Atmospheric Environment (v.45, #18)

Heterogeneous reactions of ozone with methoxyphenols, in presence and absence of light by Sopheak Net; Elena Gómez Alvarez; Sasho Gligorovski; Henri Wortham (3007-3014).
In this work, we investigated the heterogeneous reactions between gaseous ozone and seven particulate methoxyphenols, biomass tracers. The ozonolysis of silica particles coated with vanillin, vanillic acid, syringaldehyde, syringic acid, acetovanillone, acetonsyringone and coniferyl alcohol was studied successively and was carried out both in total darkness and under illumination with simulated solar light at 297 K. The condensed-phase products which emerged in such heterogeneous reactions were analyzed by gas chromatography-mass spectrometry (GC/MS).No reaction product was detected during the ozonolysis of vanillic acid, syringic acid, acetovanillone and acetosyringone under our experimental conditions.The main tranformation of pathway vanillin and syringaldehyde was the conversion of an aldehyde group to a carboxylic fonction. Thus, syringic acid and vanillic acid were respectively the main oxidation products of syringaldehyde and vanillin.The oxidation of coniferyl alcohol was relatively fast and the total degradation was observed after 16 h of ozone exposure. Five oxidation products: glycolic acid, oxalic acid, vanillin, vanillic acid and 3,4-dihydroxybenzoic acid, were identified and confirmed by their corresponding standards. It is interesting to note that 3,4-dihydroxybenzoic acid was detected only in the experiment performed under combined ozone and light exposure of the particles coated with coniferyl alcohol. Vanillin and vanillic acid also absorb light in the tropospheric actinic window and therefore they can be photochemically active which in turn can induce further modifications of the aerosol particles. A mechanistic pathway was proposed in order to elucidate the ozonolysis reaction of coniferyl alcohol and to explain the identified reaction products.► We investigated the heterogeneous reactions between ozone and seven methoxyphenols. ► The reaction products have been identified whenever it was possible. ► A reaction mechanism was proposed to elucidate the ozonolysis of coniferyl alcohol. ► These results provide essential information on the behaviour of methoxyphenols.
Keywords: Heterogeneous reaction; Photodegradation; Methoxyphenols; Silica particles; Irradiation; GC-MS;

Quantitative analysis of mineral phases in atmospheric dust deposited in the south-eastern Iberian Peninsula by J.L. Díaz-Hernández; J.D. Martín-Ramos; A. López-Galindo (3015-3024).
Particulate matter suspended in air mainly consists of a complex, multiphase system. Its nature is largely mineral at a global scale, and it has a significant physicochemical impact on the Earth’s atmosphere and on biogeochemical cycles. These mineral phases come mainly from windblown soil processes, mostly from great deserts. Despite their importance, the behaviour of their airborne components in time and space is not well known.This study found that the rate of mineral deposition over an annual cycle in the south-eastern Iberian Peninsula was 26.03 g m−2 yr−1, with maxima in spring and summer. Using powder X-Ray diffraction techniques, this value has been broken down as follows (in g m−2 yr−1): quartz (4.90), dolomite (3.36), calcite (3.28), micas (2.97), smectites (2.10), halite (1.84), kaolinite (1.82), sulphates (1.28), amorphous matter (1.15), feldspars (0.18) and graphite (0.17). Although quartz normally is the major individual component of solid particles in the atmosphere—carbonates (calcite + dolomite) can exceed quartz, and phyllosilicates can total as much as carbonates. Clay minerals correlate well with salts (sulphates and halite), and there is an antagonistic relation between sulphates and calcite. Amorphous matter consists of a mixture of metal oxides and organic compounds, among others. Graphite, a net anthropogenic constituent of atmospheric dust, only represents minor quantities.The behavioural differences of the minerals are due to their different reactivity, based on their intrinsic properties of specific surface area, deliquescence, swelling and water retention capacity, and the presence of metallic and exchangeable cations. Smectites seem to play an essential role in the atmospheric processing of SO2 and in secondary sulphate genesis.► The rate of dust deposition in the SE Iberian Peninsula was 26.03 g m−2 yr−1. ► This rate presents two maxima in spring and summer. ► The mineral phases were determined and quantified by XRD. ► These analyses found a close relationship between sulphates and smectites. ► This suggests that clays are a factor in the processing of atmospheric SO2.
Keywords: Aerosols mineralogy; Dust deposition rates; African dust; Powder X-ray diffraction; Sulphates neoformation;

Higher than expected NO x emission from trucks may affect attainability of NO2 limit values in the Netherlands by Guus J.M. Velders; Gerben P. Geilenkirchen; Ronald de Lange (3025-3033).
In past years, the European limit value for average annual NO2 concentration has been exceeded in city streets and along motorways in the Netherlands. By 2015 the limit value must be adhered to in the Netherlands. Although the total road length for which exceedance is likely is expected to decrease over the coming years, exceedances may still occur by 2015. Future NO2 concentrations not only depend on economic growth, current and proposed policies, and on inevitable meteorological fluctuations, they also depend on the effectiveness of technical measures to reduce emissions. New emission measurements for heavy-duty vehicles in the Euro-V (and Euro-III) emission standard categories, carried out under typical Dutch driving conditions, have revealed that real-world NO x emissions from these trucks are significantly higher than was previously estimated based on the reduction steps in the Euro emission standards. Emission levels were higher, by about a factor of three, along city streets, and 10 to 40% higher along motorways. These higher emission levels resulted in higher estimated national NO x emissions, increasing from 250 kt to 264 kt, compared with the national emission ceiling of 260 kt, to be adhered to by 2010. The higher emissions more than double the total road length with possible exceedance (chance >33%) of the NO2 limit value; from about 100 km to about 250 km along cities streets and motorways, by 2015. These higher emissions from trucks, therefore, affect the chances of the limit values being adhered to, in time, everywhere in the Netherlands.► We assessed the effects of new emission measurements for Euro-V trucks. ► These real-world emissions were three times larger along city streets than before. ► This more than doubles the road length with NO2 limit value exceedance by 2015. ► The attainability of the NO2 limit value in the Netherlands is thereby affected.
Keywords: Air quality; Nitrogen dioxide; Exceedance; Euro-V;

