropogenic aerosols suppress the precipitation in North China and enhance the precipitation in South China and adjacent ocean regions.► We investigate aerosol effects on summer monsoon clouds and precipitation in East Asia. ► Anthropogenic sulfate and POM suppress precipitation in North China. ► Anthropogenic BC induced precipitation change is not statistically significant. ► The effect of all anthropogenic aerosols more resembles that from anthropogenic sulfate and POM.
Keywords: Aerosol; Cloud; Precipitation; Climate; Community Atmospheric Model version 5;

Cluster analysis of roadside ultrafine particle size distributions by Kelly Sabaliauskas; Cheol-Heon Jeong; Xiaohong Yao; Yun-Seok Jun; Greg Evans (64-74).
This study reports the diurnal, seasonal, and annual variation of ultrafine particle size distributions in downtown Toronto. The k-means clustering algorithm was applied to five years of size-resolved data for particles with diameters less than 100 nm. Continuous particle number concentrations were measured 16 m from a major arterial roadway between March 2006 and May 2011 using a Fast Mobility Particle Sizer. Eight particle size distribution (PSD) types were identified. The PSD types exhibited distinct weekday–weekend and diurnal patterns. The relative frequency that each PSD occurred varied with season and wind direction and was correlated with other pollutants. These temporal patterns and correlation helped in elucidating the sources and processes that each of the eight PSD represent. Finally, similar PSD types were observed in residential areas located 6 and 15 km away from the central monitoring site suggesting that these PSD types may be generalizable to other sites. Identification of PSD types was found to be a valuable tool to support the interpretation of PSD data so as to elucidate the sources and processes contributing to ultrafine particle concentrations.► Ultrafine particles were measured for five years near a major roadway. ► The k-means clustering algorithm was applied to particle size distributions (PSD). ► Eight PSD types were found to show different temporal patterns. ► The PSD types had differing correlations with NO, NO2, PM2.5 and wind speed. ► Physical interpretations of the conditions yielding each PSD type were elucidated.
Keywords: Clustering; K-means; Ultrafine particles; Particle size distribution;

Results from air quality modeling and field measurements made as part of the Bay Region Atmospheric Chemistry Experiment (BRACE) along with related scientific literature were reviewed to provide an improved estimate of atmospheric reactive nitrogen (N) deposition to Tampa Bay, to apportion atmospheric N between local and remote sources, and to assess the impact of regulatory drivers on N deposition to Tampa Bay. Simulations using the Community Multiscale Air Quality model v4.4 modified with the University of California Davis aerosol module (CMAQ-UCD) provided a framework for this review. For 2002, CMAQ-UCD modeled atmospheric loading rates were 6910 metric tons N to the land surface of the watershed and 548 metric tons N to bay surface of the watershed, respectively. If an 18% transfer rate of atmospherically-deposited N from watershed to bay is assumed, then the corresponding atmospheric loading to Tampa Bay was 1790 metric tons N or 57% of the total N loading to the bay. From CMAQ-UCD modeling, oxidized N sources both within and outside Tampa Bay's watershed were important contributors to atmospheric N loading to the bay. Within the watershed, oxidized N emissions from mobile sources had a disproportionately larger impact than did power plant sources on atmospheric N loading. Predicted decreases in atmospheric N deposition to Tampa Bay by 2010 due to regulatory drivers were significant, and plausibly evident in recent declines in ambient air NO x concentrations in urban Tampa and St. Petersburg.► Atmospheric deposition was a significant source of N inflow to Tampa Bay. ► Mobile sources had a larger impact than did power plant sources. ► Emission sources within and outside the watershed were important contributors.
Keywords: Florida; Atmospheric deposition; Ammonia; Nitrogen oxides; CMAQ-UCD;

Road vehicle emission factors development: A review by Vicente Franco; Marina Kousoulidou; Marilena Muntean; Leonidas Ntziachristos; Stefan Hausberger; Panagiota Dilara (84-97).
Pollutant emissions need to be accurately estimated to ensure that air quality plans are designed and implemented appropriately. Emission factors (EFs) are empirical functional relations between pollutant emissions and the activity that causes them. In this review article, the techniques used to measure road vehicle emissions are examined in relation to the development of EFs found in emission models used to produce emission inventories. The emission measurement techniques covered include those most widely used for road vehicle emissions data collection, namely chassis and engine dynamometer measurements, remote sensing, road tunnel studies and portable emission measurements systems (PEMS). The main advantages and disadvantages of each method with regards to emissions modelling are presented. A review of the ways in which EFs may be derived from test data is also performed, with a clear distinction between data obtained under controlled conditions (engine and chassis dynamometer measurements using standard driving cycles) and measurements under real-world operation.► The accuracy of road emission models is directly linked to the quality of their emission factors. ► Road vehicles have a large natural variability in their emission profiles. ► Emission factors may have different resolution according to their intended use. ► Emission modellers should combine laboratory data with real-world measurements.
Keywords: Road transport; Emission inventories; Emission models; Emission factors; Chassis dynamometer; Engine dynamometer; PEMS;

Dispersion and photochemical evolution of reactive pollutants in street canyons by Kyung-Hwan Kwak; Jong-Jin Baik; Kwang-Yeon Lee (98-107).
Dispersion and photochemical evolution of reactive pollutants in street canyons with canyon aspect ratios of 1 and 2 are investigated using a computational fluid dynamics (CFD) model coupled with the carbon bond mechanism IV (CBM–IV). Photochemical ages of NO x and VOC are expressed as a function of the NO2-to-NO x and toluene-to-xylene ratios, respectively. These are found to be useful for analyzing the O3 and OH oxidation processes in the street canyons. The OH oxidation process (O3 oxidation process) is more pronounced in the upper (lower) region of the street canyon with a canyon aspect ratio of 2, which is characterized by more (less) aged air. In the upper region of the street canyon, O3 is chemically produced as well as transported downward across the roof level, whereas O3 is chemically reduced in the lower region of the street canyon. The O3 chemical production is generally favorable when the normalized photochemical ages of NO x and VOC are larger than 0.55 and 0.28, respectively. The sensitivities of O3 chemical characteristics to NO x and VOC emission rates, photolysis rate, and ambient wind speed are examined for the lower and upper regions of the street canyon with a canyon aspect ratio of 2. The O3 concentration and the O3 chemical production rate divided by the O3 concentration increase as the NO x emission rate decreases and the VOC emission rate and photolysis rate increase. The O3 concentration is less sensitive to the ambient wind speed than to other factors considered. The relative importance of the OH oxidation process compared to the O3 oxidation process increases with increasing NO x emission rate and photolysis rate and decreasing VOC emission rate. In this study, both O3 and OH oxidation processes are found to be important in street-canyon scale chemistry. The methodology of estimating the photochemical ages can potentially be adopted to neighborhood scale chemistry.► Dispersion and photochemical evolution of reactive pollutants in street canyons. ► Photochemical ages of NO x and VOC as a function of concentration ratios. ► Favorable O3 chemical production in a more aged air mass. ► Sensitivities to NO x and VOC emission rates, photolysis rate, and ambient wind speed. ► A diagram capturing the relative importance between O3 and OH oxidation processes.
Keywords: Street canyon; Dispersion; CFD model; CBM–IV; Photochemical age; Oxidation process;

Water uptake of internally mixed ammonium sulfate and dicarboxylic acid particles probed by infrared spectroscopy by Lorena Miñambres; Estíbaliz Méndez; María N. Sánchez; Fernando Castaño; Francisco J. Basterretxea (108-116).
Tropospheric aerosols are usually mixtures of inorganic and organic compounds in variable proportions, and the relative amount of organic fraction can influence the hygroscopic properties of the particles. Infrared spectra of submicrometer internally mixed dry particles of ammonium sulfate (AS) with various dicarboxylic acids (oxalic, malonic, maleic, glutaric and pimelic) have been measured in an aerosol flow tube at several solute mass ratios. The spectra show a notable broadening in the bandwidth of sulfate ion ν 3 vibrational band near 1115 cm−1 with respect to pure AS. We attribute these perturbations, that are biggest at AS/organic acid mass ratio near unity, to intermolecular interactions between inorganic ions and organic acid molecules in the internally mixed solids. The water uptake behavior of internally mixed particles has been measured by recording the infrared integrated absorbance of liquid water as a function of relative humidity (RH). The amount of water present in the particles prior to deliquescence correlates partially with the water solubilities of the dicarboxylic acids, and also with the relative magnitudes of intermolecular interactions in the internally mixed dry solids. Phase change of ammonium sulfate in the internally mixed particles with RH has been spectrally monitored, and it is shown that water uptaken before full deliquescence produces structural changes in the particles that are revealed by their vibrational spectra.► IR spectra of ammonium sulfate/dicarboxylic acids measured in an aerosol flow tube. ► Sulfate ion ν 3 vibrational band near 1115 cm−1 broadens in mixed particles. ► Perturbations in particle spectra are due to intermolecular interactions. ► Predeliquescence water content correlates with the interactions in dry particles.
Keywords: Tropospheric aerosols; Deliquescence; Hygroscopicity; Vibrational spectrum; Band broadening; Ion–molecule interactions;

Idle emission characteristics of a light-duty diesel van at various altitudes by Hang Yin; Yunshan Ge; Xin Wang; Linxiao Yu; Zhe Ji; Weicheng Chen (117-122).
As operating conditions producing severe pollution, high altitude conditions need more attention. In this present paper, regulated emissions and particulate matter distributions of a light-duty diesel van at idle at different altitudes were measured and discussed. Seven altitudes varying from 70 m to 3200 m were introduced and investigated. It was found that CO emission increased corresponding to an increase in altitude. In the cases where altitude was lower than 2400 m, the mass flows of hydrocarbon emissions were quite similar. And the hydrocarbon emissions at 3200 m were much higher than that at lower altitudes. NO x and NO emissions at high altitude were rather close to that near sea level. NO2 ascended when altitude was rising in the cases where the altitude was lower than 2400 m; but a decrease at 3200 m was observed. Generally, particulate matter positively correlated to the increased altitude despite the slight fluctuation at relatively low altitudes. Number distribution of particulate matter manifested that a majority of particles were with relatively small aerosol diameters. With the rising of altitude, the percentage of small particles became lower in the total particulate matter. In average, larger number of particles was exhausted at high altitude. With regard to mass distribution, larger particles hold larger percentages by weight than smaller particles at all investigated altitudes. The percentage shared by PM2.5 in the total particulate matter varied from 48.3% to 74.8% in the tests at different altitudes.► On-vehicle tests indicated that CO increased with altitude. ► NO x and NO emissions at high altitude were close to that near sea level. ► In average, larger number of particles was measured at higher altitudes. ► By weight, the percentage shared by PM2.5 in total PM varied from 48.3% to 74.8%.
Keywords: Emission; High altitude; Diesel engine; Idle;

