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Atmospheric Environment (v.43, #2)

Editorial board (pp. i).

Comparison of organic compositions in dust storm and normal aerosol samples collected at Gosan, Jeju Island, during spring 2005 by Gehui Wang; Kimitaka Kawamura; Meehye Lee (pp. 219-227).

To better understand the current physical and chemical properties of East Asian aerosols, an intensive observation of atmospheric particles was conducted at Gosan site, Jeju Island, South Korea during 2005 spring. Total suspended particle (TSP) samples were collected using pre-combusted quartz filters and a high-volume air sampler with the time intervals ranging from 3h to 48h. The kinds and amount of various organic compounds were measured in the samples using gas chromatography–mass spectrometry. Among the 99 target compounds detected, saccharides (average, 130±14ngm⁻³), fatty acids (73±7ngm⁻³), alcohols (41±4ngm⁻³), n-alkanes (32±3ngm⁻³), and phthalates (21±2ngm⁻³) were found to be major compound classes with polyols/polyacids, lignin and resin products, PAHs, sterols and aromatic acids being minor. Compared to the previous results reported for 2001 late spring samples, no significant changes were found in the levels of their concentrations and compositions for 4 years, although the economy in East Asia, especially in China, has sharply expanded from 2001 to 2005. During the campaign at Gosan site, we encountered two distinct dust storm episodes with high TSP concentrations. The first dust event occurred on March 28, which was characterized by a predominance of secondary organic aerosols. The second event that occurred on the next day (March 29) was found to be characterized by primary organic aerosols associated with forest fires in Siberia/northeastern China. A significant variation in the molecular compositions, which was found within a day, suggests that the compositions of East Asian aerosols are heterogeneous due to multi-contributions from different source regions together with different pathways of long-range atmospheric transport of particles.

Keywords: Organic aerosols; Water-soluble organic compounds; Hydrophobic organic compounds; Molecular compositions; Asian dust storm; Lipids; Levoglucosan

Continuous measurement of peroxyacetyl nitrate (PAN) in suburban and remote areas of western China by J.M. Zhang; T. Wang; A.J. Ding; X.H. Zhou; L.K. Xue; C.N. Poon; W.S. Wu; J. Gao; H.C. Zuo; J.M. Chen; X.C. Zhang; S.J. Fan (pp. 228-237).

Knowledge on atmospheric abundance of peroxyacetyl nitrate (PAN) is important in assessing the severity of photochemical pollution, and for understanding chemical transformation of reactive odd nitrogen and its impact on the budget of tropospheric ozone (O₃). In summer 2006, continuous measurements of PAN were made using an automatic GC–ECD analyzer with an on-line calibrator at a suburban site of Lanzhou (LZ) and a remote site of Mt. Waliguan (WLG) in western China, with concurrent measurements of O₃, total reactive nitrogen (NO_y) and carbon monoxide (CO). At LZ, several photochemical episodes were observed during the study, and the average mixing ratio of PAN (plus or minus standard deviation) was 0.76 (±0.89) ppbv with the maximum value of 9.13 ppbv, compared to an average value of 0.44 (±0.16) ppbv at remote WLG. The PAN mixing ratios in LZ exhibited strong diurnal variations with a maximum at noon, while enhanced concentrations of PAN were observed in the evening and a minimum in the afternoon at WLG. The daily O₃ and PAN concentration maxima showed a strong correlation ( r² = 0.91) in LZ, with a regression slope (PAN/O₃) of 0.091 ppbv ppbv⁻¹. At WLG, six well-identified pollution plumes (lasting 2–8 h) were observed with elevated concentrations of PAN (and other trace gases), and analysis of backward particle release simulation shows that the high-PAN events at WLG were mostly associated with the transport of air masses that had passed over LZ.

Keywords: PAN; Ozone; Lanzhou; Mt. Waliguan; Plume transport

Ambient air quality during wheat and rice crop stubble burning episodes in Patiala by Susheel K. Mittal; Nirankar Singh; Ravinder Agarwal; Amit Awasthi; Prabhat K. Gupta (pp. 238-244).

Open crop stubble burning events were observed in and around Patiala city, India. A ground level study was deliberated to analyze the contribution of wheat ( Triticum aestivum) and rice ( Oriza sativa) crop stubble burning practices on concentration levels of aerosol, SO₂ and NO₂ in ambient air at five different sites in and around Patiala city covering agricultural, commercial and residential areas. Aerosols were collected on GMF/A and QMF/A (Whatman) sheets for a 24h period throughout the year in 2007. Simultaneously, sampling of SO₂ and NO₂ was conducted and results obtained during stubble burning periods were compared to the non-stubble burning periods. Results clearly pointed out a distinct increase in aerosol, SO₂ and NO₂ levels during the crop stubble burning periods.

Keywords: Ambient air; Crop stubble burning; Aerosol; SO; ₂; NO; ₂

Air pollution dispersion within urban street canyons by Olga V. Taseiko; Sergey V. Mikhailuta; Anne Pitt; Anatoly A. Lezhenin; Yuri V. Zakharov (pp. 245-252).

A semi-empirical mathematical model, Urban Street Model (USM), is proposed to efficiently estimate the dispersion of vehicular air pollution in cities. This model describes urban building arrangements by combining building density, building heights and the permeability of building arrangements relative to wind flow. To estimate the level of air pollution in the city of Krasnoyarsk (in Eastern Siberia), the spatial distribution of pollutant concentrations off roadways is calculated using Markov's processes in USM. The USM-predicted numerical results were compared with field measurements and with results obtained from other frequently used models, CALINE-4 and OSPM. USM consistently yielded the best results. OSPM usually overestimated pollutant concentration values. CALINE-4 consistently underestimated these values. For OSPM, the maximum differences were 160% and for CALINE-4 about 400%. Permeability and building density are necessary parameters for accurately modeling urban air pollution and influencing regulatory requirements for building planning.