Development of a novel methodology for indoor emission source identification by K.H. Han; J.S. Zhang; H.N. Knudsen; P. Wargocki; H. Chen; P.K. Varshney; B. Guo (3034-3045).
The objective of this study was to develop and evaluate a methodology to identify individual sources of emissions based on the measurements of mixed air samples and the emission signatures of individual materials previously determined by Proton Transfer Reaction-Mass Spectrometry (PTR-MS), an on-line analytical device. The methodology based on signal processing principles was developed by employing the method of multiple regression least squares (MRLS) and a normalization technique. Samples of nine typical building materials were tested individually and in combination, including carpet, ceiling material, gypsum board, linoleum, two paints, polyolefine, PVC and wood. Volatile Organic Compound (VOC) emissions from each material were measured in a 50-liter small-scale chamber. Chamber air was sampled by PTR-MS to establish a database of emission signatures unique to each individual material. The same task was performed to measure combined emissions from material mixtures for the application and validation of the developed signal separation method. Results showed that the proposed method could identify the individual sources under laboratory conditions with two, three, five and seven materials present. Further experiments and investigation are needed for cases where the relative emission rates among different compounds may change over a long-term period.► Unique emission patterns appear to exist for building materials. ► These unique patterns can be established as emission signatures by PTR-MS. ► VOC mixture emissions among material mixtures appear to be superposed. ► Sorptive effects exist, but can be accurately assessed with the new technique. ► By these identified features, material emission sources can be clearly pointed out.
Keywords: Material emission signature; PTR-MS; VOC; Source identification; Signal processing;

Atmospheric deposition and corresponding variability of stemflow chemistry across temporal scales in a mid-Atlantic broadleaved deciduous forest by Delphis F. Levia; John T. Van Stan; Courtney M. Siegert; Shreeram P. Inamdar; Myron J. Mitchell; Susanna M. Mage; Patrick J. McHale (3046-3054).
Despite the fact that atmospheric deposition is widely accepted to be an important process in the biogeochemical cycling of wooded ecosystems, no single study is known that has examined stemflow chemistry in relation to atmospheric deposition across time scales, from within discrete events to season, to chronicle alterations in temporal patterns of stemflow chemistry. This research partitioned stemflow solute fluxes (K+, Na+, Mg2+, Ca2+, Cl, NO3 , and SO4 2−) from two tree species of differing canopy form and bark morphology into their leaching and dry deposition washoff components using a modified Kazda (1990) integration model at the intra-storm scale to examine differences within and among discrete rain events. Median annual stemflow concentrations in yellow poplar (Liriodendron tulipifera L.) stemflow were higher than American beech (Fagus grandifolia Ehrh.) stemflow for all ions except NO3 . Beech median enrichment ratios were larger for all monitored ions than yellow poplar. All intra-storm stemflow ionic fluxes were initially high, exponentially decaying to a steady input, typically dominated by leaching contributions. With the exception of yellow poplar stemflow Cl and NO3 fluxes and beech Na+ flux, all intra-storm mean ionic fluxes began with higher dry deposition percentages and transitioned to primarily leaching. Observations in the field implicate increased magnitude and rainfall intensity in the initiation of new stemflow flowpaths, evidenced by increased variability in the timing of stemflow ionic deposition and fluctuations in the proportion of washoff and leaching during some events. Beech stemflow fluxes were larger than yellow poplar for all ions during the leafed, leafless, and annual periods. Our results demonstrate: (1) the critical role of the initiation of new flowpaths and expansion and maturation of developed flowpaths on a tree’s surface to solute enrichment and transport to the forest floor as a canopy wets-up; and (2) the importance of temporal scale in providing important insights into some effects of stemflow on biogeochemical cycling.► First known study to examine/contrast within-event to season. ► Documents effects of new flowpaths on solute flux. ► Temporal scale affects stemflow chemistry. ► Tree species affects leaching/washoff proportions.
Keywords: Dry deposition; Canopy leaching; Sequential sampling; Deciduous canopy; Fagus grandifolia; Liriodendron tulipifera;

Gaseous and particulate emissions from rural vehicles in China by Zhiliang Yao; Hong Huo; Qiang Zhang; David G. Streets; Kebin He (3055-3061).
Rural vehicles (RVs) could contribute significantly to air pollutant emissions throughout Asia due to their considerable population, extensive usage, and high emission rates, but their emissions have not been measured before and have become a major concern for the accuracy of regional and global emission inventories. In this study, we measured CO, HC, NOx and PM emissions of RVs using a combined on-board emission measurement system on real roads in China. We also compared the emission levels of the twenty RVs to those of nineteen Euro II light-duty diesel trucks (LDDTs) that we measured for previous studies. The results show that one-cylinder RVs have lower distance-based emission factors compared to LDDTs because of their smaller weight and engine power, but they have significantly higher fuel-based PM emission factors than LDDTs. Four-cylinder RVs have equivalent emission levels to LDDTs. Based on the emission factors and the activity data obtained, we estimate that the total emissions of RVs in China in 2006 were 1049 Gg of CO, 332 Gg of HC, 933 Gg of NOx, and 54 Gg of PM, contributing over 40% to national on-road diesel CO, NOx, and PM emissions. As RVs are a significant contributor to national emissions, further research work is needed to improve the accuracy of inventories at all levels, and the government should strengthen the management of RVs to facilitate both policy making and research work.► Diesel-fueled rural vehicles (RVs) in China could produce significant emissions. ► We measured 20 RVs and compared them to Euro II light-duty diesel trucks (LDDTs). ► 4-Cylinder RVs have equivalent emission factors (EFs) to LDDTs. ► 1-Cylinder RVs have lower km-based EFs than LDDTs, but much higher fuel-based PM EFs. ► RVs contributed >40% to on-road diesel CO, NOx, and PM emissions in China in 2006.
Keywords: Rural vehicles; On-board emission measurement; Emission factors; Emission inventory; China;