Impact of the vertical mixing induced by low-level jets on boundary layer ozone concentration by Xiao-Ming Hu; Petra M. Klein; Ming Xue; Fuqing Zhang; David C. Doughty; Renate Forkel; Everette Joseph; Jose D. Fuentes (123-130).
After sunset, a stable boundary layer (SBL) develops close to the ground, while the upper region of the daytime mixed layer becomes the residual layer (RL). Mixing between the SBL and RL is often quite limited and the RL is thought to be a reservoir for daytime mixed-layer pollutants under such conditions. However, ozone (O3) profiles observed in Maryland, U.S. suggest that the RL is not always a reservoir of O3 in that region. Nocturnal low-level jets (LLJs) and/or other mechanisms are speculated to enhance vertical mixing between the SBL and RL, which influences the vertical O3redistribution. Nocturnal surface O3 maxima, an RL with reduced O3 levels, and a concurrent strong LLJ were observed in Maryland on the night of August 9–10, 2010. Surface O3 measurements in the region and three-dimensional air quality simulations suggest that horizontal advection cannot explain the nocturnal O3 maxima and concurrent decrease of O3 levels within the RL. A sensitivity study with a single column (1D) chemistry model was performed to investigate the role of LLJs in generating turbulent mixing within the nighttime boundary layer and to identify related impacts on O3 concentrations at night and on the following day. The strong shear associated with the LLJ enhanced turbulent mixing and weakened the decoupling of the RL and SBL substantially. Ozone was actively mixed down from the RL to the surface, causing secondary nocturnal surface O3 maxima. Near the surface, O3 was efficiently removed by chemical reactions and dry deposition, which resulted in lower O3 peak values on the next day.► The nocturnal O3 maxima occurred concurrently at multiple sites along the corridor. ► The residual layer (RL) is leaky in the presence of a strong low-level jet (LLJ). ► The role of LLJ-induced vertical mixing on boundary layer O3 is identified. ► Mixed-layer O3 concentration on the next day is reduced as a result of a leaky RL.
Keywords: Ozonesonde; Low-level jet; Atmospheric boundary layer; Ozone; WRF/Chem; Residual layer;

Multi-site tropospheric ozone measurements across the North Tropical Atlantic during the summer of 2010 by Gregory S. Jenkins; Miliaritiana L. Robjhon; Belay Demoz; William R. Stockwell; Seydi A. Ndiaye; Mamadou S. Drame; Moussa Gueye; Jonathan W. Smith; Yaitza Luna-Cruz; Johnathan Clark; Jalisa Holt; Casey Paulin; Ashley Brickhouse; Aneese Williams; Ajamu Abdullah; Ashford Reyes; Luis Mendes; Adriel Valentine; Moctar Camara (131-148).
Ozone soundings are launched during two Intensive Observing Periods (IOPs) from Dakar, Senegal; Sao Vicente, Cape Verde; and St James, Barbados to investigate ozone variability across the North Tropical Atlantic during June/July and August/September 2010. Two objectives of the campaign are to compare background tropospheric ozone mixing ratios and its variability associated with the Saharan Air Layer (SAL) and African Easterly Waves (AEWs) at sites located in the Eastern and Western Tropical Atlantic Ocean. During IOP1 (June–July), reduced ozone mixing ratios are found in the SAL with elevated ozone levels at the SAL's base and above it. During IOP I, the tropospheric column ozone (TCO) is higher at Cape Verde (27.5 DU) when compared to Barbados (19.8 DU). During IOP II (August–September) ozone-rich air is found above 500 hPa prior to the passage of AEW or developing tropical cyclones. The observed larger mixing ratios of middle/upper tropospheric ozone are most prominent at Dakar prior to the passage of an AEW, which we attribute to lightning NO x . During IOP II the tropospheric column ozone is highest at Dakar (30.5 DU) when compared to Cape Verde (20.2 DU) and Barbados (17.2 DU).► Higher tropospheric column ozone in the Eastern Atlantic relative to the Western Atlantic. ► Higher TCO values during (IOP I) rather than (IOP II) for Barbados and Cape Verde. ► Higher ozone mixing ratios are found in the 925–850 hPa layer in Cape Verde. ► Pre-trough conditions have higher ozone mixing ratios than post-trough conditions.
Keywords: Saharan air layer; Dust; African Easterly Waves; Lightning; Tropics;

Seasonal variations in terpene emission factors of dominant species in four ecosystems in NE Spain by Joan Llusia; Josep Peñuelas; Alex Guenther; Francesca Rapparini (149-158).
We studied the daily patterns in the rates of foliar terpene emissions by four typical species from the Mediterranean region in two days of early spring and two days of summer in 4 localities of increasing biomass cover in Northern Spain. The species studied were Thymelaea tinctoria (in Monegros), Quercus coccifera (in Garraf), Quercus ilex (in Prades) and Fagus sylvatica (in Montseny). Of the total 43 VOCs detected, 23 were monoterpenes, 5 sesquiterpenes and 15 were not terpenes. Sesquiterpenes were the main terpenes emitted from T. tinctoria. Total VOC emission rates were on average about 15 times higher in summer than in early spring. The maximum rates of emission were recorded around midday. Emissions nearly stopped in the dark. No significant differences were found for nocturnal total terpene emission rates between places and seasons. The seasonal variations in the rate of terpene emissions and in their chemical composition can be explained mainly by dramatic changes in emission factors (emission capacity) associated in some cases, such as for beech trees, with very different foliar ontogenical characteristics between spring and summer. The results show that temperature and light-standardised emission rates were on average about 15 times higher in summer than in early spring, which, corroborating other works, calls to attention when applying the same emission factor in modelling throughout the different seasons of the year.► Terpene emission factors were about 15 times higher in summer than in early spring. ► The maximum emission factors were recorded around midday. ► Minimum emission rates were recorded during the night.
Keywords: Terpenes; Emission rates; Mediterranean ecosystems; Seasonality; Climatic gradient;

Due to its proximity to the Seoul Metropolitan Area (SMA) (40 km from Seoul proper to North Korea) and the characteristics of energy consumption (coal and biomass burning as major primary energy sources), air pollutants emitted from North Korea are likely to influence the air quality in the SMA. To understand the transport of air pollutants emitted from North Korea, backward trajectories arriving in Seoul were estimated and classified into four cases depending on which area the trajectories predominantly passed through for the sampling days between 2002 and 2003. The ambient data of particulate polycyclic aromatic hydrocarbons (PAHs) were analyzed for the samples. Then, based on the contribution of biomass burning calculated by the chemical mass balance (CMB) model applied and the influence of air pollutants' emissions from North Korea to SMA is semi-quantified. The result was verified by the spatial and seasonal variations of the PAH emission in China based on the previous works. It is estimated that the influence from North Korea on the particulate PAHs concentration in Seoul was up to 20% of the observed values. Further study directions are discussed to make more quantitative and reliable estimation.► Biomass burning is one of the major energy sources in North Korea. ► The air pollutants emitted in North Korea might affect Seoul, South Korea due to its proximity. ► The contribution of the particulate PAHs from North Korea at Seoul was up to 20%.
Keywords: North Korea; Seoul Metropolitan Area; Biomass burning; PAHs; Chemical mass balance model; Backward trajectories;

An extensive measurement campaign was conducted in the Milan subway system in order to investigate PM10 concentrations, to determine its physical and elemental composition, its origins, and to attempt to quantify source contributions. The Milan subway system includes three lines and stations typically consist of two underground levels: an intermediate floor (mezzanine) where the turnstiles for accessing the platform are located, and a platform level, one floor down. Measurements were performed in two stations for each line, and both microenvironments (platform and mezzanine) were investigated in all cases. PM10 samples were collected at all twelve sites over three daily periods for nine consecutive days at each site. Particle number concentrations were also measured with Optical Particle Counters (OPC) and size-number distributions were determined. X-ray fluorescence analysis was also performed on the samples to determine element concentrations. The results indicate PM sources related with train operations as the dominant impact on particulate concentrations. Average weekday PM10 concentrations between 105 and 283 μg m 3 were observed at the platform level, while average ambient concentrations of 36 μg m 3 were observed. Fe, Ba, Sb, Mn and Cu were found to be significantly enriched. Metal particles, occurring mostly in the range of diameters between 1 and 5 μm, and therefore likely originating from mechanical processes, account for most of the PM10 mass at the platform level. Wheel, brake and track wear are found to contribute 40–73% of total PM10 mass and electric cable wear (Cu and Zn oxides) 2%–3%. Concentrations measured on the mezzanine levels are intermediate between those found in ambient air and on the platform level, with average daytime PM10 values ranging from 50 to 80 μg m−3. The situation observed on the mezzanine can well be described through an appropriate mixing of ambient and platform level air. A decreasing, albeit still significant, impact from internal sources is observed, with particulate from wheel, brake and track wear contributing an average of 2–25%, and electric cable wear 0.5–1.2%, to total PM10 mass.► PM10 levels in subway stations were studied and its elemental composition determined. ► OPC's were used to determine size distributions. ► PM10 concentrations were greater than in ambient air by up to a factor of 8. ► Source contributions were estimated on the platform and mezzanine levels. ► PM10 levels were largely due to local sources such as brake, wheel and cable wear.
Keywords: Subway; Transport microenvironment; PM10; Size distribution; Elemental composition; Source contribution;

One largely unstudied benefit of relocating families from slums to public housing is the potential improvement in indoor air quality (IAQ). We compared families that moved from slums to public housing with those that remained living in slums in Santiago, Chile in terms of fine particulate matter (PM2.5) as main indicator of change. A cross-sectional study of 98 relocated families and 71 still living in slums was carried out, obtaining indoor and outdoor samples by a Personal Environmental Monitor. Home characteristics, including indoor air pollution sources were collected through questionnaires. Multivariate regression models included the intervention (public housing or slum), indoor pollution sources, outdoor PM2.5 and family characteristics as predictors. Indoor PM2.5 concentrations were higher in slums (77.8 μg m−3 [SD = 35.7 μg m−3]) than in public housing (55.7 μg m−3 [SD = 34.6 μg m−3], p < 0.001). Differences between indoor and outdoor PM2.5 were significant only in the slum houses. The multivariate analysis showed that housing intervention significantly decreased indoor PM2.5 (10.4 μg m−3) after adjusting by the other predictors. Outdoor PM2.5 was the main predictor of indoor PM2.5. Other significant factors were water heating fuels and indoor smoking. Having infants 1–23 months was associated with a lowering of indoor PM2.5. Our results suggest that a public housing program that moves families from slums to public housing improves indoor air quality directly and also indirectly through air pollution sources.► We examine differences in the PM2.5 and their sources in slums houses and public housing. ► Indoor and outdoor PM2.5 were significantly higher in slums than in public housing. ► Differences between indoor and outdoor PM2.5 within homes were significant only in slum houses. ► Outdoor PM2.5 and the method for bathing water heating were the main predictors in all homes. ► A program that move slums families to public housing may improve indoor air quality.
Keywords: Environmental health; Indoor air quality; PM2.5; Public housing; Slums; Chile;