Keywords: Urban street canyon; Urban air pollution dispersion; Building arrangements; Air flow

Modeling toxic compounds from nitric oxide emission measurements by Daniel A. Vallero; Jeffrey Peirce; Ki Don Cho (pp. 253-261).

Determining the amount and rate of degradation of toxic pollutants in soil and groundwater is difficult and often requires invasive techniques, such as deploying extensive monitoring well networks. Even with these networks, degradation rates across entire systems cannot readily be extrapolated from the samples. When organic compounds are degraded by microbes, especially nitrifying bacteria, oxides or nitrogen (NO _x) are released to the atmosphere. Thus, the flux of nitric oxide (NO) from the soil to the lower troposphere can be used to predict the rate at which organic compounds are degraded. By characterizing and applying biogenic and anthropogenic processes in soils the rates of degradation of organic compounds. Toluene was selected as a representative of toxic aromatic compounds, since it is inherently toxic, it is a substituted benzene compound and is listed as a hazardous air pollutant under Section 12 of the Clean Air Act Amendments of 1990. Measured toluene concentrations in soil, microbial population growth and NO fluxes in chamber studies were used to develop and parameterize a numerical model based on carbon and nitrogen cycling. These measurements, in turn, were used as indicators of bioremediation of air toxic (i.e. toluene) concentrations. The model found that chemical concentration, soil microbial abundance, and NO production can be directly related to the experimental results (significant at P<0.01) for all toluene concentrations tested. This indicates that the model may prove useful in monitoring and predicting the fate of toxic aromatic contaminants in a complex soil system. It may also be useful in predicting the release of ozone precursors, such as changes in reservoirs of hydrocarbons and oxides of nitrogen. As such, the model may be a tool for decision makers in ozone non-attainment areas.

Keywords: Modeling; Organic chemicals; Soil pollution; Biodegradation; Monitoring

CFD model simulation of dispersion from chlorine railcar releases in industrial and urban areas by Steven R. Hanna; Olav R. Hansen; Mathieu Ichard; David Strimaitis (pp. 262-270).

To assist in emergency response decisions and planning in case of releases of pressurized liquefied chlorine from railroad tank cars in industrial sites and cities, the FLACS Computational Fluid Dynamics (CFD) model has been used to simulate the transport and dispersion of the dense chlorine cloud. Two accident locations are studied: an actual railcar accident at an industrial site in Festus, MO, and a hypothetical railcar accident at a rail junction in the Chicago urban area. The results show that transport of a large dense gas release at ground level in an industrial site or large city could initially extend a hundred meters or more in the upwind and crosswind directions. The dense cloud may follow terrain drainage, such as river channels. Near the source, the obstacles tend to slow down the dense gas cloud and may constrain it and cause increased concentrations. Farther downwind, the obstacles may cause enhanced mixing and dilution once the cloud has grown larger. In some cases, significant amounts of cloud mass may become “trapped” in obstacle wakes for many minutes after the main cloud has passed. Although the CFD model can account for the details of the flow and dispersion much better than standard widely-used simple dense gas models, many similarities are found among the various models in their simulated variations with downwind distance of the maximum cloud centerline concentration.

Keywords: Chlorine railcar accidents; Urban dispersion models; Dense gas dispersion models; CFD models; Festus accident; Chicago dispersion model, Urban boundary layers; Emergency response model

Seasonal variability of radon-derived fetch regions for Sado Island, Japan, based on 3 years of observations: 2002–2004 by Scott Chambers; Wlodek Zahorowski; Kiyoshi Matsumoto; Mitsuo Uematsu (pp. 271-279).

Three years of hourly atmospheric radon measurements at Sado Island (Japan) are discussed and compared with corresponding measurements at Gosan (South Korea), and Hok Tsui (China). In conjunction with back trajectory analysis, Sado radon data are used to characterise the seasonal variability in fetch regions of air masses subject to extremes of terrestrial influence. In winter, fetch regions of air masses that have experienced the greatest terrestrial influence covered southern Siberia; in summer, the terrestrial fetch was dominated by Japan; throughout the remaining months the terrestrial fetch encompassed the Korean Peninsula and far eastern China. Summer radon data are then used to estimate the radon flux from central Honshu (23.5mBqm⁻²s⁻¹), which varied regionally between 10.6 and 47.9mBqm⁻²s⁻¹. The Sado radon record reported here completes a 4-site, multi-year dataset of hourly radon concentrations across East Asia and the central Pacific (spanning 16° of latitude), which constitutes a unique evaluation tool for transport and mixing schemes of atmospheric and chemical transport models.

Keywords: Asian outflow; Atmospheric tracer; Back trajectory analysis; Fetch analysis; Radon

Performance evaluation of six different aerosol samplers in a particulate matter generation chamber by Ju-Myon Park; James C. Rock; Lingjuan Wang; Yong-Chil Seo; Amit Bhatnagar; Seongheon Kim (pp. 280-289).

The present study was carried out with the aim of evaluating the performance of six different aerosol samplers in terms of mass concentration, particle size distribution, and mass fraction for the international size-sampling conventions. The international size-sampling criteria were defined as inhalable, thoracic, and respirable mass fractions with 50% cutoff at an aerodynamic equivalent diameter of 100 μm, 10 μm, and 4 μm, respectively. Two Andersen, four total suspended particulate (TSP), two RespiCon, four PM₁₀, two DustTrak, and two SidePak samplers were selected and tested to quantitatively estimate human exposure in a carefully controlled particulate matter (PM) test chamber. The overall results indicate that (1) Andersen samplers underestimate total suspended PM and overestimate thoracic and respirable PM due to particle bounce and carryover between stages, (2) TSP samplers provide total suspended PM as reference samplers, (3) TSP samplers quantified by a coulter counter multisizer provide no information below an equivalent spherical diameter of 2 μm and therefore underestimate respirable PM, (4) RespiCon samplers are free from particle bounce as inhalable samplers but underestimate total suspended PM, (5) PM₁₀ samplers overestimate thoracic PM, and (6) DustTrak and SidePak samplers provide relative PM concentrations instead of absolute PM concentrations.

