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Atmospheric Environment (v.45, #27)

Editorial board (pp. i).

Organic compounds in PM_2.5 emitted from fireplace and woodstove combustion of typical Portuguese wood species by Cátia Gonçalves; Célia Alves; Ana Patrícia Fernandes; Cristina Monteiro; Luís Tarelho; Margarita Evtyugina; Casimiro Pio (pp. 4533-4545).

The aim of this study is the further characterisation of PM_2.5 emissions from the residential wood combustion of common woods grown in Portugal. This new research extends to eight the number of biomass fuels studied and tries to understand the differences that the burning appliance (fireplace versus woodstove) and the combustion temperature (cold and hot start) have on emissions. Pinus pinaster (Maritime pine), Eucalyptus globulus (eucalypt), Quercus suber (cork oak), Acacia longifolia (Golden wattle), Quercus faginea (Portuguese oak), Olea europea (Olive), Quercus ilex rotundifolia (Holm oak) and briquettes produced from forest biomass waste were used in the combustion tests. Determinations included fine particle emission factors, carbonaceous content (OC and EC) by a thermal–optical transmission technique and detailed identification and quantification of organic compounds by gas chromatography–mass spectrometry. Fine particle emission factors from the woodstove were lower than those from the fireplace. For both combustion appliances, the OC/EC ratio was higher in “cold start” tests (1.56 ± 0.95 for woodstove and 2.03 ± 1.34 for fireplace). These “cold start” OC/EC values were, respectively, for the woodstove and the fireplace, 51% and 69% higher than those obtained in “hot start” experiments. The chromatographically resolved organics included n-alkanes, n-alkenes, PAHs, n-alkanals, ketones, n-alkanols, terpenoids, triterpenoids, phenolic compounds, phytosterols, alcohols, n-alkanoic acids, n-di-acids, unsaturated acids and alkyl esters of acids. The smoke emission rate and composition varied widely depending on fuel type, burning appliance and combustion temperature.► Particle organic compound emissions from fireplace and woodstove were studied. ► PM_2.5 emissions were higher for the fireplace and start-up combustion conditions. ► Major organic components in smoke were anhydrosugars, acids and aliphatics. ► Retene and stigmasterol are good tracers for soft- and hardwood smoke, respectively. ► The emission database enables the calculation of weighted composite source profiles.

Keywords: Biomass burning; Fireplace; Woodstove; PM; _2.5; GC–MS; Organic tracers

Organosulfates and oxidation products from biogenic hydrocarbons in fine aerosols from a forest in North West Europe during spring by Kasper Kristensen; Marianne Glasius (pp. 4546-4556).

Organosulfates of monoterpenes and isoprene, as well as their oxidation products have been identified in biogenic secondary organic aerosols (BSOA) from both laboratory and field studies. Organosulfates provide an interesting coupling between air pollution and formation of low-volatility BSOA.HPLC quadrupole time-of-flight mass spectrometry was used to study polar acidic monoterpene and isoprene oxidation products including pinic acid, pinonic and terpenylic acid along with organosulfates and nitrooxy organosulfates in aerosols from ambient air. The method was first validated by analysis of spiked quartz filters, which showed acceptable recoveries >74% for pinic acid, pinonic acid, camphor sulphonic acid and adipic acid. Acetonitrile was identified as a better solvent than methanol for extraction and analysis of pinonic acid and adipic acid, due to improved analytical sensitivity and prevention of methyl ester formation during sample extraction.PM₁ (i.e, aerosols with an aerodynamic diameter ≤1 μm) were collected during spring 2008 in a forest in Denmark with mixed deciduous and coniferous trees. Average concentrations of the most abundant compounds were: pinic acid: 1.5 ng m⁻³, pinonic acid: 3.0 ng m⁻³, terpenylic acid: 0.8 ng m⁻³ and 3-methyl-1,2,3-butanetricarboxylic acid: 3.0 ng m⁻³.Organosulfates and nitrooxy organosulfates were identified in a majority of the daily samples and the highest levels were observed during a warm period in late spring. As a first approach, due to the lack of authentic standards, organosulfates and nitrooxy organosulfates were tentatively quantified based on the analytical response of camphor sulphonic acid. Generally the concentrations of organosulfates and nitrooxy organosulfates were lower than first generation oxidation products. The maximum concentration of a total of 10 organosulfates and nitrooxy organosulfates were found to be about three times lower than pinonic acid with a maximum concentration of 8 ng m⁻³.A correlation between total organosulfates (including nitrooxy organosulfates) and organic acids was observed with R² = 0.89, indicating that the presence of organosulfates is closely connected to photochemical oxidation and aging of volatile organic compounds.Display Omitted► We study monoterpene and isoprene oxidation products incl. organosulfates in aerosols. ► Highest levels of organosulfates and nitrooxy organosulfates in a warm period. ► Total organosulfates correlate with total organic acids.