The use of electrochemical sensors for monitoring urban air quality in low-cost, high-density networks by M.I. Mead; O.A.M. Popoola; G.B. Stewart; P. Landshoff; M. Calleja; M. Hayes; J.J. Baldovi; M.W. McLeod; T.F. Hodgson; J. Dicks; A. Lewis; J. Cohen; R. Baron; J.R. Saffell; R.L. Jones (186-203).
Measurements at appropriate spatial and temporal scales are essential for understanding and monitoring spatially heterogeneous environments with complex and highly variable emission sources, such as in urban areas. However, the costs and complexity of conventional air quality measurement methods means that measurement networks are generally extremely sparse. In this paper we show that miniature, low-cost electrochemical gas sensors, traditionally used for sensing at parts-per-million (ppm) mixing ratios can, when suitably configured and operated, be used for parts-per-billion (ppb) level studies for gases relevant to urban air quality. Sensor nodes, in this case consisting of multiple individual electrochemical sensors, can be low-cost and highly portable, thus allowing the deployment of scalable high-density air quality sensor networks at fine spatial and temporal scales, and in both static and mobile configurations.In this paper we provide evidence for the performance of electrochemical sensors at the parts-per-billion level, and then outline results obtained from deployments of networks of sensor nodes in both an autonomous, high-density, static network in the wider Cambridge (UK) area, and as mobile networks for quantification of personal exposure. Examples are presented of measurements obtained with both highly portable devices held by pedestrians and cyclists, and static devices attached to street furniture. The widely varying mixing ratios reported by this study confirm that the urban environment cannot be fully characterised using sparse, static networks, and that measurement networks with higher resolution (both spatially and temporally) are required to quantify air quality at the scales which are present in the urban environment. We conclude that the instruments described here, and the low-cost/high-density measurement philosophy which underpins it, have the potential to provide a far more complete assessment of the high-granularity air quality structure generally observed in the urban environment, and could ultimately be used for quantification of human exposure as well as for monitoring and legislative purposes.► Suitably configured electrochemical sensors can be used for air quality studies. ► Evidence of performance of electrochemical sensors at parts-per-billion levels. ► Sensors are sensitive, low noise, highly linear and generally highly selective. ► Measurement density (space and time) unachievable using current methods. ► Show low-cost air quality sensor networks are now feasible for widespread use.
Keywords: Urban air quality; Real-time measurements; Sensor networks; Air quality; Carbon monoxide (CO); Nitric oxide (NO); Nitrogen dioxide (NO2); Nitrogen oxides (NO x ); Electrochemical sensors;

Seasonal source contributions of tropospheric ozone over East Asia based on CMAQ–HDDM by Syuichi Itahashi; Itsushi Uno; Soontae Kim (204-217).
Determining the source contributions of tropospheric ozone concentration is an important issue for East Asia, due to the dramatic and rapid increase in emissions of atmospheric pollutants. To achieve this, the higher-order decoupled direct method (HDDM), a technique for efficient calculation of sensitivities, was applied in this study. Tropospheric ozone concentrations at observation sites located in remote areas of Japan were well-reproduced by Community Multi-scale Air Quality (CMAQ) model simulations, and exhibited a maximum peak in spring, a relatively small peak in autumn and a summer minimum. This seasonal pattern is a reflection of long-range transport and chemical processes, coupled with continental-oceanic air mass exchanges forced by the East Asian monsoon. For the HDDM simulation, we focused on episodic pollution events during each season of 2007 to clarify the seasonal characteristics, and then assessed source contributions paying attention to both precursor emissions (NOx and VOC) and source regions (China, central eastern China, Korea, and Japan). An ozone-sensitive regime (NOx- or VOC-sensitive regime) was also determined based on the HDDM results. This suggested a regime over East Asia that was NOx sensitive in summer, VOC sensitive in winter, and either NOx or VOC sensitive during spring and autumn. At observation sites in remote areas of Japan, by separating the precursor contribution into NOx and VOC components of ozone production, it was found that the contribution of NOx emissions was larger than that of VOC emissions in spring, autumn, and especially summer, therefore, a reduction in NOx emissions could reduce the severity of episodes of tropospheric ozone pollution in downwind areas. Due to the strong VOC-sensitive conditions in winter, NOx emissions enabled a reduction in surface ozone concentrations. In terms of the contributions attributed to source regions, the source contribution of China was relatively high during spring, but local-scale photochemical build-up was dominant during summer and autumn. It was also revealed that central eastern China (CEC), characterized by densely populated and industrialized regions, was the most significant source of Chinese source contributions in summer.► HDDM was used to estimate ozone source contributions over East Asia. ► The seasonal characteristics of ozone concentrations were well reproduced. ► An ozone-sensitive regime was diagnosed based on the HDDM results. ► NOx was identified as the most significant precursor of O3, rather than VOC. ► Ozone source contributions from China are largest during spring.
Keywords: East Asia; Tropospheric ozone; HDDM; Ozone-sensitive regime; Source–receptor relationship;

Validation of a Lagrangian particle model by Lucyna Brzozowska (218-226).
In this paper a custom-developed model of dispersion of pollutants is presented. The proposed approach is based on both a Lagrangian particle model and an urban-scale diagnostic model of the air velocity field. Both models constitute a part of an operational air quality assessment system. The proposed model is validated by comparing its computed results with the results of measurements obtained in a wind tunnel reflecting conditions of the Mock Urban Setting Test (MUST) experiment. Commonly used measures of errors and model concordance are employed and the results obtained are additionally compared with those obtained by other authors for CFD and non-CFD class models. The obtained results indicate that the validity of the model presented in this paper is acceptable.► The Lagrangian particle model is validated by being compared with the results of the experiment. ► The model is connected to a diagnostic air velocity field model. ► The results of the MUST experiment are used to be compared with the calculations. ► A new solution of particle reflection from the land surface is applied. ► Results of the calculations and measurements are in acceptable correspondence.
Keywords: Lagrangian particle model; Urban scale model; MUST experiment; Validation; Dispersion; Urban area;

The kinetics of the gas-phase reactions of selected monoterpenes and cyclo-alkenes with ozone and the NO3 radical by D.J. Stewart; S.H. Almabrok; J.P. Lockhart; O.M. Mohamed; D.R. Nutt; C. Pfrang; G. Marston (227-235).
The relative rate method has been used to measure the room-temperature rate constants for the gas-phase reactions of ozone and NO3 with selected monoterpenes and cyclo-alkenes with structural similarities to monoterpenes. Measurements were carried out at 298 ± 2 K and 760 ± 10 Torr. The following rate constants (in units of 10−18 cm3 molecule−1 s−1) were obtained for the reaction with ozone: methyl cyclohexene (132 ± 17), terpinolene (1290 ± 360), ethylidene cyclohexane (223 ± 57), norbornene (860 ± 240), t-butyl isopropylidene cyclohexane (1500 ± 460), cyclopentene (543 ± 94), cyclohexene (81 ± 18), cyclooctene (451 ± 66), dicyclopentadiene (1460 ± 170) and α-pinene (107 ± 13). For the reaction with NO3 the rate constants obtained (in units of 10−12 cm3 molecule−1 s−1) were: methyl cyclohexene (7.92 ± 0.95), terpinolene (47.9 ± 4.0), ethylidene cyclohexane (4.30 ± 0.24), norbornene (0.266 ± 0.029), cyclohexene (0.540 ± 0.017), cyclooctene (0.513 ± 0.029), dicyclopentadiene (1.20 ± 0.10) and α-pinene (5.17 ± 0.62). Errors are quoted as the root mean square of the statistical error (95% confidence) and the quoted error in the rate constant for the reference compound. Combining these results with previous studies, new recommendations for the rate constants are presented. Molecular orbital energies were calculated for each alkene and the kinetic data are discussed in terms of the deviation from the structure–activity relationship obtained from the rate constants for a series of simple alkenes. Lifetimes with respect to key initiators of atmospheric oxidation have been calculated suggesting that the studied reactions play dominant roles in the night-time removal of these compounds from the atmosphere.► Measurements of rate constants for the reactions of O3 and NO3 with terpenes and cyclo-alkenes are presented. ► The effect of structure on the observed kinetics is discussed. ► Measured values are evaluated along with literature data to give recommended rate constants. ► Atmospheric lifetimes and presented in light of this data.
Keywords: Kinetics; Ozone; Nitrate radical; Terpenes; Alkenes;

Reconciling NO x emissions reductions and ozone trends in the U.S., 2002–2006 by Wei Zhou; Daniel S. Cohan; Sergey L. Napelenok (236-244).
Dynamic evaluation seeks to assess the ability of photochemical models to replicate changes in air quality as emissions and other conditions change. When a model fails to replicate an observed change, a key challenge is to discern whether the discrepancy is caused by errors in meteorological simulations, errors in emission magnitudes and changes, or inaccurate responses of simulated pollutant concentrations to emission changes. In this study, the Community Multiscale Air Quality (CMAQ) model is applied to simulate the ozone (O3) change after the NO x SIP Call and mobile emission controls substantially reduced nitrogen oxides (NO x ) emissions in the eastern U.S. from 2002 to 2006. For both modeled and observed O3, changes in episode average daily maximal 8-h O3 were highly correlated (R 2 = 0.89) with changes in the 95th percentile, although the magnitudes of reductions increased nonlinearly at high percentile O3 concentrations. Observed downward changes in mean NO x (−11.6 to −2.5 ppb) and 8-h O3 (−10.4 to −4.7 ppb) concentrations in metropolitan areas in the NO x SIP Call region were under-predicted by 31%–64% and 26%–66%, respectively. The under-predicted O3 improvements in the NO x SIP Call region could not be explained by adjusting for temperature biases in the meteorological input, or by considering uncertainties in the chemical reaction rate constants. However, the under-prediction in O3 improvements could be alleviated by 5%–31% by constraining NO x emissions in each year based on observed NO x concentrations. This demonstrates the crucial need to accurately characterize changes in precursor emissions when dynamically evaluating a model's ability to simulate O3 responses to those changes.► CMAQ model significantly under-predicted the observed O3 change from 2002 to 2006. ► Under-prediction of O3 change was more severe at high percentile concentrations. ► The observed NO x change was significantly underestimated in NO x SIP Call. ► Under-prediction of O3 change could be alleviated by constraining NO x emissions.
Keywords: Ozone changes; Emission changes; Emission uncertainty; Chemical reaction rate; NO x SIP Call; NO x control;

Recently, mineral dust has been suspected to be one of the important environmental risk factor for meningitis epidemics in West Africa. The current study is one of the first which relies on long-term robust aerosol measurements in the Sahel region to investigate the possible impact of mineral dust on meningitis cases (incidence). Sunphotometer measurements, which allow to derive aerosol and humidity parameters, i.e., aerosol optical thickness, Angström coefficient, and precipitable water, are combined with quantitative epidemiological data in Niger and Mali over the 2004–2009 AMMA (African Monsoon Multidisciplinary Analysis) program period. We analyse how the extremely high aerosol loads in this region may influence both the calendar (onset, peaks, end) and the intensity of meningitis. We highlight three distinct periods: (i) from November to December, beginning of the dry season, humidity is weak, there is no dust and no meningitis cases; (ii) from January to April, humidity is still weak, but high dust loads occur in the atmosphere and this is the meningitis season; (iii) from May to October, humidity is high and there is no meningitis anymore, in presence of dust or not, which flow anyway in higher altitudes. More specifically, the onset of the meningitis season is tightly related to mineral dust flowing close to the surface at the very beginning of the year. During the dry, and the most dusty season period, from February to April, each meningitis peak is preceded by a dust peak, with a 0–2 week lead-time. The importance (duration, intensity) of these meningitis peaks seems to be related to that of dust, suggesting that a cumulative effect in dust events may be important for the meningitis incidence. This is not the case for humidity, confirming the special contribution of dust at this period of the year. The end of the meningitis season, in May, coincides with a change in humidity conditions related to the West African Monsoon. These results, which are interpreted in the context of recent independent epidemiological studies on meningitis highlight, (i) the particular role of dust during the dry season (low humidity conditions) on the onset and the intra-seasonal variability of the meningitis season; (ii) the specific role of high humidity at the end of the meningitis season in two Sahelian countries particularly affected by the disease.► Weak humidity is a necessary but not sufficient condition to impact on meningitis. ► Desert dust is closely linked to the onset and development of the disease. ► The mean dust/meningitis lead-time varies from 0 to 2 weeks. ► High humidity is a sufficient condition to stop the meningitis season.
Keywords: Dust; Meningitis; West Africa; Dry season;

