Atmospheric Environment (v.45, #24)

CFD model simulation of LPG dispersion in urban areas by Marco Pontiggia; Gabriele Landucci; Valentina Busini; Marco Derudi; Mario Alba; Marco Scaioni; Sarah Bonvicini; Valerio Cozzani; Renato Rota (3913-3923).
There is an increasing concern related to the releases of industrial hazardous materials (either toxic or flammable) due to terrorist attacks or accidental events in congested industrial or urban areas. In particular, a reliable estimation of the hazardous cloud footprint as a function of time is required to assist emergency response decision and planning as a primary element of any Decision Support System. Among the various hazardous materials, the hazard due to the road and rail transportation of liquefied petroleum gas (LPG) is well known since large quantities of LPG are commercialized and the rail or road transportation patterns are often close to downtown areas. Since it is well known that the widely-used dispersion models do not account for the effects of any obstacle like buildings, tanks, railcars, or trees, in this paper a CFD model has been applied to simulate the reported consequences of a recent major accident involving an LPG railcar rupture in a congested urban area (Viareggio town, in Italy), showing both the large influence of the obstacles on LPG dispersion as well as the potentials of CFD models to foresee such an influence.► We used a CFD tool to model the consequences of LPG railcar rupture in urban areas. ► We found a large influence of the obstacles on LPG dispersion in urban areas. ► We showed the potentialities of CFD models to foresee the building influence. ► The observed damaged region and the predictions of the CFD model agree quite well.
Keywords: Hazardous material transportation; Consequence analysis; CFD models; Heavy gas dispersion modeling; LPG; Viareggio accident;

A source apportionment of U.S. fine particulate matter air pollution by George D. Thurston; Kazuhiko Ito; Ramona Lall (3924-3936).
Using daily fine particulate matter (PM2.5) composition data from the 2000–2005 U.S. EPA Chemical Speciation Network (CSN) for over 200 sites, we applied multivariate methods to identify and quantify the major fine particulate matter (PM2.5) source components in the U.S. Novel aspects of this work were: (1) the application of factor analysis (FA) to multi-city daily data, drawing upon both spatial and temporal variations of chemical species; and, (2) the exclusion of secondary components (sulfates, nitrates and organic carbon) from the source identification FA to more clearly discern and apportion the PM2.5 mass to primary emission source categories. For the quantification of source-related mass, we considered two approaches based upon the FA results: 1) using single key tracers for sources identified by FA in a mass regression; and, 2) applying Absolute Principal Component Analysis (APCA). In each case, we followed a two-stage mass regression approach, in which secondary components were first apportioned among the identified sources, and then mass was apportioned to the sources and to other secondary mass not explained by the individual sources. The major U.S. PM2.5 source categories identified via FA (and their key tracers) were: Metals Industry (Pb, Zn); Crustal/Soil Particles (Ca, Si); Motor Vehicle Traffic (EC, NO2); Steel Industry (Fe, Mn); Coal Combustion (As, Se); Oil Combustion (V, Ni); Salt Particles (Na, Cl) and Biomass Burning (K). Nationwide spatial plots of the source-related PM2.5 impacts were confirmatory of the factor interpretations: ubiquitous sources, such as Traffic and Soil, were found to be spread across the nation, more unique sources (such as Steel and Metals Processing) being highest in select industrialized cities, Biomass Burning was highest in the U.S. Northwest, while Residual Oil combustion was highest in cities in the Northeastern U.S. and in cities with major seaports. The sum of these source contributions and the secondary PM2.5 components agreed well with the U.S. PM2.5 observed during the study period (mean = 14.3 μg m−3; R 2 = 0.94). Apportionment regression analyses using single-element tracers for each source category gave results consistent with the APCA estimates. Comparisons of nearby sites indicated that the PM2.5 mass and the secondary aerosols were most homogenous spatially, while traffic PM2.5 and its tracer, EC, were among the most spatially representative of the source-related components. Comparison of apportionment results to a previous analysis of the 1979–1982 IP Network revealed similar and correlated major U.S. source category factors, albeit at lower levels than in the earlier period, suggesting a consistency in the U.S. spatial patterns of these source-related exposures over time, as well. These results indicate that applying source-apportionment methods to the nationwide CSN can be an informative avenue for identifying and quantifying source components for the subsequent estimation of source-specific health effects, potentially contributing to more efficient regulation of PM2.5.► First nationwide source apportionment of U.S. PM2.5 Chemical Speciation Network data. ► A focus on primary emission tracers gave clearer source category interpretations. ► Spatial distributions of estimated impacts consistent with source interpretations. ► Co-pollutant correlations (e.g. SO2 and Hg with Coal) support source interpretations. ► Correlations with 1979–1982 IP Network results suggest spatial consistency over time.
Keywords: Chemical Speciation Network (CSN); Source apportionment; Fine particulate matter mass (PM2.5); Absolute Principal Component Analysis (APCA);

Large aerosol effects on ozone photolysis in the Mediterranean by Giampietro Casasanta; Alcide di Sarra; Daniela Meloni; Francesco Monteleone; Giandomenico Pace; Salvatore Piacentino; Damiano Sferlazzo (3937-3943).
Measurements of spectral actinic flux were performed at the ENEA Station for Climate Observations on the island of Lampedusa, in the central Mediterranean, during several periods in 2003, 2004, 2006 and 2008 with a METCON diode array spectrometer. The ozone photolysis frequency, J(O1D), was calculated from the spectral actinic fluxes. Actinic fluxes determinations were complemented with measurements of total ozone and aerosol optical depth, made with a Brewer spectrophotometer and a multi-filter rotating shadowband radiometer, respectively. As expected, J(O1D) shows a strong dependency on total ozone. The behaviour of J(O1D) was examined at fixed values of total ozone and solar zenith angle; a significant dependency on the aerosol optical depth emerges. The dependency on the aerosol optical depth at 416 nm appears to be linear, and weakly dependent on the total ozone value. The photolysis frequency amplification factor for aerosol is between 0.09 and 0.17 at 30°, and between 0.12 and 0.22 at 60° solar zenith angle, depending on total ozone. Day-to-day changes in AOD produce larger effects on J(O1D) than day-to-day changes in total ozone in summer, and comparable effects in spring. Ozone changes are dominant in autumn and winter.To our knowledge, this is the first attempt to characterize the influence of aerosol optical depth on J(O1D) from a relatively large dataset.► Measurements of J(O1D), total ozone and aerosol optical depth in the Mediterranean. ► Dependency of J(O1D) on ozone and aerosols at different solar zenith angles. ► Aerosol variability influences J(O1D) more than ozone variability in summer.
Keywords: Actinic flux; Photolysis frequency; Aerosol optical depth; Ozone; Photochemistry;