Keywords: Aerosol sampler; Particulate matter; Particle size distribution; Mass fraction; Size-sampling; Uncertainty propagation

Wintertime size distribution of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) in the urban environment: Street- vs rooftop-level measurements by Loukia P. Chrysikou; Panagiotis G. Gemenetzis; Constantini A. Samara (pp. 290-300).

The size distribution of ambient air particles and associated organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and organochlorine pesticides (OCPs) including hexachlorocyclohexanes (HCHs), DDT and metabolites, etc., was investigated at a traffic-impacted site of Thessaloniki, Greece. Investigation took place during wintertime of 2006 at two heights above ground: at the street level (1.5m) and at the rooftop level (15m). Size-resolved samples (<0.95μm, 0.95–1.5μm, 1.5–3μm, 3–7.5μm and >7.5μm) were concurrently collected from the two height levels using five-stage high volume cascade impactors. At both heights, particle mass exhibited bimodal distribution with peaks in the 0.95–1.5μm and the 3–7.5μm size fractions, whereas most organic pollutants exhibited one peak at 0.95–1.5μm. Apart from the 0.95–1.5μm fraction, particle concentrations of all size ranges were significantly higher at the street level than at the rooftop as a result of more intensive vehicular emissions and road dust resuspension. On the contrary, the concentrations of most organic pollutants did not differentiate significantly between the two elevations.

Keywords: Coarse particles; Fine size fraction; Long-range transport; Traffic emissions; Persistent organic pollutants; Semivolatile organic compounds; Thessaloniki

Intake fraction distributions for benzene from vehicles in the Helsinki metropolitan area by Miranda M. Loh; Joana Soares; Ari Karppinen; Jaakko Kukkonen; Leena Kangas; Kari Riikonen; Anu Kousa; Arja Asikainen; Matti J. Jantunen (pp. 301-310).

The intake fraction (iF) gives a measure of the portion of a source's emissions that is inhaled by an exposed population over a defined period of time. This study examines spatial and population-based iF distributions of a known human carcinogen, benzene, from a ubiquitous urban source, local vehicular traffic, in the Helsinki Metropolitan Area using three computational methods. The first method uses the EXPAND model (EXPosure to Air pollution, especially to Nitrogen Dioxide and particulate matter), which incorporates spatial and temporal information on population activity patterns as well as urban-scale and street canyon dispersion models to predict spatial population exposure distributions. The second method uses data from the personal monitoring study EXPOLIS (Air Pollution Exposure Distributions of Adult Urban Populations in Europe) to estimate the intake fractions for individuals in the study. The third method, a one-compartment box model provides estimates within an order-of-magnitude or better for non-reactive agents in an urban area. Population intake fractions are higher using the personal monitoring data method (median iF 30 per million, mean iF 39 per million) compared with the spatial model (annual mean iF 10 per million) and the box model (median iF 4 per million, mean iF 7 per million). In particular, this study presents detailed intake fraction distributions on several different levels (spatial, individual, and generic) for the same urban area.

Keywords: Intake fraction; Benzene; Traffic; Air pollution; Exposure

Atmospheric BTEX-concentrations in an area with intensive street traffic by Anna Jolanta Buczynska; Agnieszka Krata; Marianne Stranger; Ana Flavia Locateli Godoi; Velichka Kontozova-Deutsch; László Bencs; Inge Naveau; Edward Roekens; René Van Grieken (pp. 311-318).

The major threat to clean air in developed and industrializing countries is now posed by traffic emissions. The effects of traffic road modifications on the air quality are, however, rarely reported in the literature. The aim of this study was to determine the influence of the modernization and renovation of a traffic artery in the region of Mortsel (Antwerp, Belgium) on the concentration of volatile organic compounds such as: benzene, toluene, ethylbenzene and m-, p-, o-xylenes (BTEX). The original goal of the reconstruction works was to reduce the traffic lanes of one of the busiest streets in Antwerp, in order to discourage the road traffic and in consequence also to improve the air quality in this region. The average concentrations of BTEX before these works in 2003 were: 1.6, 7.0, 0.9, 2.3, and 0.9μg/m³, for benzene, toluene, ethylbenzene, m+ p xylenes, and o-xylene, respectively. However, after the completion of the works, in 2005, they were slightly higher: 2.5, 9.5, 1.6, 3.4, and 1.3μg/m³, respectively. The scatter plots of benzene against toluene, ethylbenzene and xylenes in 2003 and 2005 showed very good correlations. This fact indicated that all of the measured compounds originated from the same source, namely the road traffic. Moreover, the data obtained from an air-monitoring station at less than 6km distance from the sampling site (operated by the Flemish Environment Agency, and located in Borgerhout, Antwerp), confirmed the lack of influence of background concentrations of BTEX. The obtained results led to the conclusion that the reduction of the number of traffic lanes had apparently increased the traffic jams and also increased the emission from cars. Therefore, these modernization works had even a negative impact on the local concentration of traffic-related pollutants as BTEX.

Keywords: Benzene; Toluene; Ethylbenzene; Xylene; Passive sampler; Road traffic; Antwerp; Belgium

Prediction of particle deposition onto indoor surfaces by CFD with a modified Lagrangian method by Z. Zhang; Q. Chen (pp. 319-328).