Measurement of particle number and related pollutant concentrations in an urban area in South Brazil by D.M. Agudelo-Castañeda; E.C. Teixeira; S.B.A. Rolim; F.N. Pereira; F. Wiegand (254-262).
The purpose of the present study was to analyze atmospheric particle number concentration at Sapucaia do Sul, in the Metropolitan Area of Porto Alegre, and associate it with the pollutants NO, NO2, and O3. Measurements were performed in two periods: August to October, in 2010 and 2011. We used the following equipment: the continuous particulate monitor (CPM), the chemiluminescent nitrogen oxide analyzer (AC32M), and the UV photometric ozone analyzer (O342M). Daily and hourly particle number concentrations in fractions PR1.0 (0.3–1.0 μm), PR2.5 (1.0–2.5 μm), and PR10 (2.5–10 μm), and concentrations of pollutants NO, NO2, NO x , and O3 were measured. These data were correlated with meteorological parameters such as wind speed, temperature, relative humidity, and solar radiation. The daily variation of OX (NO2 + O3) and its relation with NO2 were also established. The results obtained for daily particle number concentration (particles L−1) showed that the area of study had higher particle number of PR2.5 and PR1.0 size ranges, with values of 19.5 and 28.51 particles L−1, respectively. Differences in particle number concentrations in PR1 and PR2.5 size ranges were found between weekdays and weekends. The daily variation per hour of concentrations of particle number, NO, and NO x showed peaks during increased traffic flow in the morning and in the evening. NO2 showed peaks at different times, with the first peak (morning) 2 h after the peak of NO, and a second peak in the evening (19:00). This is due to the oxidation of NO and to the photolysis of NO3 formed overnight. Correlation analysis suggests that there may be a relationship between the fine and ultrafine particles and NO, probably indicating that they have similar sources, such as vehicular emissions. In addition, a possible relationship of solar radiation with fine particle number concentrations, as well as with O3 was also observed. The results, too, show an inverse relationship between particle number concentration and relative humidity.► Particle number and its association with NO, NO2, NO x , O3 was analyzed. ► Results showed a higher number concentration of the PR2.5 and PR1.0 size ranges. ► Particle number concentration was influenced by emissions from motor vehicles. ► Correlation analysis suggest similar emission sources for NO, NO x and particle number. ► Results indicate influence of meteorological conditions on particle number.
Keywords: Air quality; Particle number; Nitrogen oxides; Ozone;

Impact of cement renders on airborne ozone and carbon dioxide concentrations by Sarah C. Taylor-Lange; Maria C.G. Juenger; Jeffrey A. Siegel (263-266).
The uptake of pollutants by building surfaces can potentially improve both indoor and outdoor air quality. Cement renders provide a unique opportunity for passive pollutant removal because they can cover large surface areas. This study investigated the passive removal of carbon dioxide and ozone by cement renders having varied binder compositions and curing durations. The results from this study demonstrated shorter curing durations resulted in greater pollutant uptake. However, the use of the supplementary cementitious material, metakaolin, in the cement render increased the carbon dioxide ingress while decreasing the ozone uptake. Therefore, the adaptation of the render composition for the best effective application may result in valuable indoor air quality or carbon savings consequences.
Keywords: Sustainable building materials; Passive pollutant removal; Supplementary cementitious materials; Air quality;

Following the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident on March 2011, radioactive products were released in the atmosphere. Simulations at local scale (within 80 km of FNPP1) were carried out by the Institute of Radiation Protection and Nuclear Safety (IRSN) with the Gaussian Puff model pX, during the crisis and since then, to assess the radiological and environmental consequences. The evolution of atmospheric and ground activity simulated at local scale is presented with a “reference” simulation, whose performance is assessed through comparisons with environmental monitoring data (gamma dose rate and deposition). The results are within a factor of 2–5 of the observations for gamma dose rates (0.52 and 0.85 for FAC2 and FAC5), and 5–10 for deposition (0.31 for FAC2, 0.73 for FAC5 and 0.90 for FAC10). A sensitivity analysis is also made to highlight the most sensitive parameters. A source term comparison is made between IRSN's estimation, and those from Katata et al. (2012) and Stohl et al. (2011). Results are quite sensitive to the source term, but also to wind direction and dispersion parameters. Dry deposition budget is more sensitive than wet deposition. Gamma dose rates are more sensitive than deposition, in particular peak values.► We model atmospheric dispersion of the Fukushima accident at local scale (80 km). ► Comparisons to gamma dose rate measurements are within a factor 2–5. ► Comparisons to deposition measurements are within a factor 5–10. ► Source term is the most sensitive parameter. ► Gamma dose rate, especially peak values, are more sensitive than deposition.
Keywords: Atmospheric dispersion; Radionuclides; Fukushima; Model evaluation; Gamma dose rate; Sensitivity;

Dominant global patterns of urbanization and industrialization contribute to large-scale modification of the drivers for hydrologic and biogeochemical processes, as evident in Asia, Africa, and South America which are experiencing rapid population and economic growth. One manifestation of urbanization and economic development is decreases in air quality, increases in dry/wet deposition fluxes, and growing adverse impacts on public health and ecosystem integrity. We examined available long-term (1980–2010) observational data, gathered at weekly intervals, for wet deposition at 19 urban sites in the U.S., and monitoring data (2000–2009) available for 17 urban sites at a monthly scale in East Asia. Our analyses are based on data for four constituents (SO4 2−, NO3 , Ca2+, and Mg2+); differences in atmospheric chemistry and terrestrial sources of these constituents enabled a robust comparative analysis. We examined intra-annual variability and the long-term temporal trajectories of wet deposition fluxes to discern the relative role of anthropogenic and stochastic hydro-climatic forcing. Here, we show that: (1) temporal variability in wet deposition fluxes follows an exponential probability density function at all sites, evidence that stochasticity of rainfall is the dominant control of wet deposition variability; (2) the mean wet deposition flux, μ Ω (ML−2T−1), has decreased in the U.S. over time since enactment of the Clean Air Act, with μ Ω having become homogenized across varying hydro-climatic regimes; and (3) in contrast, μ Ω values for East Asian cities are 3–10 times higher than U.S. cities, attributed to lax regulatory enforcement. Based on the observed patterns, we suggest a stochastic model that generates ellipses within which the μ Ω temporal trajectories are inscribed. In the U.S., anthropogenic forcing (regulations) is dominant in the humid regions, while variability in hydro-climatic forcing explains inter-annual variability in arid regions. Our stochastic analysis facilitates projections of the temporal trajectory shifts in wet deposition fluxes as a result of urbanization and other land-use changes, climate change, and regulatory enforcement.Display Omitted► Hydro-climatology drives inter- and intra-annual variability of wet deposition flux. ► Anthropogenic controls can homogenize wet deposition trajectories of NO3 and SO4. ► Stochastic simulations enabled evaluation of the relative importance of drivers. ► Results are useful in identifying locations that can benefit from strong regulations.
Keywords: Urbanization; Regulation; Stochastic forcing; Anthropogenic impacts;

The spatial and temporal distribution of ambient atmospheric gaseous reactive nitrogen (Nr) species concentrations (ammonia [NH3], nitrogen dioxide [NO2] and nitric acid [HNO3]) were measured at the field scale in an intensive agricultural region in southern Ontario, Canada. Atmospheric concentrations were measured with the Willems badge diffusive passive sampler (18 sites for NH3, 9 sites for NO2 and HNO3) for one year (April 2010–March 2011; under a two week measurement frequency) within a 15 km × 15 km area. Dry deposition was calculated using the inferential method and estimated across the entire study area. The spatial distribution of emission sources associated with agricultural activity resulted in high spatial variability in annual average ambient NH3 concentrations (<3–>8 μg m−3 within a 2 km distance, coefficient of variation ∼50%) and estimated dry deposition (4–13 kg N ha−1 yr−1) between sample sites. In contrast, ambient concentrations and deposition of both NO2 (∼5.2–>6.5 μg m−3; 1.0–1.5 kg N ha−1 yr−1) and HNO3 (0.6–0.7 μg m−3; 0.5–1 kg N ha−1 yr−1) had low variability (coefficient of variation <10%). The observed NH3 concentrations accounted for ∼70% of gaseous Nr dry deposition. High NH3 concentrations suggest that reduced nitrogen species (NH x ) will continue to make up an increasing fraction of Nr deposition within intensive agricultural regions in southern Ontario under legislated nitrogen oxide emission reductions. Further, estimated total inorganic Nr deposition (15–28 kg N ha−1 yr−1) may lead to potential changes in soil processes, nutrient imbalance and altered composition of mycorrhiza and ground vegetation within adjacent semi-natural ecosystems (estimated at ∼10% of the study area).► Ambient NH3 concentrations and deposition were more variable than NO2 and HNO3. ► NH3 was responsible for ∼70% of gaseous reactive nitrogen dry deposition. ► Spatial variation in NO2 concentrations was associated with agricultural traffic. ► Future policy designed to reduce N deposition should focus on reducing NH3 emissions.
Keywords: Agricultural emissions; Atmospheric concentrations; Dry deposition; Ontario Canada; Passive diffusive sampler; Willems badge;

Methyl ethyl ketone (MEK) is an abundant ketone in the urban atmosphere and croton aldehyde (CA) is a strong irritant to eye, nose, and throat. The use of plants able to absorb these compounds is one suggested mitigation method. In order to investigate this method, we determined the uptake rate of these compounds by leaves of two tree species, Castanopsis sieboldii and Viburnum odoratissimum var. awabuki. Using a flow-through chamber method, we found that these species were capable of absorbing both compounds. We also confirmed that the uptake rate of these compounds normalized to the fumigated concentration (A N) was higher at higher light intensities and that there was a linear relationship between A N and stomatal conductance (g S) for both tree species. In concentration-varying experiments, the uptake of MEK and CA seemed to be restricted by partitioning of MEK between leaf water and air. The ratio of the intercellular VOC concentration (C i) to the fumigated concentration (C a) for CA was zero, and the ratio ranged from 0.63 to 0.76 for MEK. The more efficient CA uptake ability may be the result of higher partitioning of CA into leaf water. Our present and previous results also suggest that plant MEK uptake ability was different across plant species, depending on the VOC conversion speed inside leaves.► Leaf uptake of methyl ethyl ketone (MEK) and croton aldehyde (CA) was measured. ► Uptake by urban greening trees at ppbv level was measured with PTR-MS. ► CA uptake rate was higher than that of MEK. ► MEK uptake ability was different across plant species. ► VOC dissolution into water and conversion speed inside leaves seem key factors.
Keywords: Methyl ethyl ketone; Croton aldehyde; Leaf uptake; Intercellular concentration; Proton transfer reaction mass spectrometry;