Impact of smoke from biomass burning on air quality in rural communities in southern Australia by Fabienne Reisen; C.P. (Mick) Meyer; Lachie McCaw; Jennifer C. Powell; Kevin Tolhurst; Melita D. Keywood; John L. Gras (3944-3953).
In rural towns of southern Australia, smoke from biomass burning such as prescribed burning of forests, wildfires and stubble burning is often claimed to be the major source of air pollution. To investigate the validity of this claim, ambient measurements of PM2.5 and ozone were made in two rural locations in southern Australia between 2006 and 2008. In order to distinguish PM2.5 associated with smoke from other sources of particulate pollution, PM2.5 samples were analysed for specific smoke tracers, levoglucosan, non sea-salt potassium (nssK+) and oxalate. Monitoring was also undertaken in four homes to determine the extent to which ambient pollutants from prescribed burning penetrate indoors into houses.Monitoring clearly showed that, on occasions, air quality in rural areas is significantly affected by smoke from biomass combustion with PM2.5 showing the greatest impact. Concentrations of PM2.5 increased significantly above background levels at both sites during periods of wildfire and prescribed fire leading to exceedences of the 24-h PM2.5 Air National Environment Protection Measure (NEPM) Advisory standard. The 1-h and 4-h ozone NEPM standards were exceeded only during protracted forest wildfires. The impact of prescribed burning on the indoor air quality of residences depended on the duration of the smoke event and the ventilation rate of the houses. During short-duration events indoor air quality was determined by household activities. During events that persisted for several days, indoor air quality was determined by external conditions coupled with management of household ventilation rate.► Impact of bushfire smoke on air quality is assessed for two Australian towns. ► PM2.5 increased significantly during fires, prescribed burns or woodheater usage. ► Chemical analysis confirmed that biomass combustion was a major source of PM2.5. ► Both primary and secondary emissions were observed during smoke plume events. ► Impact on indoor air depended on duration of smoke event and house ventilation rates.
Keywords: PM2.5; Levoglucosan; Ozone; Prescribed burns; Wildfires;

A Regional Air Quality Model System (RAQMS) is utilized to investigate the spatial and temporal distributions of PM10 concentrations and soil dust aerosol over East Asia in March 2010, when a severe dust storm occurred on 19–23 March. Dust aerosols were primarily generated in Gobi Deserts on 19 March and then swept across vast areas of East Asia. Model results are verified by in-situ observations from 17 Chinese cities, satellite retrievals, as well as lidar measurements in Japan. Validations demonstrate a good skill of RAQMS in representing the distribution and variation of PM10 concentrations and the major features of dust storm evolution. In general, the model performs better in the eastern parts of China than that in the western parts of China. Monthly mean surface PM10 concentrations were above 400 μg m−3 in west China, about 200–400 μg m−3 in north and central China, and below 100 μg m−3 in the remaining parts of China and East Asia. Dust aerosol was the major contributor to PM10 mass concentration during the study period, accounting for more than 80% in west China, 30%–60% in north and central China, and about 20%–40% in south China and the western Pacific regions. The dust storm in March 2010 was unique in its influential extent and transport pathway because it impacted as far as the Pearl River Delta of China, such as Hong Kong. Interannual variability of dust budget is analyzed, showing the dust emission, deposition and atmospheric loading in March 2010 are all larger than those in 2002 and 2006. Of the total dust emission (110.4 Mt), about 68% was redeposited onto the underlying surface by dry and wet deposition processes, and the remaining 32% was suspended in the atmosphere or subject to long-range transport.► Particulates during Asian dust storms in March 2010 were numerically investigated. ► Model validation demonstrated RAQMS was able to predict PM10 and dust aerosols. ► The dust storm in March 2010 was unique in the past decades. ► This dust storm exerted great impact on south China, as far as the Pearl River Delta. ► Dust emission, dry and wet depositions in March 2010 were 110.4 Mt, 48.7 Mt and 26.5 Mt.
Keywords: East Asia; PM10; Dust storm; Numerical simulation; Interannual variability;

Dust storm simulation with regional air quality model – Problems and results by Hristo Chervenkov; Hermann Jakobs (3965-3976).
On 24 March 2007 an extraordinary dust plume was observed in the troposphere over Central Europe. Satellite observations revealed its origins in a dust storm in Southern Ukraine, where large amounts of soil were resuspended from dried-out farmlands by wind gusts up to 30 m/s. Maximum particulate matter (PM10) mass concentrations between 200 and 1400 μg m−3 occurred in Slovakia, the Czech Republic, Poland, and Germany along the pathway of the plume from East to West. In order to achieve a better understanding of the processes involved and their interaction the numerical simulation of this event was done by using a new parameterization of dust emission under certain conditions. A simplified dust emission scheme, proposed by Shao (2004), is implemented in the EURAD –Model (European Air Pollution Dispersion Model). A possible way for simulation of other similar dust events in the future is described. The obtained results are compared with the available measurement data and are commented.► Numerical simulation of a dust cloud over Europe. ► Implementation of a dust activation mechanisms. ► A posteriori and a priori simulation of a dust emission event. ► Relatively good agreement of simulated particle concentrations compared with measurements.
Keywords: Air pollution; Dust emission; Case study;