Accurate prediction of particle deposition indoors is important to estimate exposure risk of building occupants to particulate matter. The prediction requires accurate modeling of airflow, turbulence, and interactions between particles and eddies close to indoor surfaces. This study used av′2¯−f turbulence model with a modified Lagrangian method to predict the particle deposition in enclosed environments. Thev′2¯−f model can accurately calculate the normal turbulence fluctuationv′2¯, which mainly represents the anisotropy of turbulence near walls. Based on the predictedv′2¯, we proposed an anisotropic particle–eddy interaction model for the prediction of particle deposition by the Lagrangian method. The model performance was assessed by comparing the computed particle deposition onto differently oriented surfaces with the experimental data in a turbulent channel flow and in a naturally convected cavity available from the literature. The predicted particle deposition velocities agreed reasonably with the experimental data for different sizes of particles ranging from 0.01μm to 50μm in diameter. This study concluded that the Lagrangian method can predict indoor particle deposition with reasonable accuracy provided the near-wall turbulence and its interactions with particles are correctly modeled.

Keywords: Particle deposition; Lagrangian method; CFD; v2f; Indoor environment

Airborne trace element pollution in 11 European cities assessed by exposure of standardised ryegrass cultures by Andreas Klumpp; Wolfgang Ansel; Gabriele Klumpp; Jörn Breuer; Philippe Vergne; María José Sanz; Stine Rasmussen; Helge Ro-Poulsen; Àngela Ribas Artola; Josep Peñuelas; Shang He; Jean Pierre Garrec; Vicent Calatayud (pp. 329-339).

Within a European biomonitoring programme, Italian ryegrass ( Lolium multiflorum Lam.) was employed as accumulative bioindicator of airborne trace elements (As, Cd, Cr, Cu, Fe, Ni, Pb, Sb, V, Zn) in urban agglomerations. Applying a highly standardised method, grass cultures were exposed for consecutive periods of four weeks each to ambient air at up to 100 sites in 11 cities during 2000–2002. Results of the 2001 exposure experiments revealed a clear differentiation of trace element pollution within and among local monitoring networks. Pollution was influenced particularly by traffic emissions. Especially Sb, Pb, Cr, Fe, and Cu exhibited a very uneven distribution within the municipal areas with strong accumulation in plants from traffic-exposed sites in the city centres and close to major roads, and moderate to low levels in plants exposed at suburban or rural sites. Accumulation of Ni and V was influenced by other emission sources. The biomonitoring sites located in Spanish city centres featured a much higher pollution load by trace elements than those in other cities of the network, confirming previously reported findings obtained by chemical analyses of dust deposition and aerosols. At some heavily-trafficked sites, legal thresholds for Cu, Pb, and V contents in foodstuff and animal feed were reached or even surpassed. The study confirmed that the standardised grass exposure is a useful and reliable tool to monitor and to assess environmental levels of potentially toxic compounds of particulate matter.

Keywords: Air quality; Biomonitoring; Trace elements; Particulate matter; Lolium multiflorum; Urban ambient air

Urban aerosol evolution and particle formation during wintertime temperature inversions by K. Frans G. Olofson; Patrik U. Andersson; Mattias Hallquist; Evert Ljungström; Lin Tang; Deliang Chen; Jan B.C. Pettersson (pp. 340-346).

Aerosol temporal and spatial distributions during wintertime temperature inversions in Gothenburg, Sweden, have been characterized by ground-based and airborne particle measurements combined with lidar measurements. Ground inversions frequently developed during evenings and nights with stable cold conditions, and the low wintertime insolation often resulted in near neutral boundary layer conditions during day-time. Under these conditions ground level aerosol concentrations peaked during morning rush hours and often remained relatively high throughout the day due to inefficient ventilation. The particle number concentrations decreased slowly with increasing altitude within the boundary layer, and measurements slightly above the boundary layer suggested limited entrainment of polluted air into the free troposphere. High concentrations of ultrafine particles were observed throughout the boundary layer up to altitudes of 1100m, which suggested that nucleation took place within the residual layer during the night and early morning. Recently formed particles were also observed around midday when the layer near ground was ventilated by mixing into the boundary layer, which indicated that ultrafine particles were either transported down from the residual layer to ground level or formed when the polluted surface layer mixed with the cleaner air above.

Keywords: Aerosol; Temperature inversion; Lidar; Wintertime; Nucleation; Gothenburg

Wind modeling of Chihuahuan Desert dust outbreaks by Nancy I. Rivera Rivera; Thomas E. Gill; Kristi A. Gebhart; Jennifer L. Hand; Max P. Bleiweiss; Rosa M. Fitzgerald (pp. 347-354).

The Chihuahuan Desert region of North America is a significant source of mineral aerosols in the Western Hemisphere, and Chihuahuan Desert dust storms frequently impact the Paso del Norte (El Paso, USA/Ciudad Juarez, Mexico) metropolitan area. A statistical analysis of HYSPLIT back trajectory residence times evaluated airflow into El Paso on all days and on days with synoptic (non-convective) dust events in 2001–2005. The incremental probability—a measure of the areas most likely to have been traversed by air masses arriving at El Paso during dusty days—was only strongly positively associated with the region west–southwest of the city, a zone of known dust source areas. Focused case studies were made of major dust events on 15 April and 15 December 2003. Trajectories approached the surface and MM5 (NCAR/Penn State Mesoscale Model) wind speeds increased at locations consistent with dust sources observed in satellite imagery on those dates. Back trajectory and model analyses suggested that surface cyclones adjacent to the Chihuahuan Desert were associated with the extreme dust events, consistent with previous studies of dust storms in the Southern High Plains to the northeast. The recognition of these meteorological patterns serves as a forecast aid for prediction of dust events likely to impact the Paso del Norte.