In this paper, we introduce a new method to estimate the change in mean mixing ratio of a target species at a receptor site due to the contribution of the long-range transport (CLRT). We applied our method to determine inter-annual and inter-seasonal variations in the CLRT of SO2 in Seoul, a major megacity in northeast Asia, during the period from 2001 to 2010. The major potential source areas of SO2 for the 2001–2010 period were located in East China according to the potential source contribution function (PSCF) maps. The CLRT of SO2 in Seoul was estimated to range from 0.40 to 1.03 ppb, which accounted for 8–21% of the ambient mean SO2 mixing ratio in Seoul. The inter-annual variations of estimated CLRT of SO2 was well correlated with those of the total emissions in China during the period of 2001–2008 (R = 0.85). We found that both local emissions from around Seoul and long-range transport from East China, especially the Shandong peninsula, affected the SO2 mixing ratio in Seoul throughout the decade of study. The CLRT of SO2 in Seoul increased after 2007 even though the total emissions of SO2 by China have been decreasing since 2006. The CLRT of SO2 in Seoul was high in spring and winter, which can be attributed to enhanced SO2 emissions in East China during these seasons and a dominant westerly wind. The CLRTs of SO2 accounted for 15, 11, 4, and 12% of the seasonal mean SO2 mixing ratio in spring, summer, fall, and winter, respectively. The uncertainty ranged from 24 to 62% of the estimated CLRT values.► We introduce a method to estimate the contribution of long range transport based on CPSCF. ► This method was applied to investigate temporal variations of SO2 mixing ratio in Seoul. ► The change in the annual SO2 mixing ratio due to long-range transport ranged from 8 to 21%. ► This change increased from 2007 to 2010 and was high in spring and winter than other seasons. ► The uncertainty ranged from 24 to 62% of the estimated changes.
Keywords: Long-range transport; Sulfur dioxide; Potential source contribution function; Receptor model;

Phenol transformation and dimerisation, photosensitised by the triplet state of 1-nitronaphthalene: A possible pathway to humic-like substances (HULIS) in atmospheric waters by Elisa De Laurentiis; Babita Sur; Marco Pazzi; Valter Maurino; Claudio Minero; Gilles Mailhot; Marcello Brigante; Davide Vione (318-327).
The nitroderivatives of polycyclic aromatic hydrocarbons are potentially important photosensitisers in the atmospheric condensed phase. Here we show that the triplet state of 1-nitronaphthalene (31NN*) is able to directly react with phenol, causing its transformation upon irradiation of 1NN in aqueous solution. Additional but less important processes of phenol degradation are reactions with •OH and 1O2, both photogenerated by irradiated 1NN. Dihydroxybiphenyls and phenoxyphenols were detected as main phenol transformation intermediates, likely formed by dimerisation of phenoxy radicals that would be produced upon phenol oxidation by 31NN*. Very interestingly, irradiation with 1NN shifted the fluorescence peaks of phenol (Ex/Em = 220–230/280–320 nm and 250–275/280–320 nm, with Ex/Em = excitation and emission wavelengths) to a region that overlaps with “M-like” fulvic substances (Ex/Em = 250–300/330–400 nm). Moreover, at longer irradiation times a further peak appeared (Ex/Em = 300–450/400–450 nm), which is in the region of HULIS fluorescence. Irradiated material was also able to photoproduce 1O2, thus showing photosensitisation properties. Therefore, compounds with fluorescence properties that closely resemble those of HULIS (they would be identified as HULIS by fluorescence if present in environmental samples) can be formed upon triplet-sensitised transformation of phenol by 1NN.Display Omitted► The triplet state of 1-nitronaphthalene oxidises phenol to phenoxyl radical. ► Dimerisation of phenoxyl yields dihydroxybiphenyls and phenoxyphenols. ► Photoprocessed material had fluorescence properties analogous to HULIS. ► The same material had photosensitising properties (1O2 photoproduction). ► Fluorescence excitation–emission matrix is very useful to study such systems.
Keywords: Photoinduced transformation; Singlet oxygen; Phenoxy radical; Kinetic modelling;

NO x emissions from a Central California dairy by Alam S. Hasson; Segun O. Ogunjemiyo; Steven Trabue; Shawn Ashkan; Kenwood Scoggin; Julie Steele; Catalina Olea; Srikar Middala; Kennedy Vu; Austen Scruggs; Laxmi R. Addala; Lucien Nana (328-336).
Concentrations of NO x (NO + NO2) were monitored downwind from a Central California dairy facility during 2011 and 2012. NO x concentrations at the dairy were significantly higher than the background levels during August 2011 primarily due to the presence of elevated NO, but were indistinguishable from background concentrations during January and April 2012. A Gaussian plume model (AERMOD) and a Lagrangian back trajectory model (Wind Trax) were used to estimate the flux of NO from the dairy during August 2011 with the assumption that emissions were primarily from animal feed. NO emissions from silage were also directly measured from feed to provide additional insight into the sources. Isolation flux chamber measurements imply an NO flux from the feed of about 1.3 × 10−3 g m−2 h−1, but these relatively low fluxes are inconsistent with the elevated NO concentrations observed during August 2011. This implies that either the flux chamber method grossly underestimates the true NO emissions from feed, or that most of the ambient NO measured at the dairy is from other sources. Emissions from farm machinery may account for the NO concentrations observed. Animal feed thus appears to be a small contributor to NO x emissions within Central California.► NO x emissions were measured from a commercial dairy. ► NO x is primarily emitted from feed as NO. ► Emissions from feed are a small fraction of total NO x emissions from the dairy. ► NO x emissions from dairies appear to be a small contributor to regional NO x levels.
Keywords: NO x ; Silage; Ozone; Confined animal facilities;

Modelling Saharan dust transport into the Mediterranean basin with CMAQ by David de la Paz; Michel Vedrenne; Rafael Borge; Julio Lumbreras; Juan Manuel de Andrés; Javier Pérez; Encarnación Rodríguez; Angeliki Karanasiou; Teresa Moreno; Elena Boldo; Cristina Linares (337-350).
The need for a better quantification of the influence of Saharan dust transport processes on the air quality modelling in the Mediterranean basin led to the formulation of a dust emission module (DEM) integrated into the Air Quality Risk Assessment System for the Iberian Peninsula (SERCA). This paper is focused on the formulation of DEM based on the GOCART aerosol model, along with its integration and execution into the air quality model. It also addresses the testing of the module and its evaluation by contrasting results against satellite products such as MODIS and CALIPSO and ground-level observations of aerosol optical thickness (AOT) and concentration levels of PM10 for different periods in July 2007. DEM was found capable of reproducing the spatial (horizontal and vertical) and temporal profiles of Saharan dust outbreaks into the Mediterranean basin and the Atlantic coast of Africa. Moreover, it was observed that its combination with CMAQ increased the correlation degree between observed and modelled PM10 concentrations at the selected monitoring locations. DEM also enhanced CMAQ capabilities to reproduce observed AOT, although significant underestimations remain. The implementation of CMAQ + DEM succeeded in capturing Saharan dust transport into the Iberian Peninsula, with contributions up to 25 and 14 μg m 3 in 1 h and 24 h average PM10 respectively. The general improvement of total PM10 predictions in Spain are however moderate. The analysis of model performance for the main PM components points out that remaining PM10 underestimation is due to dust local sources missing in the inventories and misrepresentation of organic aerosol processes, which constitutes the main areas for future improvement of CMAQ capabilities to simulate particulate matter within SERCA.► A dust emission module has been implemented in the WRF–SMOKE–CMAQ modelling system. ► The system has been applied and evaluated for Saharan dust outbreaks into Europe. ► Contributions up to 14 μg m−3 to 24-h PM10 concentration were predicted in Spain. ► Global R-value for ground-level PM10 increased from 0.33 to 0.48. ► Good agreement with satellite products although AOD is underestimated.
Keywords: Air quality modelling; Dust transport; Particulate matter; CMAQ; DEM; SERCA; Remote sensing; AOT; Saharan dust;

Reactivity of E-butenedial with the major atmospheric oxidants by Pilar Martín; Beatriz Cabañas; Inmaculada Colmenar; María Sagrario Salgado; Florentina Villanueva; Araceli Tapia (351-360).
The degradation reactions of E-butenedial with OH and NO3 radicals and Cl atoms were investigated using a relative rate method. The experiments were carried out at ∼298 ± 1 K and an atmospheric pressure of N2 or synthetic air as the bath gas. Three different sampling/detection methods have been used for the study with Cl, OH and NO3: (1) Solid-Phase Microextraction and Gas Chromatography with Flame Ionization Detection (SPME/GC-FID), (2) ‘in situ’ with long-path Fourier Transform Infrared Spectroscopy (FTIR), and (3) Tenax solid adsorbent and Gas Chromatography with Mass Spectrometry (Tenax/GC–MS) as the detection system. The measured rate coefficients for E-butenedial (cm3 molecule−1 s−1) are as follows: (1.35 ± 0.29) × 10−10 for the Cl atom, (3.45 ± 0.34) × 10−11 for the OH radical and (1.70 ± 0.83) × 10−15 for the NO3 radical. For the reaction of Cl and NO3 these are the first rate coefficient data to be reported and in the case of OH the literature value is confirmed. This study confirms that the chemical structure of the organic substances does not influence on the reactivity with Cl, has a significant effect for OH reactions and is very important for NO3 reactions. Calculated atmospheric lifetimes are in the order of days for Cl and NO3 reactions and hours for OH. In the case of Cl atoms, a lifetime of 20 h is estimated in the early morning hours in urban coastal air. These shorter lifetimes imply that the degradation reactions of E-butenedial are of great importance because their reaction products are secondary pollutants that are involved in processes such as the formation of photochemical smog or peroxyacyl nitrates (PANs).Display Omitted► The degradation reactions of E-butenedial with OH and NO3 radicals and Cl atoms were investigated. ► For the reaction of Cl and NO3, these are the first data for rate coefficients. ► The results show that the chemical structure of the organic compound is the decisive factor for NO3 reactions.
Keywords: Butenedial; Kinetic study; Atmospheric oxidants; Relative method; Atmospheric implications;

Impact of urbanization on the air pollution “holiday effect” in Taiwan by Pei-Hua Tan; Chia Chou; Charles C.-K. Chou (361-375).
The spatio-temporal characteristics of the “holiday effect”, defined as the difference in air pollutant concentrations between the holiday (Chinese New Year) and non-holiday periods during 1994–2008, and its association with the degree of urbanization in Taiwan are examined. Daily surface measurements of six major pollutants from 54 monitoring stations of the Taiwan Environmental Protection Administration are used. Holiday effects are found for almost all air pollutants in all divisions and individual stations. A widespread holiday effect with consistent signs suggests a high degree of urbanization over Taiwan. Holiday effects are stronger in the west than in the east, due to urban-rural differences, and have a distinct north–south difference in the west, due to different emission sources. In the spatial distribution, as the population (motor vehicle) number in the division increases, holiday effects of NO x , CO and NMHC are intensified. Holiday effects of pollutants can also be stronger when the associated dominant anthropogenic sources in the division have larger emissions. Both imply the association of a stronger holiday effect with a higher degree of urbanization in the division. In the temporal variation, on the other hand, holiday effects and pollutant concentrations tend to weaken and reduce in almost all the urban divisions for all six pollutants except O3. These weakening trends imply possible contributions of other effects, such as the mature state of urbanization for the urban division, the effective pollution-control measures and behavioral pattern changes.► Holiday effects are found for almost all six pollutants over six divisions in Taiwan. ► The strength of the holiday effect is mainly caused by the degree of urbanization. ► The degree of division urbanization is related to population and vehicle number. ► The degree of division urbanization is related to anthropogenic sources. ► Normalized holiday effects by population are mostly weakening in the urban divisions.
Keywords: Holiday effect; Weekend effect; Urbanization; Anthropogenic source; Sector emission; Industrialization;