Keywords: Secondary organic aerosol; Monoterpenes; Isoprene; HPLC-MS

Biogenic emissions from Citrus species in California by Silvano Fares; Drew R. Gentner; Jeong-Hoo Park; Elena Ormeno; John Karlik; Allen H. Goldstein (pp. 4557-4568).

Biogenic Volatile Organic Compounds (BVOC) emitted from plants are the dominant source of reduced carbon chemicals to the atmosphere and are important precursors to the photochemical production of ozone and secondary organic aerosols. Considering the extensive land used for agriculture, cultivated Citrus plantations may play an important role in the chemistry of the atmosphere especially in regions such as the Central Valley of California. Moreover, the BVOC emissions from Citrus species have not been characterized in detail and more species-specific inputs for regional models of BVOC emissions are needed. In this study, we measured the physiological parameters and emissions of the most relevant BVOC (oxygenated compounds, monoterpenes, and sesquiterpenes) for four predominant Citrus species planted in California ( Citrus sinensis var. ‘Parent Navel’, Citrus limon var. ‘Meyer’, Citrus reticulata var. ‘W. Murcott’ and ‘Clementine’). We used two analytical techniques to measure a full range of BVOC emitted: Proton Transfer Reaction Mass Spectrometry (PTR-MS) and gas chromatography with mass spectrometry. Methanol, followed by acetone and acetaldehyde, were the dominant BVOC emitted from lemon and mandarin trees (basal emission rates up to 300 ng(C) g(DW)⁻¹ h⁻¹), while oxygenated monoterpenes, monoterpenes, and sesquiterpenes were the main BVOC emitted from orange trees (basal emission rates up to = 2500 ng(C) g(DW)⁻¹ h⁻¹). Light and temperature-dependent algorithms were better predictors of methanol, acetaldehyde, acetone, isoprene and monoterpenes for all the Citrus species. Whereas, temperature-dependent algorithms were better predictors of oxygenated monoterpenes, and sesquiterpenes. We observed that flowering increased emissions from orange trees by an order of magnitude with the bulk of BVOC emissions being comprised of monoterpenes, sesquiterpenes, and oxygenated monoterpenes. Chemical speciation of BVOC emissions show that the various classes of terpene emissions among all Citrus species are dominated by ocimenes, β-caryophyllene, and linalool, respectively. In addition to utilizing our reported emission factors in BVOC emission models, we recommend that future BVOC emission models consider additional emissions from flowering and harvest, which occur seasonally and may have a significant impact on regional atmospheric chemistry.► We measured physiological parameters and emissions of BVOC for four Citrus species. ► Methanol, acetone and acetaldehyde, were the dominant BVOC emitted from lemons and mandarins. ► Oranges emitted mainly oxygenated monoterpenes, monoterpenes, and sesquiterpenes. ► Flowering increased emissions from orange trees by an order of magnitude. ► Ocimenes, β-caryophyllene, and linalool were the main terpenes emitted from Citrus.

Keywords: BVOC emissions; OVOC; Terpene; Basal emission rate; Citrus

Sources of gaseous oxidized mercury and mercury dry deposition at two southeastern U.S. sites by Peter S. Weiss-Penzias; Mae S. Gustin; Seth N. Lyman (pp. 4569-4579).

Wet deposition measurements have shown that relative to other parts of the US, the southeastern region has the highest mercury (Hg) inputs. The source of this Hg has been investigated by multiple researchers and is suggested to be derived from local, regional and global sources. Here we focus on trying to understand potential sources of Hg to this area during periods dominated by dry gaseous oxidized mercury (GOM) deposition. Dry deposition of GOM to a surrogate surface was measured in conjunction with speciated atmospheric Hg, and ancillary parameters from September 2007 through September 2008 at two sites located within 25 km of coal-fired power plants (CFPPs) near Yorkville, GA, and Pensacola, FL. Mean weekly GOM dry deposition, daily GOM, and daily sulfur dioxide (SO₂) concentrations were significantly ( P < 0.01) higher at the Yorkville site by factors of 1.5, 2.0, and 1.8, respectively. At both sites, GOM and SO₂ concentrations were significantly correlated ( P < 0.05) in every season on hourly and daily time scales. Wind rose diagrams showed significantly enhanced GOM and SO₂ concentrations when air moved to the sites from the direction of the nearest CFPPs. Most periods of enhanced GOM concentrations ([GOM] > 98th percentile), were also associated with NO_y/SO₂ ratios that were within 25% of that reported for the local CFPPs ( N = 27 of 33 at Yorkville, N = 18 of 26 at Pensacola). During these events, termed Category 1, mean GOM/SO₂ enhancement ratios were 2.4 ± 0.1 and 2.3 ± 0.1 pg m⁻³ ppb⁻¹ for Yorkville and Pensacola, respectively (range = 0.5 to 5.5). The remaining events at both sites (termed Category 2) displayed significantly lower SO₂ concentrations, yet GOM concentrations were not significantly different compared to Category 1 events. The potential sources of GOM during the Category 2 events at OLF were investigated using gridded frequency distributions (GFD) of 72-h atmospheric back trajectories. During these periods there was a greater component of air mass transport from the free troposphere, and less precipitation along the trajectory paths compared to GFDs for Category 1 events. GFDs developed for the weeks when GOM dry deposition was in the upper quartile at both sites simultaneously revealed a similar pattern to the GFDs of Category 2 GOM concentration events, that is, greater free tropospheric transport and relatively little precipitation. Although dry deposition inputs are thought to represent <15% of total annual Hg deposition in this region, we suggest that a significant portion of this Hg could be derived from sources outside the local area.► Emissions from the nearest coal-fired utilities influenced gaseous oxidized Hg. ► SO₂/NO_x ratios were useful for identifying sources of gaseous oxidized Hg. ► Evidence for significant Hg dry deposition from regional or global sources.