A diagnostic model evaluation effort has been performed to focus on photochemical ozone formation and the horizontal transport process since they strongly impact the temporal evolution and spatial distribution of ozone (O3) within the lower troposphere. Results from the Community Multiscale Air Quality (CMAQ) modeling system are evaluated against surface and upper air measurements from field studies during summer 2002 when several high O3 episodes occurred in the eastern United States. Modeled O3 and winds are compared to research aircraft measurements and wind profiler data, respectively, to investigate whether model underestimates of daily maximum 8-h ozone concentrations during high O3 episodes might be attributable to discrepancies in either or both of these modeled processes. Comparisons of 10 AM surface O3 concentrations, which are representative of O3 levels in the residual layer aloft, revealed that model underestimation was greater at higher observed ozone levels. Mid-morning vertical ozone profiles corroborated this surface-level finding, as modeled concentrations tended to be lower than observed O3 aloft. Net ozone production efficiency (OPE) results suggested photochemical ozone formation was comparable between the model and observations with composite OPE values of 6.7 and 7.6, respectively, within the afternoon planetary boundary layer. Evaluation of wind profiles revealed modeled wind speeds with the base four-dimensional data assimilation (FDDA) approach underestimated observed speeds by more than 2 m s−1 and direction was biased by about 20° in the nocturnal residual layer aloft as coarse resolution analysis fields involved in FDDA were found to inhibit modeled winds. These differences could produce large spatial displacements in modeled and observed ozone patterns within the region. Although sensitivity simulation results with the WRF meteorological model with FDDA using all available upper air profile observations displayed improvements in capturing wind fields aloft, CMAQ maximum 8-h O3 results using the improved wind fields also underestimated observations.► Ozone production efficiency and horizontal transport were investigated. ► Modeled maximum O3 concentrations underestimated observed values aloft. ► Modeled O3 production efficiency was comparable to observed values aloft. ► Modeled wind speeds underestimated measurements in the nocturnal low level jet. ► Data assimilation of observed wind profiles improved modeled winds aloft at night.
Keywords: Diagnostic model evaluation; Photochemical modeling; Residual layer ozone; Nocturnal low level jet; Four-dimensional data assimilation;

Very little is currently known about the relationship between exposure to different sources of ambient ultrafine particles (PM0.1) and human health effects. If human health effects are enhanced by PM0.1’s ability to cross cell membranes, then more information is needed describing the sources of ultrafine particles that are deposited in the human respiratory system. The current study presents results for the source apportionment of airborne particulate matter in six size fractions smaller than 1.8 μm particle diameter including ultrafine particles (PM0.1) in one of the most polluted air basins in the United States. Size-resolved source apportionment results are presented at an urban site and rural site in central California’s heavily polluted San Joaquin Valley during the winter and summer months using a molecular marker chemical mass balance (MM–CMB) method. Respiratory deposition calculations for the size-resolved source apportionment results are carried out with the Multiple Path Particle Dosimetry Model ( v 2.0), including calculations for ultrafine (PM0. 1) source deposition.Diesel engines accounted for the majority of PM0.1 and PM1.8 EC at both the urban and rural sampling locations during both summer and winter seasons. Meat cooking accounted for 33–67% and diesel engines accounted for 15–21% of the PM0.1 OC at Fresno. Meat cooking accounted for 22–26% of the PM0.1 OC at the rural Westside location, while diesel engines accounted for 8–9%. Wood burning contributions to PM0.1 OC increased to as much as 12% of PM0.1 OC during the wintertime. The modest contribution of wood smoke reflects the success of emissions control programs over the past decade. In contrast to PM0.1, PM1.8 OC had a higher fraction of unidentified source contributions (68–85%) suggesting that this material is composed of secondary organic aerosol (SOA) or primary organic aerosol (POA) that has been processed by atmospheric chemical reactions. Meat cooking was the largest identified source of PM1.8 organic carbon (OC) at the Fresno site (12–13%) while diesel engines were the largest identified PM1.8 OC source at the rural site (5–8%). Wood burning contributions to PM1.8 OC increased during the wintertime at both sites (6–9%) but were relatively small during the summertime (∼1%).As expected, diesel engines were the dominant source of PM0.1 EC respiratory deposition at both the urban and rural site in both summer and winter (0.01–0.03 μg PM0.1 EC deposited per m3 air inhaled). Meat cooking accounted for 0.01–0.025 μg PM0.1 OC deposited per m3 air inhaled while diesel fuel accounted for 0.005–0.013 μg PM0.1 OC deposited per m3 air inhaled. Minor contributions from wood burning, motor oil, and gasoline fuel were calculated at levels <0.005 μg PM0.1 OC deposited per m3 air inhaled at both urban and rural locations during winter and summer seasons. If the burden of PM0.1 deposited in the respiratory system is relevant for human health effects, then future toxicology studies should be carried out at PM0.1 concentrations and source mixtures equivalent to those measured in the current study.► We performed PM0.1 source apportionment using molecular markers. ► Source contributions by season/location are presented for the San Joaquin Valley. ► We calculated PM source respiratory deposition using size-resolved apportionment. ► PM0.1 EC deposition is dominated by diesel engines at all locations in all seasons. ► PM0.1 OC deposition has large contributions from SOA, meat cooking, and diesel.
Keywords: PM0.1; Ultrafine particles; CMB; San Joaquin Valley; MOUDI;