Assessment of human exposure level to PM10 in China by Xingqin An; Qing Hou; Nan Li; Shixian Zhai (376-386).
Epidemiological studies have found that atmospheric particulate matter, especially PM10 (inhalable particulate matter with aerodynamic diameter less than or equal to 10 μm) is one of the pollutants that are harmful to human health. In recent years, particulate matter pollution in China is becoming increasingly serious and PM10 has become the primary pollutant in Beijing and other cities. Therefore, it is necessary to carry out studies and a health damage assessment of PM10. In human health damage assessment, measuring human exposure level to PM10 is required and crucial to provide accurate exposure data for the exposure–response relationship, and also for the accurate quantitative assessment of human exposure.The spatial distribution of particle concentration in China is variable because of spatial differences in the local economic level and the geographical environment. Along with the accelerating urbanisation in China, city population density is high, and the population distribution is variable between and within cities, thus resulting in different population numbers exposed to different concentration ranges. Therefore, an accurate assessment of China's level of exposure to particulate matter is a priority and the basis for assessing the damage to public health caused by particle pollution.Using high accuracy population and PM10 monitoring data, this study analysed the human exposure to PM10 in different regions and typical cities of China. The results show that for most areas of China, the population-weighted PM10 exposure concentration is slightly higher than the annual mean concentration, meaning that more of the population is exposed to high concentrations, and most of the population is exposed to levels that meet the second national standard (between 40 and 100 μg m−3), occupying about 83.7% of population and 76.3% of area in China. The population exposure to PM10 is higher in two types of typical regions and cities: areas with dense human populations such as Jingjinji, Beijing and Tianjin, and areas with more sand dust and factories such as Northwest and Chongqing.► PM10 is one of the pollutants that are harmful to human health. ► In human health damage assessment, the level of human exposure to PM10 is crucial. ► The spatial distributions of PM10 and the human population in China are uneven. ► This study assessed the level of human exposure to PM10 in China. ► In China, most of the population is exposed to a level that meets the second national standard.
Keywords: PM10; Human exposure level to PM10 pollution; Assessment; Heath loss;

A Multisite-Multipollutant Air Quality Index by A. Plaia; F. Di Salvo; M. Ruggieri; G. Agró (387-391).
In this paper, starting from a multivariate spatio-temporal array, containing air pollution data collected for the main pollutants at different monitoring sites over a 1-year period, a new approach is proposed to get a Multipollutant-Multisite Air Quality Index (AQI) time series. A two steps aggregation, related to space and to pollutants, is considered. For the first aggregation (spatial synthesis) a PCA is performed on data array opportunely rearranged, while the index I 2, proposed in Ruggieri and Plaia (2011), is used for the second aggregation (pollutant synthesis), obtaining the new index I 2 MS . Daily data of four air pollutants from the city of Palermo (Italy) are analyzed to test the performance of the new index. The index I 2 MS overcomes the main issues presented by other indices, many of which based on AQI computed by US EPA, considering the highest pollutant concentration adequately standardized. The comparison carried out shows that the index here proposed has a better performance than the synthesis by median-maximum (Bruno and Cocchi, 2002) and the synthesis by PCA along both space and pollutant dimensions, if the conjoint effect on air quality of the different pollutants has to be taken into account.► We propose a Multipollutant-Multisite Air Quality Index. ► We take into account the adverse effects due to the coexistence of all the pollutants. ► The methodology is based on an asymmetrical approach to cope with the three-mode array synthesis. ► The index behavior along the year is free from ambiguity and eclipsicity problems.
Keywords: Air Quality Index; Three-way array;

Ice nucleation activity of bacteria isolated from cloud water by Muriel Joly; Eléonore Attard; Martine Sancelme; Laurent Deguillaume; Caroline Guilbaud; Cindy E. Morris; Pierre Amato; Anne-Marie Delort (392-400).
Some Gamma–Proteobacteria can catalyze ice formation thereby potentially contributing to the induction of precipitation in supercooled clouds and subsequently to bacterial deposition. Forty-four bacterial strains from cloud water were screened for their capacity to induce freezing. Seven strains (16%) were active at −8 °C or warmer and were identified as Pseudomonas syringae, Xanthomonas spp. and Pseudoxanthomonas sp. Phylogenetic analysis revealed that the P. syringae strains in clouds at the Puy de Dôme belonged to clades that are among the most infrequently detected in the environment, while widespread clades were absent suggesting some extent of selection or unusual biogeography of the bacteria at the sampling site. Three strains induced freezing at −3 °C while the others nucleated ice at −4 °C to −6 °C. The freezing profiles revealed that the peaks of activity were centered around −3.5 °C, −5 °C and/or −8.5 °C depending on the strain. The frequency of ice-nuclei (IN) per cell at −6 °C was generally below 0.5% and reached up to 4.2% in one strain. We estimated that clouds influenced by vegetated areas would carry between less than 1 and ∼500 bacterial IN mL−1 of water active between −3 °C and −10 °C depending on the season. These data will contribute to modeling the impact of bacterial IN on precipitation at regional scales.► Ice nucleation active (INA) bacteria were recovered from cloud water. ► 16% of the Pseudomonas-like bacteria isolated from cloud water are INA. ► Active strains identified as Pseudomonas syringae, Xanthomonas sp. and Pseudoxanthomonas sp. ► The P. syringae strains from clouds belong to rare clades in the environment. ► The number of bacterial IN in cloud water was estimated to fall between 0 and 500/mL.
Keywords: Cloud; Bacteria; Ice nucleation; Pseudomonas; Bioprecipitation;

Observation of NO3 radicals over Shanghai, China by Shanshan Wang; Chanzhen Shi; Bin Zhou; Heng Zhao; Zhuoru Wang; Suna Yang; Limin Chen (401-409).
Observation of NO3 radicals was performed in an urban area of Shanghai using differential optical absorption spectroscopy from August 15 to October 7, 2011. With the exception of the data below detection limit (0.008 ppb), the observed mean NO3 mixing ratios were 0.016 ± 0.009 ppb with a range of 0.008–0.095 ppb. Under the assumption of steady state, the NO3 production rates were calculated ranging from 0.08 ppb h 1 to 12.85 ppb h 1 throughout the observation, and averaged at 1.10 ± 1.09 ppb h 1. The calculated NO3 lifetime was determined from several seconds to a maximum value of about 900 s with an average of 102 ± 93 s. The correlation of NO3 production rates to the NO2 mixing ratio is about 0.76, which is larger than the correlation between NO3 production rates and O3. Hence, the variability of NO3 production mainly results from the ambient NO2 levels present. The positive correlation between the NO3 mixing ratio and its production rates indicates that direct sinks cannot be neglected for the nitrate radical's chemistry in Shanghai, especially under polluted conditions. However, the slope of the logarithmic correlation between NO3 lifetime and NO2 mixing ratio is very close to −1, which means that the NO3 removal strongly depends on the indirect loss process. Moreover, the atmospheric suspended particles have an important impact on NO3 lifetime because the NO3 removal was accelerated by the high PM10 concentration, e.g. all NO3 lifetime are below 30 s when the PM10 concentration was larger than 0.05 mg m 3 during the severe pollution episode in October.► The nighttime NO3 radical was observed by DOAS for two months in Shanghai, China. ► The variability of NO3 production comes mainly from the ambient NO2 levels. ► The NO3 lifetime was effected by the PM10 due to the indirect losses process.
Keywords: NO3 radical; DOAS; Shanghai;

The possible cause and sources of enhanced ozone at upper tropospheric equatorial Africa, observed by cruise Measurements of OZone by Airbus In Service Aircraft (MOZAIC) during the Northern Hemisphere winter in 1996 and 1997 on flight routes from Johannesburg to Vienna, are investigated. Two enhanced ozone events over upper tropospheric equatorial Africa are identified from MOZAIC observations on April 6, 1996 and March 27, 1997. High resolution ECMWF reanalysis GOME ozone has exhibited enhancement as well during these periods suggesting that the two events are not isolated small scale events but part of a larger scale process. As a result, the source and mechanisms of ozone increase over the region are further analysed using reanalysis data from ECMWF, outgoing long wave radiation (OLR) from NOAA and Meteosat images from NASA, International Satellite Cloud Climatology Project. Equivalent latitude computed from potential vorticity has shown that massive mid- and high-latitude stratospheric ozone rich airmass is funnelled into lower latitude troposphere through troughs extending from large amplitude planetary waves towards equator. The Space-time Fourier decomposition of meridionally averaged zonal wind has revealed that these planetary wave activities are linked to waves with zonal wavenumber 1–2, which prevail during Northern Hemisphere winter. Additional analysis to understand the mechanisms of ozone enhancement was made using Multitaper Method–Singular Value Decomposition (MTM–SVD) spectral approach. The analysis confirms that ozone enhancement over the region is dependent on the relative position of positive PV and direction of wind anomalies. The high relative humidity measured simultaneously with ozone onboard MOZAIC, Meteosat imageries and circulation during the events have shown presence of deep convection. The coherent variation of OLR and ozone found over 8-day temporal cycle determined from MTM–SVD has indicated existence of OLR negative forcing in the upper troposphere and positive forcing in the lower stratosphere. These results show coupling of PV intrusion and deep convection over continental equatorial Africa in the same manner as the climatologically preferred intrusion over mid-ocean in eastern pacific. Moreover, the results enrich previous understanding with purely observational high resolution MOZAIC and ERA-Interim datasets, and statistical method.► Ozone enhancement over upper troposphere equatorial Africa is investigated. ► Planetary waves are responsible for ozone enhancements over equatorial Africa. ► The MTM–SVD method reveals how planetary waves modulate stratospheric PV intrusion. ► Coupling of PV intrusion and deep convection during ozone enhancement is confirmed. ► PV and wind anomalies during a particular cycle of the wave govern ozone variation.
Keywords: Ozone enhancement; Stratospheric intrusion; Convection; MTM–SVD; Troposphere;