Keywords: Dust; Chihuahuan Desert; MM5; HYSPLIT; Trajectory

A study of the ozone formation by ensemble back trajectory-process analysis using the Eta–CMAQ forecast model over the northeastern U.S. during the 2004 ICARTT period by Shaocai Yu; Rohit Mathur; Daiwen Kang; Kenneth Schere; Daniel Tong (pp. 355-363).

The integrated process rates (IPRs) estimated by the Eta–CMAQ model at grid cells along the trajectory of the air mass transport path were analyzed to quantitatively investigate the relative importance of physical and chemical processes for O₃ formation and evolution over the northeastern U.S. during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) period. The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model is used to determine the back trajectory of air masses reaching the northeast by linking a downwind receptor to upwind source areas. The process analysis is applied to a high O₃ episode occurring on July 22, 2004 at three selected sites in the northeastern U.S. The process analysis at the location of the site shows that during the daytime, the O₃ concentrations in the surface layer are mainly enhanced by the vertical diffusion of O₃-rich air from aloft, followed by horizontal advection (HADV) and chemical production (CHEM), whereas dry deposition (DDEP) and vertical advection (ZADV) mainly deplete O₃ concentrations at the sites of Valley Central (VC), NY and Castle Spring (CS), NH. By integrating the effects of each process over the depth of the daytime planetary boundary layer (PBL), it was found that at the VC site, CHEM and HADV contributed about 53% and 41%, respectively, to O₃ levels within the PBL. This confirms the significance of regional transport of O₃ from the industrialized areas into the Northeast. On the other hand, the process analysis results for O₃ formation in moving air masses indicate that on July 22, large chemical production of O₃ along the transport path over the polluted urban regions leads to significant increase in O₃ in the air mass reaching the VC site, whereas the low chemical production of O₃ along the transport path over the low emission regions leads to the low O₃ concentration at the site of Belleayre Mountain (BM), NY. The dramatic buildup of O₃ concentration from 50ppb to 102ppb in the air masses before reaching the VC site after 12:00 EST on 7/22 indicates the significant impact of pollution from the northeastern urban corridor at this site. On the basis of the results at the CS site, it was found that high NO_x emissions along the transport path led to large chemical production of O₃ in the air mass reaching the CS site on July 22. In contrast, the low chemical production of O₃ associated with low emission (relatively clean conditions) along the transport path over the northern portions of the domain is responsible for the low O₃ concentration at the CS site on July 26.

Keywords: Ozone formation; Eta–CMAQ model; Process analysis; Back trajectory

Investigation of the positive artifact formation during sampling semi-volatile aerosol using wet denuders by A. Khlystov; M. Lin; M.A. Bolch; Y. Ma (pp. 364-370).

Wet denuders are used in several steam-based semi-continuous aerosol monitors to avoid gaseous absorption artifacts and pre-humidify the air stream, while simultaneously allowing measurements of water-soluble gaseous species. Unlike dry denuders, wet denuders saturate the sample air stream with water vapor, which can lead to re-partitioning of water-soluble volatile species to the aerosol phase, thereby causing a positive artifact in aerosol measurements. This paper investigates the magnitude of the positive artifact formation occurring in wet denuders using modeling techniques. Gaseous nitric acid was used as an example of volatile water-soluble gas in both flat and annular wet denuders. We have also verified the occurrence of the positive artifact in a flat wet denuder through a laboratory experiment. The model results indicate that the magnitude of the artifact is rather limited under typical conditions being less than 2.5% of ambient nitric acid concentration for the flat denuder and less than 0.6% for the annular denuder. The magnitude of the artifact increases with condensational sink of the aerosol (i.e. with the mean aerosol size and number concentration) and aerosol water solubility. While the artifact is relatively small in the absolute sense, it could be substantial for aerosol nitrate measurements, especially in ammonia limited conditions, when the concentration of the nitric acid is high and the concentration of nitrate is low. Therefore, we recommend that the artifact is assessed regularly by replacing the wet denuder with a dry denuder.

Keywords: Sampling artifact; Wet-wall denuders; Nitrate

A new metric space incorporating radon-222 for generation of back trajectory clusters in atmospheric pollution studies by Jagoda Crawford; Wlodek Zahorowski; David D. Cohen (pp. 371-381).

A novel metric space for the clustering of back trajectories to be used in fine particle aerosol data analysis was proposed and evaluated. The metric is based on spatial and non-spatial variables incorporating great-circle distance, altitude and radon-222.Its performance was examined using the intra-cluster variation of measured and fingerprint apportioned aerosol mass as the quantitative criterion. The new metric was demonstrated to perform better than those based on great-circle distance, or a great-circle distance and altitude alone. The same criterion was applied to investigate the clustering performance as a function of the length of its back trajectories. The optimum back trajectory length was found to be dependent on the pollution source being considered.Performance tests, as well as the application of the new metric space to re-analysis of previously published results, were based on a three year long dataset comprising co-located aerosol fine particles (PM_2.5) collection and hourly measurements of radon-222 concentration.The new metric space can easily be redefined to include other trace species.

Keywords: Aerosol; Back trajectory cluster analysis; Radon

Experimental testing of an annular denuder and filter system to measure gas–particle partitioning of semivolatile bifunctional carbonyls in the atmosphere by Ricardo Ortiz; Kenji Enya; Kazuhiko Sekiguchi; Kazuhiko Sakamoto (pp. 382-388).