Intercomparison study of NOx passive diffusion tubes with chemiluminescence analysers and evaluation of bias factors by Araceli Sánchez Jiménez; Mathew R. Heal; Iain J. Beverland (3062-3068).
Passive diffusion tubes (PDTs) are an inexpensive and simple method to monitor air pollutants. Numerous studies have investigated the performance of PDTs for NO2 but little attention has been paid to PDTs for NOx. The aim of this study was to evaluate the performance of NOx PDTs in three different urban environments. Duplicate NOx and NO2 PDTs were co-located with chemiluminescence analysers at kerbside, urban centre and background sites in the city of Glasgow for twelve 1-week exposures. PDT measurements generally showed good temporal correlations with NOx and NO2 determined by the continuous analysers. However detailed evaluation showed PDT measurements were variously influenced by factors causing bias, according to individual site characteristics: positive bias in both NOx and NO2 PDTs due to wind-associated shortening of diffusion path; positive bias in NO2 PDTs due to within-tube chemical reaction between NO and O3; and, where NO concentrations were high, negative bias in NOx PDTs assumed due to incomplete oxidation of NO by the in-cap oxidising granules. In conclusion, where ambient NOx is low (less than a few tens of μg m−3), and PDTs are in sheltered locations, NOx PDTs should perform well over 1-week exposures; however substantial negative bias for NOx PDTs is expected in polluted roadside environments for exposures of several weeks as is usually the case in ambient air quality deployment. Observations from this study suggest that sheltering PDTs from high wind is important to minimise positive bias due to wind-associated shortening of the diffusion path.► Passive diffusion tubes (PDTs) are a simple method to monitor air pollutants. ► NO2 and NOx PDTs are influenced by factors causing bias. ► Negative bias in total NOx capture was observed in polluted environments. ► Positive bias for PDTs was observed in PDTs exposed to wind. ► At low NOx concentrations and when PDTs are sheltered, PDTs should perform well.
Keywords: Nitrogen oxides; Passive samplers; Diffusion tubes; Air pollution monitoring;

A study of air pollution of city clusters by Huiwang Gao; Jing Chen; Bin Wang; Sai-Chun Tan; Colin M. Lee; Xiaohong Yao; Han Yan; Jinhui Shi (3069-3077).
In this study, the Daily Air Pollution Index (DAPI) of 81 cities in China from June 2004 to June 2007 was used to study the air pollution of city clusters. The cluster analysis results show that the DAPI of 71 cities in central and eastern China is divided into two regimes, the north regime and the south regime along the Qinling Mountain and its eastern extension to Huaihe River. The demarcation line between the two DAPI regimes duplicates the demarcation line of the watersheds of the northern and southern climate regions and the DAPI were statistically negatively correlated to the amount of rainfall, suggesting that precipitation could play an important role in determining the two regimes. The two DAPI regimes are further divided into seven city clusters mainly associated with emissions of air pollutants and geographical distributions, supported by remote sensing data of aerosol optical depth (AOD), NO2 and emission inventories of air pollutants in China in 2006 reported in the literature. DAPI of three city clusters in the north regime exhibited a similar seasonal trend, which was different from the four city clusters in the south regime. Outbreak of dust storms in spring and an increase in emissions due to space-heating impacted the DAPI of the northern city clusters. Although the consumption of electricity decreased in the southern city clusters in the fall and winter, the reported high DAPI were the result of less rainfall and poor dispersion. Seven cities in western China were classified into two groups associated with their unique nature environments. Three cities were not allocated to any of the city clusters.► The Daily Air Pollution Index (DAPI) in China is divided into seven city clusters. ► The seven city clusters was supported by remote sensing data of aerosol optical depth and NO2. ► The cause of the high DAPI in the three northern city clusters is different from that in others. ► In general, the spatial scale of regional air pollution is about 500 km.
Keywords: Air pollution index; China; Dust storm; PM10; City clusters;

Impacts of emission changes on sulfate aerosols in Alaska by Trang T. Tran; Greg Newby; Nicole Mölders (3078-3090).
WRF/Chem simulations were performed using the meteorological conditions of January 2000 and alternatively the emissions of January 1990 and 2000 to examine whether increases in emissions may have caused the increasing trends in observed sulfate-aerosol concentrations at coastal Alaska sites. The analysis focused on six regions in Alaska that are exposed differently to the main emission sources. Meteorological observations at 59 sites and aerosol measurements at three sites showed that WRF/Chem captured the meteorological situation over Alaska well and simulated the aerosol concentrations acceptably. Except for the region adjacent to the Arctic Ocean that is influenced by local SO2-emissions, Alaska SO2 and SO4 2−-aerosol distributions are affected by long-range transport of SO2 from ship emissions and/or emissions in Canada and southern Siberia. Local changes in emissions between 1990 and 2000 are not the main cause for concentrations changes in the six regions. The increases of SO4 2−-aerosols and SO4 2−-in-cloud along the Gulf of Alaska are caused by increased ship or Canadian emissions. The study provides evidence that the increased ship and Canadian emissions during the last decades can cause increases in sulfate aerosols.► Ship, Canadian emissions affect SO4 2−-aerosol in coastal Alaska except the Arctic Ocean. ► Local-emission changes are not the main cause of concentration changes in Alaska. ► Increased ship, Canadian emissions caused the observed increases in SO4 2−-aerosols. ► Sulfate concentration changes lead to changes in the meteorological conditions.
Keywords: Long-range transport; Sulfate aerosols; Emissions; Alaska;