Characteristics of airborne ultrafine and coarse particles during the Australian dust storm of 23 September 2009 by E.R. Jayaratne; G.R. Johnson; P. McGarry; H.C. Cheung; L. Morawska (3996-4001).
Particle number concentrations and size distributions, visibility and particulate mass concentrations and weather parameters were monitored in Brisbane, Australia, on 23 September 2009, during the passage of a dust storm that originated 1400 km away in the dry continental interior. The dust concentration peaked at about mid-day when the hourly average PM2.5 and PM10 values reached 814 and 6460 μg m−3, respectively, with a sharp drop in atmospheric visibility. A linear regression analysis showed a good correlation between the coefficient of light scattering by particles (Bsp) and both PM10 and PM2.5. The particle number in the size range 0.5–20 μm exhibited a lognormal size distribution with modal and geometrical mean diameters of 1.6 and 1.9 μm, respectively. The modal mass was around 10 μm with less than 10% of the mass carried by particles smaller than 2.5 μm. The PM10 fraction accounted for about 68% of the total mass. By mid-day, as the dust began to increase sharply, the ultrafine particle number concentration fell from about 6 × 103 cm−3 to 3 × 103 cm−3 and then continued to decrease to less than 1 × 103 cm−3 by 14 h, showing a power-law decrease with Bsp with an R 2 value of 0.77 (p < 0.01). Ultrafine particle size distributions also showed a significant decrease in number during the dust storm. This is the first scientific study of particle size distributions in an Australian dust storm.► We present the physical characteristics of particles in an Australian dust storm. ► First scientific study of particle size distributions in an Australian dust storm. ► Ultrafine particles in the environment are removed by dust particles. ► A good correlation between light scattering coefficient and both PM10 and PM2.5. ► The PM10 fraction accounted for about 68% of the total mass of dust particles.
Keywords: Dust storm; Particle concentration; Particle size; Visibility; Air pollution;

Long-term effects of ozone on CO2 exchange in peatland microcosms by Jaana K. Haapala; Sami K. Mörsky; Riikka Rinnan; Sanna Saarnio; Pertti J. Martikanen; Toini Holopainen; Jouko Silvola (4002-4007).
Effects of elevated tropospheric ozone concentration on the CO2 exchange of peatland microcosms and the photosynthetic capacity of the dominating sedge, Eriophorum vaginatum, were studied in a four-year open-field experiment. The net ecosystem CO2 exchange and the dark respiration rate of the microcosms were measured with the closed chamber method. The CO2 assimilation rate and chlorophyll fluorescence (maximal photochemical efficiency of PSII, Fv /Fm ) of E. vaginatum leaves were also measured. The gross photosynthesis rate of the microcosms was transiently decreased by ozone exposure during the first year. During the fourth year, the gross photosynthesis and dark respiration rate were both slightly increased by ozone exposure but this was due to the increased density of sedge leaves and no difference was found in Fv /Fm . In overall, chronic ozone exposure had only slight effect on the CO2 exchange of the peatland microcosms.► Moderately increased O3 has minor effects on the CO2 exchange of fen ecosystems. ► Chronic O3 exposure has variable effects on the Fv /Fm of Eriophorum vaginatum. ► CO2 assimilation rate of E. vaginatum is not affected by increased O3.
Keywords: Peatland; Ozone; CO2 exchange; Photosynthesis; Eriophorum vaginatum;

Waste combustion as a source of ambient air polybrominated diphenylethers (PBDEs) by Barbara Wyrzykowska-Ceradini; Brian K. Gullett; Dennis Tabor; Abderrahmane Touati (4008-4014).
The first comprehensive set of U.S. data on polybrominated diphenylether (PBDE) concentrations from municipal waste combustion (MWC), with more than 40 PBDE congeners reported, was compared to ambient air levels of PBDEs in the U.S. The PBDE profiles in the raw MWC flue gas reflected the historical production and usage pattern of PBDE-based flame retardants in North America, which favored Penta- and Deca- BDE formulations. The pattern of selected, routinely measured in the environment, PBDEs (TeBDE-47, PeBDE-99, PeBDE-100, HxBDE-153 and DcBDE-209) was similar in the MWC emissions and profiles most commonly reported for the U.S. atmosphere.The mean Σ PBDE concentrations in the clean flue gases collected from the stack were 0.13 and 1.7 ng dscm−1 during the steady state and transients of MWC, respectively (which was 98.6% reduction compare to the levels in the raw flue gases). The major PBDE congeners in the MWC flue gases were those typically found in PBDE technical mixes (TeBDE-47, PeBDE-99, PeBDE-100, HxBDE-153, HpBDE-183, OcBDE-197, NoBDE-206, NoBDE-207, NoBDE-208, DcBDE-209). The profile of the PBDEs in the raw flue gas was dominated by heavier congeners, especially DcBDE-209, while the profile of the stack flue gases profile was dominated by the lighter congeners (TeBDE-47, PeBDE-99, PeBDE-100 accounted for around 80% of total stack emissions). Some of the MWC flue gas samples exhibited enrichment of lower brominated congeners that are minor or not present in the technical mixtures, suggesting that debromination occurs during combustion. Congeners substituted in non- and mono-ortho positions (TeBDE-77, PeBDE-126, HxBDE-156 and -169) were detected mostly during the transients of MWC.► First report on pre- and post-air pollution control system (APCS) levels of PBDEs in the flue gas of a U.S. MWC. ► Pre-APCS PBDE profiles resemble PBDE historical usage in North America. ► Post-APCS PBDE profile is similar to air-profiles commonly reported for North America
Keywords: Municipal waste combustion; MWC; Ambient air; Polybrominated diphenylether; PBDE emissions; Homologue and isomer patterns;