Source apportionment of particulate pollutants in the atmosphere over the Northern Yellow Sea by L. Wang; J.H. Qi; J.H. Shi; X.J. Chen; H.W. Gao (425-434).
Atmospheric aerosol samples were collected over the Northern Yellow Sea of China during the years of 2006 and 2007, in which the Total Carbon (TC), Cu, Pb, Cd, V, Zn, Fe, Al, Na+, Ca2+, Mg2+, NH4 +, NO3 , SO4 2−, Cl, and K+ were measured. The principle components analysis (PCA) and positive matrix factorization (PMF) receptor models were used to identify the sources of particulate matter. The results indicated that seven factors contributed to the atmospheric particles over the Northern Yellow Sea, i.e., two secondary aerosols (sulfate and nitrate), soil dust, biomass burning, oil combustion, sea salt, and metal smelting. When the whole database was considered, secondary aerosol formation contributed the most to the atmospheric particle content, followed by soil dust. Secondary aerosols and soil dust consisted of 65.65% of the total mass of particulate matter. The results also suggested that the aerosols over the North Yellow Sea were heavily influenced by ship emission over the local sea area and by continental agricultural activities in the northern China, indicating by high loading of V in oil combustion and high loading of K+ in biomass burning. However, the contribution of each factor varied greatly over the different seasons. In spring and autumn, soil dust and biomass burning were the dominant factors. In summer, heavy oil combustion contributed the most among these factors. In winter, secondary aerosols were major sources. Backward trajectories analysis indicated the 66% of air mass in summer was from the ocean, while the air mass is mainly from the continent in other seasons.► Principle component analysis and positive matrix factorization are used. ► Secondary aerosols and soil dust are the dominant sources. ► Aerosols are influenced by ship emission and continental agricultural activities. ► Seasonal variations of the contributions of the sources are exhibited.
Keywords: Atmospheric aerosol; Receptor models; Source apportionment; Northern Yellow Sea;

The surface ozone concentrations at the Tanah Rata regional Global Atmosphere Watch (GAW) station, Malaysia (4°28′N, 101°23′E, 1545 m above Mean Sea Level (MSL)) from June 2006 to August 2008 were analyzed in this study. Overall the ozone mixing ratios are very low; the seasonal variations show the highest mixing ratios during the Southwest monsoon (average 19.1 ppb) and lowest mixing ratios during the spring intermonsoon (average 14.2 ppb). The diurnal variation of ozone is characterised by an afternoon maximum and night time minimum. The meteorological conditions that favour the formation of high ozone levels at this site are low relative humidity, high temperature and minimum rainfall. The average ozone concentration is lower during precipitation days compared to non-precipitation days. The hourly averaged ozone concentrations show significant correlations with temperature and relative humidity during the Northeast monsoon and spring intermonsoon. The highest concentrations are observed when the wind is blowing from the west. We found an anticorrelation between the atmospheric pressure tide and ozone concentrations. The ozone mixing ratios do not exceed the recommended Malaysia Air Quality Guidelines for 1-h and 8-h averages. Five day backward trajectories on two high ozone episodes in 07 August 2006 (40.0 ppb) and 24 February 2008 (45.7 ppb) are computed using the HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model to investigate the origin of the pollutants and influence of regional transport. The high ozone episode during 07 August 2006 (burning season during southwest monsoon) is mainly attributed to regional transport from biomass burning in Sumatra, whereas favourable meteorological conditions (i.e. low relative humidity, high temperature and solar radiation, zero rainfall) and long range transport from Indo-China have elevated the ozone concentrations during 24 February 2008.► Overall ozone mixing ratios are low, with seasonal averages of 14.2–19.1 ppb. ► High temperature, low RH and minimum rainfall favour the formation of high O3 levels. ► Diurnal variation is characterized by an afternoon maximum and a night time minimum. ► High O3 on 07 August 2006 is mainly due to regional transport from biomass burning. ► Meteorological conditions and long range transport have elevated O3 on 24 Feb 2008.
Keywords: Surface ozone; Biomass burning; Meteorological conditions; Regional transport; Backward trajectory;

Impact of biogenic volatile organic compounds on ozone production at the Taehwa Research Forest near Seoul, South Korea by So-Young Kim; Xiaoyan Jiang; Meehye Lee; Andrew Turnipseed; Alex Guenther; Jong-Choon Kim; Suk-Jo Lee; Saewung Kim (447-453).
The importance of biogenic volatile organic compounds (BVOCs) in understanding of air-quality and climate on regional to global scales has been highlighted in a number of modeling and observational studies. At the same time, another important emerging research topic in atmospheric chemistry is the regional and global impacts of fast growing East Asian megacities. These two research topics must be integrated in order to adequately understand and address air quality challenges emerging from Eastern Asian megacities surrounded by planted or natural forest areas. We present initial measurement results for May, June and September 2011 from the Taehwa Research Forest (TRF) which has been developed to serve as a long term observatory for investigating biosphere–atmosphere interactions at the edge of the Seoul Metropolitan Area (population of ∼23.5 million). The comprehensive measurement datasets of ozone and its precursors such as CO, NO x , SO2 and VOCs shows that high ozone episodes in the suburban site could not be explained by just anthropogenic pollutants alone. In addition, isoprene (C5H8) and monoterpenes (C10H16) were observed as two of the most important OH chemical sinks inside of the forest canopy. In order to understand the impacts of these BVOCs on ozone and related photochemistry, we conducted model sensitivity simulations using a coupled meteorology-chemistry model (WRF-Chem) for conditions including with and without BVOC emissions. The modeling results suggest that BVOC emissions could enhance regional daytime ozone production from 5 to 20 ppbv. The observed temporal variations in ozone correspond well with the variations in BVOCs, which likely reflects the influence of BVOCs on ozone formation. These findings strongly suggest that interactions between anthropogenic pollutants and BVOCs must be understood and quantified in order to assess photochemical ozone formation in the regions surrounding East Asian megacities.► We presented trace gas observation results in the suburban Seoul Metropolitan Area. ► We found BVOCs are dominant OH chemical sinks. ► WRF-Chem modeling results show that BVOCs are important ozone precursors.
Keywords: Tropospheric ozone; BVOCs; WRF-Chem; Asian megacities;

Impact of elevated aerosol layer on the cloud macrophysical properties prior to monsoon onset by S. Dipu; Thara V. Prabha; G. Pandithurai; J. Dudhia; G. Pfister; K. Rajesh; B.N. Goswami (454-467).
Atmospheric aerosols alter the radiation balance by absorption/scattering of solar radiation, and indirectly by modifying the cloud microphysical properties. Observations during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) provide a unique opportunity to investigate the aerosol–cloud interaction in a dry to wet transition phase prior to the onset of southwest monsoon. It is observed that aerosol loading increased over the central Indian region in spite of the increase in surface rainfall. This aerosol loading was observed mainly in the 2–5 km level above surface. The origin and influence of elevated aerosol layer have been investigated with the help of WRF-Chem simulations by conducting sensitivity experiments for dust emissions, modified based on the satellite observations. To enhance the dust emissions, the erodible fraction over the Thar Desert region is enhanced to an average factor of 1.7 based on TOMS aerosol index (AI) and USGS land use category, which contributed to enhanced dust emissions by a factor of 1.25 over the study region. This enhancement of dust emission from Thar Desert can result in an increased radiative heating due to elevated aerosol layers, which leads to an increase in the ice mixing ratio and ice water content in the regions of dry to wet transition. It is shown that even natural dust emissions (without changes in anthropogenic emissions) may also influence the spatial and temporal distribution of cloud and precipitation and the hydrological cycle.► Elevated aerosol layer (2–5 km) was caused by increased dust emission from Thar Desert. ► Interaction of aerosol via radiation parameterization is investigated with WRF-Chem. ► Enhanced dust emission leads to increased heating in the elevated aerosol layers. ► Enhanced dust emissions may influence cloud distribution and precipitation patterns.
Keywords: Elevated aerosol layer; Aerosol optical depth; Atmospheric heating; Aerosol flux; Cloud macrophysics;

Determination of isoprene-derived secondary organic aerosol tracers (2-methyltetrols) by HPAEC-PAD: Results from size-resolved aerosols in a tropical rainforest by Zhi-Sheng Zhang; Guenter Engling; Chuen-Yu Chan; Yi-Hong Yang; Mang Lin; Si Shi; Jun He; Yi-De Li; Xue-Mei Wang (468-476).
Secondary organic aerosol (SOA) formed from oxidation of isoprene, the most abundant nonmethane hydrocarbon in the atmosphere, has been estimated to contribute significantly to the global aerosol burden. Measurement of isoprene-derived SOA molecular markers has become an effective method for the investigation of biogenic aerosol contributions in the atmosphere. The primary goals of this work are to present a new method based on high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) to quantify diastereoisomeric 2-methyltetrols (2-methylerythritol and 2-methylthreitol), marker compounds of isoprene-derived SOA, and thus to obtain better understanding regarding their abundance and size distribution specifically in a rainforest area. The 2-methyltetrol data, along with water-soluble inorganic ion concentrations, were obtained from size-segregated samples collected at a tropical rainforest site in South China during the period from May to June, 2010. The concentrations of 2-methyltetrols from selected samples measured by HPAEC-PAD showed good agreement with those measured by GC/MS. Overall, the HPAEC-PAD method provides a simple and fast, yet selective and sensitive, alternative to GC/MS for 2-methyltetrol determination, allowing for more efficient analysis of large sample numbers. The size distributions of 2-methylerythritol and 2-methylthreitol both exhibited a unimodal pattern, peaking in the particle size range of 0.44–1.0 μm, where their average concentrations were 11.7 and 4.2 ng m 3, respectively. A strong correlation between 2-methylerythritol and 2-methylthreitol was observed among the entire set of size-segregated samples, indicating their photochemical origin and similar formation mechanism regardless of particle sizes. Compared to the results obtained from previous chamber studies, the similar isomeric fraction of 2-methyltetrols obtained in this study and other field studies confirms their formation through photooxidation of isoprene.► New HPAEC-PAD method for identifying and quantifying 2-methyltetrols. ► The size distributions of 2-methyltetrols exhibited a unimodal pattern. ► 2-Methylerythritol correlated excellently with 2-methylthreitol. ► Similar isomeric fractions of the 2-methyltetrols were obtained in all particle sizes.
Keywords: 2-Methylerythritol; 2-Methylthreitol; Biogenic SOA; Isoprene; Particle size distribution;

Meteorological overview and plume transport patterns during Cal-Mex 2010 by Naifang Bei; Guohui Li; Miguel Zavala; Hugo Barrera; Ricardo Torres; Michel Grutter; Wilfredo Gutiérrez; Manuel García; Luis Gerardo Ruiz-Suarez; Abraham Ortinez; Yaneth Guitierrez; Carlos Alvarado; Israel Flores; Luisa T. Molina (477-489).
Cal-Mex 2010 Field Study is a US-Mexico collaborative project to investigate cross-border transport of emissions in the California-Mexico border region, which took place from May 15 to June 30, 2010. The current study presents an overview of the meteorological conditions and plume transport patterns during Cal-Mex 2010 based on the analysis of surface and vertical measurements (radiosonde, ceilometers and tethered balloon) conducted in Tijuana, Mexico and the modeling output using a trajectory model (FLEXPRT-WRF) and a regional model (WRF). The WRF model has been applied for providing the meteorological daily forecasts that are verified using the available observations. Both synoptic-scale and urban-scale forecasts (including wind, temperature, and humidity) agree reasonably well with the NCEP-FNL reanalysis data and the measurements; however, the WRF model frequently underestimates surface temperature and planetary boundary layer (PBL) height during nighttime compared to measurements. Based on the WRF-FLEXPART simulations with particles released in Tijuana in the morning, four representative plume transport patterns are identified as “plume-southeast”, “plume-southwest”, “plume-east” and “plume-north”, indicating the downwind direction of the plume; this will be useful for linking meteorological conditions with observed changes in trace gases and particular matter (PM). Most of the days during May and June are classified as plume-east and plume-southeast days, showing that the plumes in Tijuana are mostly carried to the southeast and east of Tijuana within the boundary layer during daytime. The plume transport directions are generally consistent with the prevailing wind directions on 850 hPa. The low level (below 800 m) wind, temperature, and moisture characteristics are different for each plume transport category according to the measurements from the tethered balloon. Future studies (such as using data assimilation and ensemble forecasts) will be performed to improve the temperature, wind and PBL simulations.► We present meteorological overview and plume transport patterns during Cal-Mex 2010. ► We identify four plume transport patterns along the California-Mexico border. ► The plume transport directions are mostly consistent with the wind directions on 850 hPa.
Keywords: California-Mexico border; Meteorology; Plume transport; Emissions;