This paper presents the characteristics of particulate matter (PM) mass concentrations (PM10, PM2.5 and PM1) emitted from heterogeneous traffic in Chennai city during monsoon, winter and summer seasons of the year 2007–2009. The heterogeneous traffic characteristics at the study region indicated dominance of 2-wheelers (58%) followed by the 4-wheelers (29%), 3-wheelers (7%), light commercial vehicle (LCV = 2%) and heavy commercial vehicle (HCV = 4%). The vintage analysis of vehicles showed that 11, 24, 35 and 30% of the total vehicles were 15, 10, 5 and <5 years old vehicles, respectively (75% of LCV and 70% of HCV were older than 10 years and 82% of 2W, 85% of 3W, 79% of 4W were less than 10 years old). The travel demand on weekdays at the study site revealed that 60% of the traffic in the morning peak hours was due to school and office trips and 40% was due to the business trips. During the weekends the peak rush hours traffic was dominated by travel demand (90%) due to tourists and pleasure trips.At study region, the PM10 comprised a large fraction of PM2.5 (56% of PM2.5 and 44% of PM1), while PM2.5 comprised a large proportion of PM1 (81%). The estimated PM2.5/PM10 ratios for monsoon, winter and summer seasons were ranged between 0.44–0.62, 0.66–0.76 and 0.62–0.73, respectively. The high PM2.5/PM10 ratios (R 2 = 0.92–0.98) during peak hour indicated significant contribution from the vehicular emissions. Further, it was found that the PM10, PM2.5 and PM1 concentrations at the study site followed similar trend of 2W, 3W, 4W and HCV during morning peak hour traffic.The analysis of 24-hr average PM10, PM2.5 and PM1 concentrations showed maximum during monsoon (188.75 ± 71.75, 83.91 ± 33.18, 65.81 ± 28.47 μg/m3) and winter (134.58 ± 64.55, 72.95 ± 39.27, 59.00 ± 31.15 μg/m3) and minimum during summer (75.96 ± 43.15, 42.16 ± 19.76, 31.40 ± 16.05 μg/m3) seasons. The 24-hr average PM10 and PM2.5 indicated maximum violations of Indian national ambient air quality standards (NAAQS for PM10 = 100 μg/m3 and PM2.5 = 60 μg/m3) during winter and monsoon seasons and minimum during summer season. Further, PM10 and PM2.5 concentrations were well above the prescribed 24-hr average world health organization (WHO) standards (PM10 = 50 μg/m3; PM2.5 = 25 μg/m3).The PM chemical characterization indicated the dominance of soil based crustal elements (74%) – Al, Ca, Fe and Mg, salt based components (16%)-Na and K, and the other trace elements (10%) such as B, Ba, Cd, Co, Cr, Cu, Ga, Mn, Mo, Ni, Pb, Rb, Se, Sr, Te, V and Zn. The percentage composition of different group of elements indicates traffic as the main source of PM emission at the study site. The ion analysis showed presence of more anions (36–82%) than the cations (18–64%). SO4 2− and Mg2+ were major constituents.► Assessment of PM10 and PM2.5 concentrations emitted by heterogeneous traffic. ► Characterization of heterogeneous traffic on weekdays and weekends. ► Influence of traffic characteristics and meteorology on the PM10 and PM2.5 concentrations. ► Chemical characterization of PM mass for monsoon, winter and summer seasons. ► Urban roadside PM mass consists of 74% crustal, 16% marine and 10% other trace elements.
Keywords: Chemical composition; Coarse particles; Fine particles; Heterogeneous traffic; Vehicle speed; Particulate matter; Standards;

The unique OMI HCHO/NO2 feature during the 2008 Beijing Olympics: Implications for ozone production sensitivity by J.C. Witte; B.N. Duncan; A.R. Douglass; T.P. Kurosu; K. Chance; C. Retscher (3103-3111).
In preparation of the Beijing Summer Olympic and Paralympics Games, strict emission control measures (ECMs) were imposed between July and September 2008 on motor vehicle traffic and industrial emissions to improve air quality. We estimated changes in the chemical sensitivity of ozone production to these ECMs using Ozone Monitoring Instrument (OMI) column measurements of formaldehyde (HCHO) and nitrogen dioxide (NO2), where their ratio serves as a proxy for the sensitivity. During the ECMs, OMI NO2 significantly decreased, subsequently increasing the HCHO/NO2. For the first half of the ECM time period, the ratios maintained values greater than two indicating that ozone production became primarily NO x -limited. In contrast, ozone production was predominantly volatile organic compound (VOC)-limited or mixed VOC-NO x -limited during the same period in the preceding three years. After the ECMs were lifted, NO2 and HCHO/NO2 returned to their previous values. The 2005–2008 OMI record shows that this transition to a predominantly NO x -limited regime during the ECMs was unique. Meteorological factors likely explain the variability in HCHO/NO2, particularly the transition to a mixed NO x -VOC-limitation in mid-August during the Olympics, where ozone production became sensitive to both NO x and VOCs until the end of the ECMs. The mixed VOC-NO x -limited regime observed during the Paralympics is also unique because previous years show that Beijing in September is predominantly VOC-limited. Beijing’s large-scale tree-planting program was expected to increase levels of biogenic VOCs, but this is not supported by OMI HCHO data. However, MODIS vegetation indices show a small increase in vegetation cover from 2003 leading up to the Games in 2008. After the Games, however, there was a downturn in the indices (2009 and 2010) to levels similar to 2006.► Beijing was in a rare NO x -limited regime due to strict traffic controls on NO x . ► The VOC-NO x -limited regime in September 2008 is unique in the OMI data record. ► Meteorological factors likely explain the transition in regimes during the ECMs.
Keywords: Air quality; OMI; NO2; HCHO; Beijing Olympics;