We have examined the interactions of gaseous pollutants and primary aerosols that can produce secondary inorganic aerosols. The specific objective was to estimate degradation rates of precursor gases (NH3, NO2 and SO2) responsible for formation of secondary inorganic aerosols. A Teflon-based outdoor environmental chamber facility (volume 12.5 m3) was built and checked for wall losses, leaks, solar transparency and ability to simulate photochemical reactions. The chamber was equipped with state-of-the-art instrumentation to monitor concentration–time profiles of precursor gases, ozone, and aerosol. A total of 14 experimental runs were carried out for estimating the degradation of precursor gases. The following initial conditions were maintained in the chamber: NO2  = 246 ± 104 ppb(v), NH3  = 548 ± 83 ppb(v), SO2  = 238 ± 107 ppb(v), O3  = 50 ± 11 ppb(v), PM2.5 aerosol = 283438 ± 60524 No./litre. The concentration–time profile of gases followed first-order decay and were used for estimating degradation rates (NO2  = 0.26 ± 0.15 h−1, SO2  = 0.31 ± 0.17 h−1, NH3  = 0.35 ± 0.21 h−1). We observed that degradation rates showed a statistical significant positive correlation (at 5% level of significance) with the initial PM2.5 levels in the chamber (coefficient of correlation: 0.63 for NO2; 0.62 for NH3 and 0.51 for SO2), suggesting that the existing surface of the aerosol could play a significant role in degradation of precursor gases. One or more gaseous species can be adsorbed on to the existing particles and these may undergo heterogeneous or homogeneous chemical transformation to produce secondary inorganic aerosols. Through correlation analysis, we have observed that degradation rates of precursor gases were dependent on initial molar ratio of (NH3)/(NO2  + SO2), indicative of ammonia-rich and ammonia-poor situations for eventual production of ammonium salts.► Degradation rates of precursor gases were estimated as: NO = 0.45 ± 0.11 h−1, NO2  = 0.26 ± 0.15 h−1, SO2  = 0.31 ± 0.17 h−1, NH3  = 0.35 ± 0.21 h−1. ► Existing surface of primary aerosol plays a significant role in degradation of precursor gases. ► A higher initial molar ratio of (NH3) to (NO2  + SO2) leads to greater degradation rates of the precursor gases.
Keywords: Ammonium salts; Secondary inorganic aerosols; Degradation rate; Wall losses; Photolysis rate;

Fine particulate concentrations on sidewalks in five Southern California cities by Marlon G. Boarnet; Douglas Houston; Rufus Edwards; Marko Princevac; Gavin Ferguson; Hansheng Pan; Christian Bartolome (4025-4033).
This research provides an exploratory examination of the factors associated with fine particle concentrations in intersection and sidewalk microenvironments in five study areas in the Los Angeles region. The study areas range from low-density, auto-oriented development patterns to dense urban areas with mid- and high-rise buildings. Average concentrations of FPDT (fine particle concentrations measured with DustTrak Aerosol Monitors) ranged from about 20 to 70 μg m−3 across study areas during stationary and mobile (walking) monitoring in morning, midday, and evening periods. Results suggest that fine particle concentrations are highly variable on urban sidewalks. A regression analysis shows that concentrations are associated with traffic and the proximate built environment characteristics after accounting for meteorological factors, time of day, and location in the region. Regressions show higher concentrations were associated with lower wind speeds and higher temperatures, higher adjacent passenger vehicle traffic, higher ambient concentrations, and street canyons with buildings of over five stories. Locations in street canyons with 2–5 story buildings and with more paving and open space had lower concentrations after accounting for other factors. The associations with traffic and built environment variables explained a small amount of the variation in FPDT concentrations, suggesting that future research should examine the relative role of localized traffic and built environment characteristics compared to regional ambient concentrations and meteorology.► Concentrations of fine particle (FPDT) are highly variable on urban sidewalks. ► FPDT concentrations ranged from about 20 to 70 μg m−3 across five study areas study. ► Sidewalk concentrations were associated with wind speeds, temperatures, traffic, and building and open space patterns. ► Findings raise concerns for development near high-traffic arterials.
Keywords: Particulate matter; Exposure; Sidewalk; Built Environment; Air quality; Pollution;

Fog water chemistry in Shanghai by Pengfei Li; Xiang Li; Chenyu Yang; Xinjun Wang; Jianmin Chen; Jeffrey L. Collett (4034-4041).
With the aim of understanding the fog chemistry in a Chinese megacity, twenty-six fog water samples were collected in urban Shanghai from March 2009 to March 2010. The following parameters were measured: pH, electrical conductivity (EC), ten inorganic major ions ( SO 4 2 − , NO 3 − , NO 2 − , F, Cl, Na+, K+, Ca2+, Mg2+, NH 4 + ) and four major organic acids (CH3COO, HCOO, C 2 O 4 2 − , MSA). The total ionic concentration (TIC) and EC of fog samples were one or two orders of magnitude higher than those often found in Europe, North America and other Asian countries. Pollutants were expected to be mainly from local sources, including factories, motor vehicle emissions and civil construction. Non-local sources such as moderate- and long-range transport of sea salt also contributed to pollution levels in fog events as indicated by back trajectory analysis. The pH of the fog water collected during the monitoring period varied from 4.68 to 6.58; acidic fogs represented about 30.8% of the total fog events during this period. The fog water was characterized by high concentrations of SO 4 2 − (20.0% of measured TIC), NO 3 − (17.1%), NH 4 + (28.3%) and Ca2+ (14.4%). SO 4 2 − and NO 3 − , the main precursors of fog acidity, were related to burning fossil fuels and vehicle emissions, respectively. NH 4 + , originating from the scavenging of gaseous ammonia and particulate ammonium nitrate and ammonium sulfate, and Ca2+, originating from the scavenging of coarse particles, acted as acid neutralizers and were the main cause for the relatively high pH of fogs in Shanghai. The ratio of ( SO 4 2 −  +  NO 3 − )/( NH 4 +  + Ca2+) was lower than 1, indicating the alkaline nature of the fog water. A high ratio of NO 3 − / SO 4 2 − and low ratio of HCOO/CH3COO were consistent with large contributions from vehicular emissions that produce severe air pollution in megacities.► Systematic studies of urban fog chemistry were conducted firstly in Shanghai. ► Fog with acidic property represented about 30.8% of total fog events. ► Complex relationship was observed between fog ion content and fog microphysics.
Keywords: Fog chemistry; Air mass trajectories; Microphysics; Shanghai;