An annular denuder and filter-pack system was tested in combination with the use of the in-tube and on-fiber O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA)-derivatization technique to simultaneously sample and measure gaseous and particulate concentrations of semivolatile bifunctional carbonyl compounds in the atmosphere. Ozone was denuded from the sampling air to avoid oxidation and PFBHA was used as the sorbent by coating the sampling denuders and impregnating the filters. The collection efficiency of the system was evaluated under different conditions in photochemical smog chamber experiments and in field samplings of urban and suburban atmospheres. The effects of concentration level, temperature, and humidity on the collection efficiency were assessed. The system showed average collection efficiencies in one denuder from 81% for pyruvic acid and 82% for glyoxylic acid to 87% for hydroxyacetone and dihydroxyacetone. The capacity of the filters to collect the gaseous fraction that cannot be collected in the denuders was also evaluated, and the system allows a correction for this artifact. The application of this method to chamber experiments and field samplings offers an easy-to-apply technique with good results that can be used to evaluate the partition mechanisms of these compounds in the atmosphere.

Keywords: Gas–particle partitioning; Bifunctional carbonyl compounds; Atmospheric sampling; Collection efficiency

Large emissions of sesquiterpenes and methyl chavicol quantified from branch enclosure measurements by Nicole C. Bouvier-Brown; Rupert Holzinger; Katrin Palitzsch; Allen H. Goldstein (pp. 389-401).

Multiple field studies have suggested chemistry within a forest canopy is poorly understood due to inadequate detection and quantification of reactive biogenic emissions, such as terpenes. To measure emission rates of terpenes at Blodgett Forest, a coniferous forest in the Sierra Nevada mountains of California, we placed enclosures over branches of the dominant species at the site – Ponderosa pine, manzanita, and ceanothus – in the summer of 2005. Zero air, with ambient CO₂ concentrations, flowed through the chamber system and volatile organic compound (VOC) emission measurements were made by proton transfer reaction mass spectrometry (PTR-MS), solid phase microextraction (SPME) on fibers followed by direct injection into a gas chromatograph with an ion trap mass spectrometer (GC-ITMS), and by in situ GC with a flame ionization detector (GC-FID). We show that previously undetected sesquiterpenes and methyl chavicol significantly contribute to the total reactive biogenic emission profile from this field site.

Keywords: Terpenes; Branch enclosure; Sesquiterpenes; Methyl chavicol; SPME fibers; BVOC emission rates

Modeling wintertime particulate matter formation in central California by Betty K. Pun; Rochelle T.F. Balmori; Christian Seigneur (pp. 402-409).

A wintertime episode during the 2000 California Regional PM Air Quality Study (CRPAQS) was simulated with the air quality model CMAQ–MADRID. Model performance was evaluated with 24-h average measurements available from CRPAQS. Modeled organic matter (OM) was dominated by emissions, which were probably significantly under-represented, especially in urban areas. In one urban area, modeled daytime nitrate concentrations were low and evening concentrations were high. This diurnal profile was not explained by the partition of nitrate between the gas and particle phases, because gaseous nitric acid concentrations were low compared to PM nitrate. Both measured and simulated nitrate concentrations aloft were lower than at the surface at two tower locations during this episode. Heterogeneous reactions involving NO₃ and N₂O₅ accounted for significant nitrate production in the model, resulting in a nighttime peak. The sensitivity of PM nitrate to precursor emissions varied with time and space. Nitrate formation was on average sensitive to NO _x emissions. However, for some periods at urban locations, reductions in NO _x caused the contrary response of nitrate increases. Nitrate was only weakly sensitive to reductions in anthropogenic VOC emissions. Nitrate formation tended to be insensitive to the availability of ammonia at locations with high nitrate, although the spatial extent of the nitrate plume was reduced when ammonia was reduced. Reductions in PM emissions caused OM to decrease, but had no effect on nitrate despite the role of heterogeneous reactions. A control strategy that focuses on NO _x and PM emissions would be effective on average, but reductions in VOC and NH₃ emissions would also be beneficial for certain times and locations.

Keywords: Ammonium nitrate; Organic matter; Control strategy; CMAQ (Community Multiscale Air Quality model); MADRID (Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution)

Impact of different nitrogen emission sources on tree physiology as assessed by a triple stable isotope approach by M.R. Guerrieri; R.T.W. Siegwolf; M. Saurer; M. Jäggi; P. Cherubini; F. Ripullone; M. Borghetti (pp. 410-418).

The importance that nitrogen (N) deposition has in driving the carbon (C) sequestration of forests has recently been investigated using both experimental and modeling approaches. Whether increased N deposition has positive or negative effects on such ecosystems depends on the status of the N and the duration of the deposition. By combining δ¹³C, δ¹⁸O, δ¹⁵N and dendrochronological approaches, we analyzed the impact of two different sources of NO_x emissions on two tree species, namely: a broadleaved species ( Quercus cerris) that was located close to an oil refinery in Southern Italy, and a coniferous species ( Picea abies) located close to a freeway in Switzerland. Variations in the c_i/ c_a ratio and the distinction between stomatal and photosynthetic responses to NO_x emissions in trees were assessed using a conceptual model, which combines δ¹³C and δ¹⁸O. δ¹⁵N in leaves, needles and tree rings was found to be a bioindicator of N input from anthropogenic emissions, especially at the oil refinery site. We observed that N fertilization had a stimulatory effect on tree growth near the oil refinery, while the opposite effect was found for trees at the freeway site. Changes in the c_i/ c_a ratio were mostly related to variations in δ¹³C at the freeway site and, thus, were driven by photosynthesis. At the oil refinery site they were mainly related to stomatal conductance, as assessed using δ¹⁸O. This study demonstrates that a single method approach does not always provide a complete picture of which physiological traits are more affected by N emissions. The triple isotope approach combined with dendrochronological analyses proved to be a very promising tool for monitoring the ecophysiological responses of trees to long-term N deposition.