Sources and transport of black carbon at the California–Mexico border by Christopher A. Shores; Michael E. Klapmeyer; Marina E. Quadros; Linsey C. Marr (490-499).
At international border areas that suffer from poor air quality, assessment of pollutant sources and transport across the border is important for designing effective air quality management strategies. As part of the Cal-Mex 2010 field campaign at the US–Mexico border in San Diego and Tijuana, we measured black carbon (BC) concentrations at three locations in Mexico and one in the United States. The measurements were intended to support the following objectives: to characterize the spatial and temporal variability in BC, to estimate the BC emission inventory, to identify potential source areas of BC emissions, and to assess the cross-border transport of BC. Concentrations at Parque Morelos, the campaign's supersite, averaged 2.2 μg m−3 and reached a maximum value of 55.9 μg m−3 (1-min average). Sharp, regularly occurring peaks around midnight were suggestive of clandestine industrial activity. BC concentrations were more than two times higher, on average, in Tijuana compared to San Diego. BC and carbon monoxide (CO) were strongly correlated at the three sites in Mexico. The ΔBC/ΔCO ratio of 5.6 ± 0.5 μg m−3 ppm−1 in Tijuana, or 4.7 ± 0.5 μg m−3 ppm−1 when adjusted for seasonal temperature effects to represent an annual average, was comparable to that in other urban areas. Tijuana's emissions of BC were estimated to be 230–890 metric tons per year, 6–23% of those estimated for San Diego. Large uncertainties in this estimate stem mainly from uncertainties in the CO emission inventory, and the lower end of the estimate is more likely to be accurate. Patterns in concentrations and winds suggest that BC in Tijuana was usually of local origin. Under typical summertime conditions such as those observed during the study, transport from Tijuana into the US was common, crossing the border in a northeasterly direction, sometimes as far east as Imperial County at the eastern edge of California.Display Omitted► We measured black carbon at four locations in San Diego and Tijuana. ► Black carbon was at least two times higher in Tijuana than in San Diego. ► We estimated Tijuana's black carbon emissions to be 230–890 metric tons per year. ► Emissions of black carbon were sometimes transported from Tijuana into the US.
Keywords: Black carbon; Elemental carbon; United States; Mexico; Border; Emissions; Carbon monoxide; Trajectory analysis;

Submicron organic aerosol in Tijuana, Mexico, from local and Southern California sources during the CalMex campaign by S. Takahama; A. Johnson; J. Guzman Morales; L.M. Russell; R. Duran; G. Rodriguez; J. Zheng; R. Zhang; D. Toom-Sauntry; W.R. Leaitch (500-512).
The CalMex campaign was conducted from May 15 to June 30 of 2010 to study the properties and sources of air pollution in Tijuana, Mexico. In this study, submicron organic aerosol mass (OM) composition measured by Fourier Transform Infrared Spectroscopy (FTIR), Aerosol Chemical Speciation Monitor (ACSM), and X-ray spectromicroscopy are combined with statistical analysis and measurements of other atmospheric constituents. The average (±one standard deviation) OM concentration was 3.3 ± 1.7 μg m−3. A large source of submicron aerosol mass at this location was determined to be vehicular sources, which contributed approximately 40% to the submicron OM; largely during weekday mornings. The O/C ratio estimated from ACSM measurements was 0.64 ± 0.19; diurnal variations in this value and the more oxygenated fraction of OM as determined from Positive Matrix Factorization and classification analyses suggest the high degree of oxygenation originates from aged OM, rather than locally-produced secondary organic aerosol. A large contribution of this oxygenated aerosol to Tijuana from various source classes was observed; some fraction of this aerosol mass may be associated with non-refractory components, such as dust or BC. Backtrajectory simulations using the HYSPLIT model suggest that the mean wind vector consistently originated from the northwest region, over the Pacific Ocean and near the Southern California coast, which suggests that the origin of much of the oxygenated organic aerosol observed in Tijuana (as much as 60% of OM) may have been the Southern California Air Basin. The marine aerosol contribution to OM during the period was on average 23 ± 24%, though its contribution varied over synoptic rather than diurnal timescales. BB aerosol contributed 20 ± 20% of the OM during the campaign period, with notable BB events occurring during several weekend evenings.► Submicron OM composition was measured by FTIR, ACSM, and STXM during CalMex. ► The average OM concentration was 3.3 μg m 3 in Tijuana. ► Vehicular sources contributed approximately 40% to the submicron OM. ► OM measured in Tijuana was highly oxygenated (average O/C = 0.6). ► The Southern California Air Basin was a large source of oxygenated OM in Tijuana.
Keywords: Organic aerosol; Air pollution; FTIR; ACSM; STXM; NEXAFS; Border region;

Measurements of formaldehyde at the U.S.–Mexico border during the Cal-Mex 2010 air quality study by Jun Zheng; Renyi Zhang; Jessica P. Garzón; María E. Huertas; Misti Levy; Yan Ma; Ricardo Torres-Jardón; Luis G. Ruiz-Suárez; Lynn Russell; Satoshi Takahama; Haobo Tan; Guohui Li; L.T. Molina (513-520).
Ambient formaldehyde (HCHO), along with other volatile organic compounds (VOCs), was measured using proton-transfer reaction mass spectrometry (PTR-MS) at a ground site along the U.S.–Mexico border during the Cal-Mex 2010 air quality study. During the observation period, the HCHO mixing ratio varied between 1.0 ppbv and 13.7 ppbv. On average, a daily maximum of 6.3 ± 2.6 ppbv occurred around 10 AM and a minimum of 2.8 ± 1.3 ppbv was observed around midnight. The early onset of the HCHO daily maximum (∼ 3 h before the solar noon) indicated the presence of primary HCHO sources and a fast photolysis loss of HCHO, consistent with a measured low ratio of HCHO to acetaldehyde of (2.5 ± 0.8). Using the simulated photolysis rates, we estimated the contribution of OH radical production from HCHO photolysis relative to that from O3 photolysis, with a ratio from 0.8 to 18 and the highest values around traffic rush hours. Hence, our results indicate that HCHO plays a dominant role in regulating the OH radical budget in the area.► We measured HCHO using a fast-response PTR-MS during the Cal-Mex 2010 study. ► Up to 13.7 ppbv HCHO was measured along the Tijuana–San Diego boarder. ► In this study, HCHO was found to be more important than O3 in contributing to OH radicals and the local photochemistry.
Keywords: Formaldehyde; PTR-MS; Ozone; Hydroxyl radical; Secondary organic aerosol; Air pollution;

Volatile organic compounds in Tijuana during the Cal-Mex 2010 campaign: Measurements and source apportionment by Jun Zheng; Jessica P. Garzón; María E. Huertas; Renyi Zhang; Misti Levy; Yan Ma; José I. Huertas; Ricardo T. Jardón; Luis G. Ruíz; Haobo Tan; Luisa T. Molina (521-531).
As part of the Cal-Mex 2010 air quality study, a proton transfer reaction–mass spectrometer (PTR-MS) was deployed at the San Diego–Tijuana border area to measure volatile organic compounds (VOCs) from 15 May to 30 June 2010. The major VOCs identified during the study included oxygenated VOCs (e.g., methanol, acetaldehyde, acetone, and methyl ethyl ketone) and aromatics (e.g., benzene, toluene, C8- and C9-aromatics). Biogenic VOCs (e.g., isoprene) were scarce in this region because of the lack of vegetation in this arid area. Using an U.S. EPA positive matrix factorization model, VOCs together with other trace gases (NO x , NO z and SO2) observed in this border region were attributed to four types of sources, i.e., local industrial solvent usage (58% in ppbC), gasoline vehicle exhaust (19% in ppbC), diesel vehicle exhaust (14% in ppbC), and aged plume (9% in ppbC) due to regional background and/or long-range transport. Diesel vehicle emission contributed to 87% of SO2 and 75% of NO x , and aged plume contributed to 92% of NO z . An independent conditional probability function analysis of VOCs, wind direction, and wind speed indicated that the industrial source did not show a significant tendency with wind direction. Both gasoline and diesel engine emissions were associated with air masses passing through two busy cross-border ports. Aged plumes were strongly associated with NW wind, which likely brought in aged air masses from the populated San Diego area.► Major VOCs at the San Diego–Tijuana border area included OVOCs and aromatics. ►VOCs were attributed to solvent usage, gas/diesel vehicle exhausts, and aged plume. ► Diesel vehicle emission contributed to 87% of SO2 and 75% of NOx. ► Aged plumes were associated with NW wind from air masses of the San Diego area.
Keywords: VOCs; Trace gases; PTR-MS; Positive matrix factorization; Conditional probability function; Tijuana;

NO2 fluxes from Tijuana using a mobile mini-DOAS during Cal-Mex 2010 by Claudia Rivera; Hugo Barrera; Michel Grutter; Miguel Zavala; Bo Galle; Naifang Bei; Guohui Li; Luisa T. Molina (532-539).
NO2 fluxes were measured using a mobile mini-DOAS during Cal-Mex 2010 field study, between May 15 and June 30, 2010, from the urban area of Tijuana, Baja California as well as the Rosarito power plant. The average calculated NO2 fluxes were 328 ± 184 (269 ± 201) g s 1, and 23.4 ± 4.9 (12.9 ± 11.9) g s 1 for Tijuana urban area and Rosarito power plant, respectively, using model based wind fields and onsite measurements (in parenthesis). Wind speed and wind direction data needed to estimate the fluxes were both modeled and obtained from radiosondes launched regularly during the field campaign, whereas the mixing layer height throughout the entire field campaign was measured using a ceilometer. Large variations in the NO2 fluxes from both the Tijuana urban area and Rosarito power plant were observed during Cal-Mex 2010; however, the variability was less when model based wind fields were used. Qualitative comparisons of modeled and measured plumes from the Tijuana urban area and Rosarito power plant showed good agreement.► Average NO2 flux from the Tijuana urban area was 328 ± 184 g s−1. ► Average NO2 flux from the Rosarito power plant was 23.4 ± 4.9 g s−1. ► Rosarito power plant flux has a minor contribution of NO2 in regional air quality. ► Less variability of NO2 fluxes was found when using modeled winds. ► Comparison of measured and simulated distribution of plumes had good agreement.
Keywords: Tijuana; Cal-Mex 2010; Nitrogen dioxide; Fluxes; DOAS;