Aerosol black carbon at five background measurement sites over Finland, a gateway to the Arctic by A.-P. Hyvärinen; P. Kolmonen; V.-M. Kerminen; A. Virkkula; A. Leskinen; M. Komppula; J. Hatakka; J. Burkhart; A. Stohl; P. Aalto; M. Kulmala; K.E.J. Lehtinen; Y. Viisanen; H. Lihavainen (4042-4050).
Aerosol equivalent black carbon (BCe) was measured at five different background stations in Finland, with the longest data set from Hyytiälä, December 2004–December 2008. Measurements were conducted either with an aethalometer or a Multi-Angle Absorption Photometer, MAAP. Measured black carbon concentrations were highest in Virolahti in southeastern Finland, with annual averages ranging from 385 to 460 ng m−3, followed by Hyytiälä (250–370 ng m−3), Utö (230–270 ng m−3), Puijo (225–230 ng m−3), and Pallastunturi (60–70 ng m−3) in northern Finland. The BCe fractions of measured PM2.5 concentrations were generally between 5 and 10%, with highest fractions at Virolahti close to the Eastern border. At all the stations, the highest concentrations were observed during the spring and the winter, and the lowest concentrations during the summer. The seasonal cycle could generally be attributed to the reaching of long-range-transported black carbon. Additional reasons were increasing domestic wood burning and reduced boundary-layer height during winter, and a more effective vertical mixing during summer. The highest concentrations for each station occurred with southerly winds, and on the basis of trajectory analyses, the source areas of BCe resided mostly in Central and Eastern Europe. Occasionally the long-range-transported BCe concentrations were elevated for short periods to fulfill the characteristics of pollution episodes. From these episodes, about 62% were a result of non-fire anthropogenic sources and 36% due to open biomass burning sources. Episodes from the biomass burning sources were most often observed during the spring.► Long term black carbon measurements reported in and close to the Arctic. ► Trans-boundary transport a major source of black carbon in clean areas. ► Open biomass burning a reason for transported black carbon in Northern Europe.
Keywords: Background aerosols; Black carbon; Aethalometer; Multi-angle absorption photometer;

Multi-year inventories of biomass burning emissions were established in the Pearl River Delta (PRD) region for the period 2003–2007 based on the collected activity data and emission factors. The results indicated that emissions of sulfur dioxide (SO2), nitrogen oxide (NOx), ammonia (NH3), methane (CH4), organic carbon (OC), non-methane volatile organic compounds (NMVOC), carbon monoxide (CO), and fine particulate matter (PM2.5) presented clear declining trends. Domestic biofuel burning was the major contributor, accounting for more than 60% of the total emissions. The preliminary temporal profiles were established with MODIS fire count information, showing that higher emissions were observed in winter (from November to March) than other seasons. The emissions were spatially allocated into grid cells with a resolution of 3 km × 3  km, using GIS-based land use data as spatial surrogates. Large amount of emissions were observed mostly in the less developed areas in the PRD region. The uncertainties in biomass burning emission estimates were quantified using Monte Carlo simulation; the results showed that there were higher uncertainties in organic carbon (OC) and elemental carbon (EC) emission estimates, ranging from −71% to 133% and −70% to 128%, and relatively lower uncertainties in SO2, NOx and CO emission estimates. The key uncertainty sources of the developed inventory included emission factors and parameters used for estimating biomass burning amounts.► Declining trends in biomass burning emissions are found in the PRD region. ► Domestic biofuel is the largest contributor to the PRD biomass burning emissions. ► Gridded biomass burning emissions are developed and ready for model use. ► Higher emissions occur in winter and most distributed over the less developed areas. ► Higher uncertainties in OC and EC emission estimates.
Keywords: Pearl River Delta region; Biomass burning emission inventory; Temporal characteristics; Spatial characteristics; Uncertainty;

Impact of fair-weather cumulus clouds and the Chesapeake Bay breeze on pollutant transport and transformation by Christopher P. Loughner; Dale J. Allen; Kenneth E. Pickering; Da-Lin Zhang; Yi-Xuan Shou; Russell R. Dickerson (4060-4072).
Two fine-scale meteorological processes, fair-weather cumulus cloud development and a bay breeze, are examined along with their impacts on air chemistry. The impact of model resolution on fair-weather cumulus cloud development, transport of pollutants through clouds, sulfur dioxide to sulfate conversion in clouds, and the development of the Chesapeake Bay breeze are examined via 13.5, 4.5, 1.5, and 0.5 km resolution simulations covering the Washington – Baltimore area. Results show that as the resolution increases, more pollutants are transported aloft through fair-weather cumulus clouds causing an increase in the rate of oxidation of sulfur dioxide to sulfate aerosols. The high resolution model runs more nearly match observations of a local pollutant maximum near the top of the boundary layer and produce an increase in boundary layer venting with subsequent pollutant export. The sensitivity of sulfur dioxide to sulfate conversion rates to cloud processing is examined by comparing sulfur dioxide and sulfate concentrations from simulations that use two different methods to diagnose clouds. For this particular event, a diagnostic method produces the most clouds and the most realistic cloud cover, has the highest oxidation rates, and generates sulfur dioxide and sulfate concentrations that agree best with observations. The differences between the simulations show the importance of accurately simulating clouds in sulfate simulations. The fidelity of the model’s representation of the bay breeze is examined as a function of resolution. As the model resolution increases, a larger temperature gradient develops along the shoreline of the Chesapeake Bay causing the bay breeze to form sooner, push farther inland, and loft more pollutants upward. This stronger bay breeze results in low-level convergence, a buildup of near surface ozone over land and a decrease in the land-to-sea flux of ozone and ozone precursors as seen in measurements. The resulting 8 h maximum ozone concentration over the Bay is 10 ppbv lower in the 0.5 km simulation than in the 13.5 km simulation.► As model resolution increases, more SO2 is oxidized in fair-weather cumulus clouds. ► More planetary boundary layer venting as the model resolution increases. ► A stronger bay breeze forms with higher resolution simulations. ► As resolution increases, higher ozone concentrations at bay breeze convergence zone. ► Coarse resolution simulations produce higher ozone concentrations over the water.
Keywords: Community Multiscale Air Quality (CMAQ) model; Horizontal grid resolution; Fair-weather cumulus clouds; Sulfur dioxide; Bay breeze; Ozone;