Keywords: Dendrochronology; N deposition; NO; _x; emissions; Picea abies; Quercus cerris; Stable isotopes

Modeling air quality during the California Regional PM₁₀/PM_2.5 Air Quality Study (CPRAQS) using the UCD/CIT source-oriented air quality model – Part III. Regional source apportionment of secondary and total airborne particulate matter by Qi Ying; Jin Lu; Michael Kleeman (pp. 419-430).

A comprehensive air quality modeling project was carried out to simulate regional source contributions to secondary and total (=primary+secondary) airborne particle concentrations in California's Central Valley. A three-week stagnation episode lasting from December 15, 2000 to January 7, 2001, was chosen for study using the air quality and meteorological data collected during the California Regional PM₁₀/PM_2.5 Air Quality Study (CRPAQS). The UCD/CIT mechanistic air quality model was used with explicit decomposition of the gas phase reaction chemistry to track source contributions to secondary PM. Inert artificial tracers were used with an internal mixture representation to track source contributions to primary PM. Both primary and secondary source apportionment calculations were performed for 15 size fractions ranging from 0.01 to 10μm particle diameters. Primary and secondary source contributions were resolved for fugitive dust, road dust, diesel engines, catalyst equipped gasoline engines, non-catalyst equipped gasoline engines, wood burning, food cooking, high sulfur fuel combustion, and other anthropogenic sources.Diesel engines were identified as the largest source of secondary nitrate in central California during the study episode, accounting for approximately 40% of the total PM_2.5 nitrate. Catalyst equipped gasoline engines were also significant, contributing approximately 20% of the total secondary PM_2.5 nitrate. Agricultural sources were the dominant source of secondary ammonium ion. Sharp gradients of PM concentrations were predicted around major urban areas. The relative source contributions to PM_2.5 from each source category in urban areas differ from those in rural areas, due to the dominance of primary OC in urban locations and secondary nitrate in the rural areas. The source contributions to ultra-fine particle mass PM_0.1 also show clear urban/rural differences. Wood smoke was found to be the major source of PM_0.1 in urban areas while motor vehicle sources were the major contributor of PM_0.1 in rural areas, reflecting the influence from two major highways that transect the Valley.

Keywords: Source apportionment; Secondary particulate matter; Ultra-fine particulate matter; Central California; CRPAQS

Determination of primary and secondary sources of organic acids and carbonaceous aerosols using stable carbon isotopes by Rebeka Fisseha; Matthias Saurer; Maya Jäggi; Rolf T.W. Siegwolf; Josef Dommen; Sönke Szidat; Vera Samburova; Urs Baltensperger (pp. 431-437).

Stable carbon isotope ratio ( δ¹³C) data can provide important information regarding the sources and the processing of atmospheric organic carbon species. Formic, acetic and oxalic acid were collected from Zurich city in August–September 2002 and March 2003 in the gas and aerosol phase, and the corresponding δ¹³C analysis was performed using a wet oxidation method followed by isotope ratio mass spectrometry. In August, the δ¹³C values of gas phase formic acid showed a significant correlation with ozone (coefficient of determination ( r²)=0.63) due to the kinetic isotope effect (KIE). This indicates the presence of secondary sources (i.e. production of organic acids in the atmosphere) in addition to direct emission. In March, both gaseous formic and acetic acid exhibited similar δ¹³C values and did not show any correlation with ozone, indicating a predominantly primary origin. Even though oxalic acid is mainly produced by secondary processes, the δ¹³C value of particulate oxalic acid was not depleted and did not show any correlation with ozone, which may be due to the enrichment of¹³C during the gas - aerosol partitioning.The concentrations and δ¹³C values of the different aerosol fractions (water soluble organic carbon, water insoluble organic carbon, carbonate and black carbon) collected during the same period were also determined. Water soluble organic carbon (WSOC) contributed about 60% to the total carbon and was enriched in¹³C compared to other fractions indicating a possible effect of gas - aerosol partitioning on δ¹³C of carbonaceous aerosols. The carbonate fraction in general was very low (3% of the total carbon).

Keywords: Stable isotopes; Organic acids; Carbonaceous aerosol; ¹³; C

Biomass consumption and CO₂, CO and main hydrocarbon gas emissions in an Amazonian forest clearing fire by T.G. Soares Neto; J.A. Carvalho Jr.; C.A.G. Veras; E.C. Alvarado; R. Gielow; E.N. Lincoln; T.J. Christian; R.J. Yokelson; J.C. Santos (pp. 438-446).

Biomass consumption and CO₂, CO and hydrocarbon gas emissions in an Amazonian forest clearing fire are presented and discussed. The experiment was conducted in the arc of deforestation, near the city of Alta Floresta, state of Mato Grosso, Brazil. The average carbon content of dry biomass was 48% and the estimated average moisture content of fresh biomass was 42% on wet weight basis. The fresh biomass and the amount of carbon on the ground before burning were estimated as 528tha⁻¹ and 147tha⁻¹, respectively. The overall biomass consumption for the experiment was estimated as 23.9%. A series of experiment in the same region resulted in average efficiency of 40% for areas of same size and 50% for larger areas. The lower efficiency obtained in the burn reported here occurred possibly due to rain before the experiment. Excess mixing ratios were measured for CO₂, CO, CH₄, C₂–C₃ aliphatic hydrocarbons, and PM_2.5. Excess mixing ratios of CH₄ and C₂–C₃ hydrocarbons were linearly correlated with those of CO. The average emission factors of CO₂, CO, CH₄, NMHC, and PM_2.5 were 1,599, 111.3, 9.2, 5.6, and 4.8gkg⁻¹ of burned dry biomass, respectively. One hectare of burned forest released about 117,000kg of CO₂, 8100kg of CO, 675kg of CH₄, 407kg of NMHC and 354kg of PM_2.5.