Methyl chloride and C2–C5 hydrocarbon emissions from dry leaf litter and their dependence on temperature by Leonie Derendorp; Rupert Holzinger; Asher Wishkerman; Frank Keppler; Thomas Röckmann (3112-3119).
Emissions of methyl chloride and several C2–C5 hydrocarbons from dry leaf litter at temperatures in the range 20–100 °C are reported for different plant species. The emission rates of ethane, ethene, propane, propene, n-pentane and methyl chloride increased with temperature. Hydrocarbon emission rates up to 0.88 ng gdw−1 h−1 were measured at 20 °C, while methyl chloride emission rates between 0.03 and 0.85 ng gdw−1 h−1 were observed at this temperature. At 70 °C emission rates increased up to 650 ng gdw−1 h−1 for C2–C5 hydrocarbons and up to 18 μg gdw−1 h−1 for methyl chloride. The Arrhenius relation can be used to describe the temperature dependence of methyl chloride emissions, while for hydrocarbon emissions deviations from this relation were observed. The emissions were not due to enzymatic activity, which was indicated by emission rates that continuously increased with increasing temperature, and activation energies higher than 50 kJ mol−1. At constant temperature, the emission rate of both methyl chloride and hydrocarbons from dry leaf litter decreased in time. At high temperatures (80–100 °C) this was noticeable on a timescale of hours, while at low temperatures (20–30 °C) the decrease was very slow and only visible on a timescale of months. Emission of methyl chloride from leaf litter might be significant for its global budget, while temperature induced hydrocarbon emissions from leaf litter are likely insignificant.► Methyl chloride and C2–C5 hydrocarbons are emitted by leaf litter. ► Emission rates increase with temperature. ► Methyl chloride from leaf litter is a significant source of chloride in the atmosphere.
Keywords: Methyl chloride; C2–C5 hydrocarbons; Leaf litter;

Long-term trend in emission ratio of NO2 to NO x was analyzed in relation to traffic activities using ambient monitoring data (such as NO, NO2, NO x , and O3) collected from 7 urban roadside (U-RS) and 7 urban background (U-BG) locations in Seoul, Korea over a 14-year period (1996–2009). In general, the temporal trend of NO2 concentrations is less distinctive than that of NO x , whether being downward or upward. However, if their differences are checked statistically, only a few cases were significant. The mean emission ratio of NO2 to NO x values varied slightly across the 7 U-RS sites during the study period, ranging from 0.11 to 0.19. In general, there was no significant annual trend in the relative fraction of NO2 in NO x emission (f(NO2)) at U-RS, with an exceptional downward trend at one site (slope of −0.008 y−1). On the other hand, diurnal variations of f(NO2) were likely affected by driving conditions and fuel type of vehicles. The overall results of this study suggest that NO x emission control strategies such as diesel particulate filter (DPF) or diesel oxidation catalyst (DOC) should have direct influences on f(NO2) values at U-RS sites.► Emission ratios of NO2 to NO x across the urban sites ranged from 0.11 to 0.19. ► There was no significant annual trend in the emission ratio. ► Temporal trend of NO2 concentrations is less distinctive than that of NO x . ► NO x emission control strategies should have direct influences on the ratio.
Keywords: NO2/NO x emission ratio; Roadside; Emission; Seoul; Mann–Kendall test;

Quantification of the effects of molecular marker oxidation on source apportionment estimates for motor vehicles by Anirban A. Roy; Kristina M. Wagstrom; Peter J. Adams; Spyros N. Pandis; Allen L. Robinson (3132-3140).
Molecular markers are individual organic compounds used in receptor models to apportion fine particulate matter to sources. These models currently assume that molecular markers are chemically stable; however, recent laboratory experiments suggest they may be significantly oxidized on atmospherically relevant time scales. To investigate the effects of photo-oxidation, we extended a 3-D chemical transport model (PMCAMx) to simulate norhopane concentrations over the eastern United States during July 2001. Norhopane is an important molecular marker for motor vehicle exhaust. We examined eight different simulation scenarios, using different combinations of reaction rates and source profiles. The simulations including norhopane oxidation better reproduced the observed spatial patterns of norhopane concentrations than the non-reactive cases. Chemical mass balance (CMB) analysis was performed using the PMCAMx-predicted motor vehicle norhopane and elemental carbon (EC) concentrations to quantify the bias caused by oxidation on source apportionment estimates. Norhopane oxidation caused CMB to underestimate total vehicle OC by 10–50%, with larger biases in rural areas. This underestimation was largely due to changes in the amount of OC apportioned to gasoline vehicles which was reduced by as much as 100%. The OC apportioned to diesel vehicle emissions was relatively insensitive to norhopane reaction. Therefore, oxidation can substantially alter CMB estimates regarding the relative importance of gasoline and diesel vehicle emissions.► Extended a chemical transport model, simulating automobile tracer concentrations. ► Considered one non-reactive and three reactive cases. ► Tracer reaction caused underestimation of motor vehicle organic carbon. ► This was primarily to underestimation of the gasoline contribution ► Contribution of diesel was relatively unchanged.
Keywords: Norhopane; Organics; Gasoline; Diesel; Oxidation; Biases;