Comparisons of JU2003 observations with four diagnostic urban wind flow and Lagrangian particle dispersion models by Steven Hanna; John White; James Trolier; Rebecca Vernot; Michael Brown; Akshay Gowardhan; Hadassah Kaplan; Yehuda Alexander; Jacques Moussafir; Yansen Wang; Chatt Williamson; John Hannan; Elizabeth Hendrick (4073-4081).
Urban wind flow and dispersion models are needed that can satisfactorily account for the effects of the three-dimensional (3D) building geometries but which run much faster than Computational Fluid Dynamics (CFD) models. With sufficient speed, the models can be used for rapid response and for applications where many simulations must be performed in a short time period. To satisfy this need, several diagnostic wind flow models have been developed for urban areas, where mass-consistent principles are used in combination with local wind observations to solve for the mean wind flow on domains of size ranging from a few hundred meters to several kilometers on a side, within which detailed 3-D building geometries are defined. Simple assumptions about vortex flow structures are parameterized near buildings and in street canyons. The wind flow results are used as inputs to a Lagrangian particle dispersion model (LPDM), where the needed turbulent velocities and time scales are parameterized using standard boundary layer profile formulas combined with special relations around buildings. As part of a collaborative study, with the intent of advancing each model, the developers of four of these models have run their models for two tracer releases (one daytime and one nighttime) during the Joint Urban 2003 (JU2003) field experiment. The four models are: QUIC by Los Alamos National Laboratory (LANL), 3DWF by the Army Research Laboratory, the urban Lagrangian model by the Israel Institute for Biological Research (IIBR), and Microswift/Spray (MSS) by Aria Technologies and SAIC. The comparison uses nearly identical domains and grid systems, and all models use the same input wind profile. The simulated patterns of wind fields and tracer contours are in good qualitative agreement. For wind speed near the surface, the mean model biases are less than about 20% and RMS errors are about 1–2 m s−1. For tracer concentrations, the four models give similar quantitative results, where the mean relative biases suggest that the individual models can be sometimes as much as a factor of two high or low, and where the scatter suggests that, for all models, about 30 or 40% of the simulations are within a factor of two of observations. In most cases, the observed plume is broader than the simulated plume, and the models are biased toward slight underestimation of the dispersion of the plumes to the tall rooftops.► We compare four similar Rockle-type urban wind and dispersion models. ► The models are applied to two Joint Urban 2003 (JU2003) field trials. ► The four models have similar performance for winds and SF6 tracer gas. ► Improvements are needed in vertical mixing around tall buildings.
Keywords: Urban dispersion models; Urban wind flow; JU2003;

Multi-day ozone production potential of volatile organic compounds calculated with a tagging approach by T.M. Butler; M.G. Lawrence; D. Taraborrelli; J. Lelieveld (4082-4090).
Calculation of the ozone production potential of Volatile Organic Compounds (VOC) has traditionally been performed using so-called incremental reactivity techniques, requiring multiple photochemical model runs in which the combined direct and indirect effects on ozone from slight perturbations to each VOC are investigated in turn. A new approach to this problem is presented here using an extensively tagged chemical mechanism, in which the direct effects of VOC on ozone are calculated using a single model run. The results of this approach are consistent with previous work, but deliver much more detailed information about the VOC intermediate oxidation products involved in the production of ozone. We show that different classes of VOC exhibit very different temporal evolution in their ozone production potential, with alkenes and reactive aromatic VOC producing ozone rapidly, while the ozone production potential of alkanes increases in the day after they are emitted. We suggest that this is related to the rate at which these different classes of compounds are able to produce very small oxidation fragments. This multi-day ozone production potential has implications for emission control strategies for the management of air quality in polluted regions. We also relate the ozone production potential of VOC to the OH reactivity of the VOC oxidation intermediates, and show that the early oxidation products of alkenes and reactive aromatics are more efficient at producing ozone in their subsequent reactions with OH than similarly reactive alkanes. While this study examines idealised chemical conditions in a box model, the techniques employed here could potentially be adapted to the study of a wider range of atmospheric conditions using three-dimensional air quality models.► New method for calculating Ozone Production Potential of VOC. ► Greater level of mechanistic detail than previous techniques. ► Ozone production related to structure and reactivity of VOC oxidation intermediates. ► Ozone Production Potential of alkanes increases over multiple days.
Keywords: Air quality; Ozone; VOC; Photochemical modelling;