Keywords: Biomass consumption; Gas emission; Amazonian forest fires

MM5 simulations for air quality modeling: An application to a coastal area with complex terrain by Sang-Mi Lee; Marko Princevac; Satoru Mitsutomi; Joe Cassmassi (pp. 447-457).

A series of modifications were implemented in MM5 simulation in order to account for wind along the Santa Clarita valley, a north–south running valley located in the north of Los Angeles. Due to high range mountains in the north and the east of the Los Angeles Air Basin, sea breeze entering Los Angeles exits into two directions. One branch moves toward the eastern part of the basin and the other to the north toward the Santa Clarita valley. However, the northward flow has not been examined thoroughly nor simulated successfully in the previous studies. In the present study, we proposed four modifications to trigger the flow separation. They were (1) increasing drag over the ocean, (2) increasing soil moisture content, (3) selective observational nudging, and (4) one-way nesting for the innermost domain. The Control run overpredicted near-surface wind speed over the ocean and sensible heat flux, in an urbanized area, which justifies the above 1st and 2nd modification. The Modified run provided an improvement in near-surface temperature, sensible heat flux and wind fields including southeasterly flow along the Santa Clarita valley. The improved MM5 wind field triggered a transport to the Santa Clarita valley generating a plume elongated from an urban center to the north, which did not exist in MM5 Control run. In all, the modified MM5 fields yielded better agreement in both CO and O₃ simulations especially in the Santa Clarita area.

Keywords: Flow separation; Sea breeze; Upslope flow; Mesoscale modeling; MM5; CAMx; Local thermal circulation

Southeastward transport of Asian dust: Source, transport and its contributions to Taiwan by Tsun-Hsien Liu; Fujung Tsai; Shih-Chieh Hsu; Che-Wei Hsu; Chein-Jung Shiu; Wei-Nei Chen; Jien-Yi Tu (pp. 458-467).

Atmospheric Aluminum measured in northern Taiwan from 2003 to 2006 is used as a dust tracer, from which dust concentrations are derived, and major Asian dust events are determined. The source locations for the major dust events are traced back and identified, and the processes leading to the southeastward transport of Asian dust is investigated. The derived dust concentrations are compared to the local PM₁₀ (particle with size less than 10μm) concentrations, and the impacts of Asian dust on the air quality of Taiwan are quantified.According to the backward trajectory and dust observation analyses, most of the southeastward transport of major Asian dust events originate from Mongolia and Inner Mongolia in northern China, and only one out of 16 events is generated from western China. Modeling studies and weather analyses of dust events suggest that the southeastward transport of Asian dust is usually generated behind a surface front and transported downwind behind the associated upper level trough. The associated upper level trough is usually deep, in which the northwesterly wind behind the trough favors the southeastward transport of dust to lower latitudes. Dust transported to Taipei generally occur during periods of large-scale subsidence.Asian dust contributes about 15μgm⁻³ of aerosol particles to northern Taiwan during winter monsoon, which accounts for about 24–30% of the PM₁₀ concentrations to the northern Taiwan. The contributions of Asian dust are raised pronouncedly to about 60–70% during major dust events. The impacts of Asian dust on Taiwan's air quality are most substantial in December. The Asian dust impacts decrease in other months, but still remain at around 30% in the late winter to early spring.

Keywords: Asian dust; Southeastward transport; Aluminum; Upper level trough

Measurement of the vertical profile of atmospheric SO₂ during the heating period in Beijing on days of high air pollution by Sun Yang; Wang Yuesi; Zhang Changchun (pp. 468-472).

This paper reports altitude-resolved concentrations of sulfur dioxide (SO₂) and particulate matter up to 10 microns in diameter (PM₁₀) in the planetary boundary layer of major urban centers during extreme pollution episodes. The concentration of SO₂ was observed continuously from November 24, 2004, to December 4, 2004, in Beijing during the heating period. Fluorescence SO₂ analyzers were used to measure the atmospheric SO₂ concentrations. Four SO₂ analyzers were placed at 4 different levels (8m, 47m, 120m, and 280m) of the 325-m high meteorological observation tower of the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences. A maximal SO₂ concentration of 172.3ppb was measured during this pollution episode, and SO₂ concentration increased with altitude and reached its maximal value at ∼50m. The study also analyzed the meteorological situation before, during, and after the pollution episode.

Keywords: SO; ₂; Vertical profile; Atmospheric environmental pollution

Dispersion of pollutants over land–water–land interface: Study using CALPUFF model by S. Indumati; R.B. Oza; Y.S. Mayya; V.D. Puranik; H.S. Kushwaha (pp. 473-478).

The CALMET/CALPUFF modeling system is used to study atmospheric dispersion of pollutant over land–water–land interface. It is shown that the default scheme used by CALMET/CALPUFF to handle inhomogeneous surfaces does not take care of the different turbulence characteristics over such surfaces. An alternative method is suggested to incorporate different turbulent characteristics over inhomogeneous surfaces by using the appropriate atmospheric stability category over different surfaces. The results show that the presence of water body can increase the ground level concentration by a factor of up to 50 for the width of water body varying from 1 km to 5 km. It is also shown that the effect of water body on the ground level concentration decreases as the distance from the water body increases. The present study showed that for land–water interface, the realistic specification of turbulence characteristics over inhomogeneous surfaces significantly changes the estimation of ground level concentration as compared to the default scheme available in the CALMET/CALPUFF modeling system and is expected to give realistic results.

Keywords: Dispersion; Overwater; Complex terrain; Gaussian model; Atmospheric stability

Some comments on “The flow across a street canyon of variable width—Part 2: Scalar dispersion from a street level line source” by Serge Simoëns and James M. Wallace by Nils Mole (pp. 479-479).

Response to comment of Nils Mole by Serge Simoens (pp. 479-480).

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Atmospheric Environment (v.43, #2)