Diazinon is a member of the organophosphorus class of insecticides. It has been regarded as an important atmospheric pollutant because of its high detection-frequency in the air and potential adverse effects on humans and wildlife. In this paper, the reaction mechanism and possible degradation products for the OH-initiated atmospheric degradation of diazinon were investigated and the rate constants of crucial elementary steps over the temperature range of 180–370 K were predicted. Present results show that OH addition to C4 atom in the pyrimidyl ring, H abstraction from the –CH– moiety as well as OH addition to P atom are the dominant pathways for the reaction of diazinon with OH radicals. The dominant degradation products are diazoxon, SO2, P3, CH3CHO, P4, CH3CO, P14 as well as CH3CHCH3. This work provides a comprehensive investigation of the OH-initiated atmospheric degradation of diazinon and should help to clarify its potential risk to non-targets.► OH addition to C4 atom is the dominant pathway for OH addition to the pyrimidyl ring. ► H abstraction from the –CH– moiety is the dominant abstraction pathway. ► OH addition to P atom is another dominant pathway. ► The dominant products are diazoxon, SO2, P3, CH3CHO, P4, CH3CO, P14 and CH3CHCH3.
Keywords: Diazinon; OH radicals; Atmospheric oxidation; Reaction mechanism; Degradation products; Kinetics;

Hydrogen peroxide generation from α- and β-pinene and toluene secondary organic aerosols by Ying Wang; Hwajin Kim; Suzanne E. Paulson (3149-3156).
In-particle reactions and reactivity may play a role in aerosol aging as well as health effects associated with airborne particles. One of the main hypotheses for the species causing the observed health effects are reactive oxygen species including hydrogen peroxide (H2O2). Secondary organic aerosol (SOA), the focus of this study, comprises a major fraction of ambient fine mode particle mass. SOA were generated via dark ozonolysis of α-pinene and β-pinene, as well as photochemical oxidation of α-pinene, β-pinene, and toluene. SOA particles generate a considerable amount of H2O2 in the aqueous phase. For the same particle mass, α-pinene and β-pinene SOA have higher H2O2 generation ability than toluene SOA. H2O2 levels were sensitive to the pH of the particle extraction solutions, decreasing as the pH was increased. SOA samples aged for 20 h in room air lost a significant fraction of the H2O2 compared to fresh samples. The H2O2 is likely from decomposition or hydrolysis of hydroxyhydroperoxides, peroxic acids and related species for α- and β-pinene SOA, and from redox cycling of quinoid compounds for toluene SOA. This study provides the first quantitative measurement of H2O2 for a range of SOA particle types.► H2O2 associated with SOA and its possible generation mechanism(s) are investigated. ► Pinene SOA generates the most H2O2 followed by ambient aerosol and toluene SOA. ► H2O2 varies with extraction solution composition and temperature of SOA generation. ► H2O2 generation depends on the freshness of SOA particles and extraction time length. ► We find evidence for quinone redox cycling for toluene but not pinene SOA.
Keywords: Reactive oxygen species; Hydrogen peroxide; Organic hydroperoxides; Quinones; Secondary organic aerosol; Aging; Health effects;

Isoprene and monoterpene emissions from a mixed temperate forest by Q. Laffineur; M. Aubinet; N. Schoon; C. Amelynck; J.-F. Müller; J. Dewulf; H. Van Langenhove; K. Steppe; M. Šimpraga; B. Heinesch (3157-3168).
We measured the isoprene and monoterpene fluxes continuously above a mixed forest site at Vielsalm in the eastern part of Belgium, using the disjunct eddy covariance technique with proton transfer reaction-mass spectrometry. Simultaneously, we also measured the carbon dioxide fluxes in order to deduce the gross primary production. The measurements were conducted from July to September 2009.During the day, the seasonal evolution of the isoprene/monoterpene emissions was studied using a monthly temperature and light dependence function deduced from our results to standardize the fluxes. A seasonal decrease in the standard emission factors was observed, probably linked to acclimation or senescence. The standard emission factor for isoprene fluxes (30 °C, 1000 μmol m−2 s−1) fell from 0.91 ± 0.01 to 0.56 ± 0.02 μg m−2 s−1 and for monoterpene fluxes from 0.74 ± 0.03 to 0.27 ± 0.03 μg m−2 s−1.During the night, a slight positive flux of monoterpenes was observed that seemed to be driven by air temperature. The standard emission factor (30°C) for nighttime monoterpene fluxes was equal to 0.093 ± 0.019 μg m−2 s−1.Finally, we studied the seasonal evolution of the relationship between the gross primary production and the isoprene/monoterpenes fluxes. A linear relationship was observed, highlighting the strong link between carbon assimilation and isoprene/monoterpene emissions.► Isoprene and monoterpene emissions during the day are driven by temperature and light. ► Monoterpene emissions during the night seems to be driven by temperature. ► Isoprene and monoterpene emissions are strongly linked with the carbon assimilation. ► Standard emission factors for isoprene and monoterpenes decrease with the season. ► Standard emission factor seems to be linked to the friction velocity.
Keywords: Forest ecosystems; Monoterpenes; Isoprene; Disjunct eddy covariance; PTR-MS;