Dynamic evaluation of regional air quality model’s response to emission reductions in the presence of uncertain emission inventories by Sergey L. Napelenok; Kristen M. Foley; Daiwen Kang; Rohit Mathur; Thomas Pierce; S. Trivikrama Rao (4091-4098).
A method is presented and applied for evaluating an air quality model’s changes in pollutant concentrations stemming from changes in emissions while explicitly accounting for the uncertainties in the base emission inventory. Specifically, the Community Multiscale Air Quality (CMAQ) model is evaluated for its ability to simulate the change in ozone (O3) levels in response to significant reductions in nitric oxide (NOx = NO + NO2) emissions from the NOx State Implementation Plan (SIP) Call and vehicle fleet turnover between the years of 2002 and 2005. The dynamic model evaluation (i.e., the evaluation of a model’s ability to predict changes in pollutant levels given changes in emissions) differs from previous approaches by explicitly accounting for known uncertainties in the NOx emissions inventories. Uncertainty in three sectors of NOx emissions is considered – area sources, mobile sources, and point sources – and is propagated using sensitivity coefficients calculated by the decoupled direct method in three dimensions (DDM-3D). The change in O3 levels between 2002 and 2005 is estimated based on differences in the empirical distributions of the modeled and observed data during the two years. Results indicate that the CMAQ model is able to reproduce the observed change in daily maximum 8-hr average O3 levels at more than two-thirds of Air Quality System (AQS) monitoring locations when a relatively moderate amount of uncertainty (50%) is assumed in area and mobile emissions of NOx together with a low amount of uncertainty (3%) in the utility sector (elevated point sources) emissions. The impact of other sources of uncertainty in the model is also briefly explored.► We evaluate CMAQ modeled change in surface O3 between the summers of 2002 and 2005. ► We consider uncertainties in area, mobile and point NOx emission inventories. ► Assuming moderate amount of uncertainty, the model predicts observed changes well. ► Other sources of the discrepancy in the modeled and observed signals are explored. ► Dynamic evaluation methodology is flexible for extension to other uncertainties.
Keywords: Dynamic model evaluation; CMAQ; Direct decoupled method; DDM; Air quality modeling; Ozone; Uncertainty; Sensitivity;

An improved line source model for air pollutant dispersion from roadway traffic by Régis Briant; Irène Korsakissok; Christian Seigneur (4099-4107).
Gaussian plume models, which are widely used to model atmospheric dispersion, provide an exact analytical solution for line sources such as roads only when the wind direction is perpendicular to the road. Some approximations have been developed to provide an analytical formula for a line source when the wind direction is not perpendicular to the road; however, such formulas lead to some error and the solution diverges when the wind direction is parallel to the road. A novel approach that reduces the error in the line source formula when the wind direction is not perpendicular to the road is presented here. Furthermore, a combination of analytical and numerical line source solutions is used to better represent cases where the wind direction becomes parallel to the road. The improved model was implemented in the Polyphemus modeling platform and it was successfully evaluated against a reference solution as well as observations obtained near a roadway in eastern France.► Improved line source model for Gaussian dispersion models. ► Correction of errors downwind of the line source section. ► Correction of errors downwind of the line source extremities. ► Line source/discretized source combination when the wind becomes parallel to the source.
Keywords: Gaussian plume model; Line source; Polyphemus;

Land use regression (LUR) models have been widely used to estimate traffic air pollution within cities taking into account spatial distributions. These prediction models are often built based on multiple linear models that regress concentrations measured at locations spread over the cities on land use characteristics of those locations. Land use variables, as for example total road length, are sometimes calculated in buffers of different radii that are then included simultaneously in the models to account for a distance gradient in the influence of sources. Some authors favour including disjoint concentric ring areas instead. Both approaches have their advantages, and adjacent concentric rings tend to make model interpretation more intuitive. The present paper shows that both approaches lead to identical models and the results achieved from one can be used to calculate the results of the other when absolute continuous exposure measures such as total length of roads are used. It is also shown that this holds as well for relative exposures such as area-weighted total length of streets, inhabitants per area, and % urban area, where the measure of interest is scaled by a constant factor.
Keywords: Land use regression models; Buffers; Donuts; Rings; Air pollution;

Persistent daily new particle formation at a mountain-top location by A. Gannet Hallar; Douglas H. Lowenthal; Galina Chirokova; Randolph D. Borys; Christine Wiedinmyer (4111-4115).
Frequent new particle formation (NPF) has been observed regularly at Storm Peak Laboratory, a high elevation mountain-top observatory in Colorado. These events occurred during 52% of the 474 measurement days from 2001 to 2009, consistently during the mid-afternoon throughout the spring, summer and winter months. Average growth rates, condensation sinks, and formation rates are presented. The strongest correlation factor with NPF is ultraviolet radiation. The events are not related to increased ozone concentration or pre-existing aerosol surface area, implying that nucleation can occur anywhere in the free troposphere.
Keywords: Atmospheric aerosols; High elevation; New particle formation;

Using data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) program, report that average fine particulate light absorbing carbon (LAC) concentrations in California decreased by about 50% from 0.46 μg m-3 in 1989 to 0.24 μg m-3 in 2008. They attribute most of the LAC decline in California to reductions in the state’s diesel emissions. These findings are encouraging, but in this comment we call attention to a significant methodological issue that can arise in any long-term trends analysis using IMPROVE data. In the Bahadur et al. analysis, LAC data from eighteen remote monitoring sites were aggregated with data from three urban sites that only operated for 1–8 years. The large absolute decrease of 0.22 μg m-3 they reported in the statewide California average was largely driven by one urban site, South Lake Tahoe (Tahoe), which was dropped from the network in mid-1997. LAC concentrations at Tahoe were an order of magnitude higher than those at nearby Bliss State Park indicative of large local source contributions. The exclusion of the three locally influenced urban sites substantially reduces the magnitude of the decreasing LAC trends shown in Bahadur et al., though this does not necessarily invalidate the paper’s conclusion that LAC is broadly decreasing and diesel emission controls are likely to be responsible for part of this decrease. Control of emissions from wood-burning stoves may also have contributed to decreases in LAC and other particulate compounds; like diesel emission controls, this too is an important regulatory success.
Keywords: Light absorbing carbon; Long-term trends;

We address possible sampling biases reported in an earlier work () relating decreases in black carbon (BC) measurements from the IMPROVE monitoring network to diesel emissions in California. A decrease in average BC concentration of between 40 and 60% is found at each site between 1988 and 2007, consistent with the statewide average of 50%. No significant regional biases are found to drive these trends, which are independent of latitude as well as mean BC concentration. We find no reason to revise the conclusions of Bahadur et al, in response to the methodological issues raised by Schichtel et al.