Influence of seasonal factors on the atmospheric particle number concentration and size distribution in Madrid by F.J. Gómez-Moreno; M. Pujadas; J. Plaza; J.J. Rodríguez-Maroto; P. Martínez-Lozano; B. Artíñano (3169-3180).
The ambient particle number concentration and size distribution have been measured in an urban background site in Madrid, a continental Mediterranean area, over more than two years (Oct 2006–Dec 2008). The objective was to study the sources and processes affecting or contributing to fine and ultrafine particles in this area. They have been measured with a TSI–SMPS (15–600 nm) instrument and with a modified Vienna type DMA (3–80 nm) and a CPC 3025 (TSI) during 6 months. The average particle number concentration was lower than in other sites as it is an urban background site and because of its location in the Mediterranean area. The particle number concentrations have shown a clear seasonal influence: maximum values were observed every year in the period November–January, coinciding with atmospheric stagnant conditions and pollution episodes, while minimum values were measured in springtime, a period in which wind speed produced high atmospheric dilution. The Aitken and accumulation modes have shown similar seasonal behavior, with two maxima related to vehicle emissions. The nucleation mode had a third maximum observed at noon during spring and summer. The size distributions were bimodal during most of the time: the first mode was centered on 20–50 nm and was associated with fresh particles related to vehicle exhaust emission; the second mode, between 50 and 160 nm, mainly corresponded to the evolution of the first mode. The evolution of the size distributions reveals a marked annual cycle related to the season, with an increase of median diameters during summer and a decrease during winter. Different evolutions of particle size distribution corresponding to different meteorological and seasonal scenarios were identified. The influence of higher wind speeds on particle size distribution has been confirmed to cause a decrease in the particle number concentration and in the size distribution mode. Particle nucleation is not a frequent phenomenon in this measurement site, where 63 events per year have been observed. They mainly occurred during spring and summer periods, with the minimum number during winter. This suggests that insolation and temperature are important variables in nucleation. Class Ia nucleation events mainly occurred during spring and summer. High wind speeds were important during class II events, as the particles suffered low growth or lost their semivolatile compounds.► The particle number concentrations have shown a clear seasonal influence. ► Different meteorological and seasonal scenarios gave different distribution evolutions. ► High wind speeds cause decrease in particle concentration and size distribution mode. ► Particle nucleation mainly occurred during spring and summer periods.
Keywords: Fine particles; Particle number concentration; Particle size distribution; Nucleation mode; Particle nucleation; Wind speed;

The environmental fate of the cyclic methylsiloxanes (cVMS), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), is important because of the potential for long range transport. Previous studies of environmental loss pathways for D4 and D5 focused on gas phase oxidation, establishing OH as the main gas phase reactant, with OH kinetics consistent with atmospheric lifetimes of approximately 7 days (D5) and 11 days (D4) at an OH concentration of 106  molec cm−3. In this work, we assess the sensitivity of atmospheric lifetime to other factors using an atmospheric compartment model. D4 and D5 were modeled using an compartment model to simulate emission, decay, and atmospheric transport across a range of three levels of population density corresponding to urban, transition, and rural areas of a developed mid-latitude location. The model was used to examine sensitivities of lifetime and concentration of D4 and D5 to spatial variation in OH, variation in the time-of-day for cVMS emissions, and to relative humidity dependent heterogeneous uptake and/or reactions on mineral dusts. cVMS lifetime was found to be insensitive to urban OH concentrations because of limited residence time, and somewhat sensitive to enhanced OH levels in the transition area between the urban and rural compartments. Realistic aerosol loadings and heterogeneous kinetics consistent with recent laboratory results (uptake coefficients of 1.2 × 10−5 and 1.6 × 10−5 for D4 and D5, respectively) were calculated to reduce the cVMS lifetime by ∼3% under dry conditions but have negligible effects at RH values above 50%. The heterogeneous kinetics were based on experimental observations of uptake to mineral aerosol surfaces at cVMS loadings well above ambient levels. The compartment model, after tuning of residence times and ceiling heights to reproduce benzene observations, predicted D5 at 6, 28, and 50 ng m−3 in areas with low, medium, and high population densities, respectively. Corresponding D4 concentration predictions were 5, 20, and 33 ng m−3. These values were compared with available measurements.► Atmospheric fate of methylsiloxanes D4 and D5 is probed using a multiple box model. ► Parameterized heterogeneous reactions between cVMS and aerosols are modeled. ► Implication of aerosol reactions is minor compared to the dominant OH oxidation. ► Modeled cVMS concentrations range from 5–50 ng m−3 depending on population density.
Keywords: Volatile methyl siloxanes; Heterogeneous decay; Heterogeneous chemistry; Persistent organic pollutants; Octamethylcyclotetrasiloxane; Decamethylcyclopentasiloxane; D4; D5; Tropospheric lifetime;

Lake Michigan air quality: The 1994–2003 LADCO Aircraft Project (LAP) by Theresa Foley; Eric A. Betterton; P.E. Robert Jacko; John Hillery (3192-3202).
The goal of the 1994 to 2003 LADCO Airplane Project (LAP) was to study ozone formation over Lake Michigan so that equitable regional control strategies could be devised. This paper for the first time documents LAP in the peer-reviewed literature. found that the atmosphere over Lake Michigan is stable in the summer due to the airwater temperature difference, which creates an efficient reaction chamber for ozone formation. They also hypothesize that the southwest winds characteristic of ozone-conducive conditions transport ozone further north over the lake before it crosses the shoreline onto land. This statistical analysis of LAP data support the hypothesis of Dye et al.Below 200 m above the lake, ozone formation is VOC-limited in the morning and becomes NOx limited in the afternoon. Above 200 m, ozone formation is NOx-limited throughout the day. The onshore NOx and VOC diurnal cycles peak during the early morning rush hour and are clearly linked to traffic patterns. Over the lake, VOC and NOy concentrations peak during the mid-morning rather than the early morning, supporting the hypothesis that the land breeze transports VOC and NOy over the lake. The diurnal NOx pattern over Lake Michigan is less clearly defined than the VOC pattern possibly as a result of emissions from five coal-burning power plants located on the Lake Michigan shoreline.Using a “photochemical clock” model, we estimate the climatological average hydroxyl radical concentration over the lake to be (9.43 ± 5.88) × 106 molecule cm−3 near Chicago and (8.43 ± 3.68) × 106 molecule cm−3 near Milwaukee.► First report of a decade-long airplane study of ozone episodes over Lake Michigan. ► O3 formation is first VOC-limited then NOx-limited at low altitudes over the lake. ► Average HO concentration was estimated using a “photochemical clock”.
Keywords: Ozone; Ozone trends; Lake breeze; Photochemical clock; Photochemical age;

Reactive nitrogen compounds (RNCs) in exhaust of advanced PM–NOx abatement technologies for future diesel applications by Norbert V. Heeb; Yan Zimmerli; Jan Czerwinski; Peter Schmid; Markus Zennegg; Regula Haag; Cornelia Seiler; Adrian Wichser; Andrea Ulrich; Peter Honegger; Kerstin Zeyer; Lukas Emmenegger; Thomas Mosimann; Markus Kasper; Andreas Mayer (3203-3209).
Long-term exposure to increased levels of reactive nitrogen compounds (RNCs) and particulate matter (PM) affect human health. Many cities are currently not able to fulfill European air quality standards for these critical pollutants. Meanwhile, promising new abatement technologies such as diesel particle filters (DPFs) and selective catalytic reduction (SCR) catalysts are developed to reduce PM and RNC emissions. Herein, effects of a urea-based SCR system on RNC emissions are discussed and we quantified the highly reactive intermediates isocyanic acid (HNCO) and ammonia (NH3), both potential secondary pollutants of the urea-based SCR chemistry. A diesel engine (3.0 L, 100 kW), operated in the ISO 8178/4 C1, cycle was used as test platform. A V2O5-based SCR catalyst was either applied as such or down-stream of a high oxidation potential-DPF (hox-DPF). With active SCR, nitric oxide (NO) and nitrogen dioxide (NO2) conversion efficiencies of 0.86–0.94 and 0.86–0.99 were obtained. On the other hand, mean HNCO and NH3 emissions increased to 240–280 and 1800–1900 mg h−1. On a molar basis, HNCO accounted for 0.8–1.4% and NH3 for 14–25% of the emitted RNCs. On roads, SCR systems will partly be inactive when exhaust temperatures drop below 220 °C. The system was active only during 75% of the test cycle, and urea dosing was stopped and restarted several times. Consequently, NO conversion stopped but interestingly, NO2 was still converted. Such light-off and shutdown events are frequent in urban driving, compromising the overall deNOx efficiency. Another important effect of the SCR technology is illustrated by the NH3/NO2 ratio, which was >1 with active SCR, indicating that exhaust is basic rather than acidic after the SCR catalyst. Under these conditions, isocyanic acid is stable. The widespread use of various converter technologies already affected RNC release. Diesel oxidation catalysts (DOCs) and hox-DPFs increased NO2 emissions, three-way catalysts (TWCs) those of NH3. The investigated SCR technology substantially lowered NO and NO2 emissions, while NH3 levels were comparable to those of TWC vehicles (300–1500 mg h−1). If applied in the future, the combined DPF/SCR technology will change ambient RNC levels, PM compositions and atmospheric redox- and acid/base-chemistry in traffic-affected areas.► Advanced PM–NOx abatement technologies for diesel engines affect RNC emissions. ► When active, urea-based SCR catalysts lower NO and NO2 emissions by ∼90%. ► Isocyanic acid and ammonia were identified as secondary pollutants of the SCR system. ► Combined deNOx/DPF systems have the potential to lower urban RNC and PM levels. ► They also change ambient air redox and acid/base chemistry in traffic-affected areas.
Keywords: Diesel particulate filter; DeNOx technology; Selective catalytic reduction; Isocyanic acid; Secondary NO2 formation; Heavy duty vehicles;

Near-source observations of aerosol size distributions in the eruptive plumes from Eyjafjallajökull volcano, March–April 2010 by E. Ilyinskaya; V.I. Tsanev; R.S. Martin; C. Oppenheimer; J. Le Blond; G.M. Sawyer; M.T. Gudmundsson (3210-3216).
Near-source observations of aerosol size distributions (<15 km from the vent) were made during three eruptive phases of Eyjafjallajökull in 2010: Phase I - basaltic lava fountaining; Phase II - andesitic phreatomagmatic explosions (mass discharge rate of the order of 105 kg s−1); Phase III - andesitic magmatic explosions (mass discharge rate of the order of 103 kg s−1). Two methods were used, photometric measurements of suspended aerosol mass (with size distributions retrieved by inverse modelling); and sampling of ash fallout during Phases II and III (with size distributions measured by laser diffraction). The suspended aerosol in Phase I plume was dominated by particles sized <0.4 μm in diameter. During Phase II, the suspended aerosol mass contained a high number of fine (<1 μm) particles (an internal mixture of soluble aerosol and very fine ash) and coarse (>1 μm) ash particles. The number ratio of fine and coarse particles fluctuated strongly within short intervals of time, indicative of changes in the fragmentation energy. The near-source ash fallout was poorly sorted (interquartile range 16–80 μm); ash grains sized <1 μm contributed up to 7% of total volume. The high water content and electrical charge in the plume are believed to have enhanced the deposition through particle aggregation. In Phase III the eruption became drier and less explosive. The suspended aerosol mass was strongly bimodal with high particle abundances at ∼0.2 and 1–4 μm. The ash fallout was better sorted (interquartile range 60–120 μm; no grains <1 μm), attributed to limited particle aggregation.► Suspended and deposited particle matter during eruption of Eyjafjallajökull. ► Measurements made <15 km from the vent in April 2010. ► Aerosol optical thickness measured by Sun photometry and retrieved for size distributions. ► Grain size distribution measured in ash fallout. ► Fluctuations in the number ratio of fine and coarse particles indicative of eruptive energy.
Keywords: Volcanic plume; Aerosol; Ash; Sun photometry; Eyjafjallajokull;