Keywords: Gaseous oxidized mercury; Dry deposition; Coal-fired power plants; Emissions sources; Sulfur dioxide; Southeastern United States; Pollution transport

The weekly cycle of ambient concentrations and traffic emissions of coarse (PM₁₀–PM_2.5) atmospheric particles by I. Barmpadimos; M. Nufer; D.C. Oderbolz; J. Keller; S. Aksoyoglu; C. Hueglin; U. Baltensperger; A.S.H. Prévôt (pp. 4580-4590).

The aim of the study is to investigate the existence of a weekly cycle of coarse mode (PM₁₀–PM_2.5) atmospheric particles, to compare this weekly cycle to the weekly cycle of PM_2.5 and to compare the strength of the coarse mode weekly cycle in different seasons and different wind speed, wind direction and precipitation conditions. In addition, an estimate of the contribution of traffic to the total ambient coarse mode particulate matter in Zurich, Switzerland is provided by estimating the weekly cycle of coarse mode traffic emissions and by comparing it to the weekly cycle of ambient concentrations. The coarse mode data used in the study are the result of simultaneous daily measurements of PM₁₀ and PM_2.5 at seven sites located in Switzerland. The measurements cover a period of 7–12 years for six stations and 3 years for one station. It is found that a coarse mode weekly cycle is present in various types of urban and rural stations. Ambient concentrations on weekdays are higher than on Sundays by a factor of 1.53 on average over all urban and suburban sites and by a factor of 1.32 on average over all rural sites. Moreover, the relative increase of coarse mode ambient concentrations on weekdays compared to Sundays was larger than the relative increase of PM_2.5 concentrations by a factor of 2.7 on average over all urban and suburban sites, whereas no considerable difference was found at the rural sites. A calculation of coarse mode traffic emissions for an urban scenario was carried out using traffic-induced dust resuspension and brake wear emission factors for light and heavy duty vehicles and traffic counts from Zurich, Switzerland. It is shown that coarse mode emissions on weekdays are greater than on Sundays by a factor of 2.0. The contribution of traffic to coarse mode urban ambient concentrations was estimated to be 53% (34%–78%) on Sundays and 70% (57%–86%) on weekdays. It is deemed however that these numbers are somewhat overestimating the traffic contribution because of lack of information on construction activities around the considered sites.► A weekly cycle of PM_coarse (PM₁₀–PM_2.5) is found at urban, rural and rural elevated sites. ► The weekly cycle of PM_coarse is similar to the weekly cycle of PM_2.5. ► Weekday/weekend concentration ratios are higher for PM_coarse than for PM_2.5. ► Traffic emissions contribute an estimated 34%–78% to PM_coarse on Sundays and 57%–86% on weekdays.

Keywords: Coarse mode; Weekly cycle; Traffic emissions

Surface ozone in the White Mountains of California by Joel D. Burley; Andrzej Bytnerowicz (pp. 4591-4602).

Surface ozone concentrations are presented for four high-elevation sites along a north–south transect along the spine of the White Mountains and a fifth site located at lower elevation approximately 15km to the west on the floor of the Owens Valley. The ozone data, which were collected from mid-June through mid-October of 2009, include results from two sites, White Mountain Summit (4342m elevation) and Barcroft Station (3783m), that are believed to be higher in elevation than any previously investigated sampling locations in North America. Average daily ozone values from the five sampling sites display similar day-to-day and week-to-week temporal fluctuations, which suggest that the sites are experiencing the same regional-scale background patterns in air quality and meteorology. Ozone concentrations increase with increasing elevation, consistent with findings from prior studies in Europe and North America. A linear elevation gradient of +0.0042ppbm⁻¹ is obtained for July 15–August 15, but analogous gradients for August 15–September 15 and September 15–October 15 show reduced linearity and possibly the onset of a plateau in ozone concentrations for elevations above 2000m. Average diurnal cycle magnitudes decrease with increasing elevation, falling from ∼25–35ppb for the Owens Valley site to ∼3–7ppb at three of the four high-elevation sites. Diurnal cycle magnitudes decrease (or remain roughly constant) at the non-Summit sites during the progression from mid-July to mid-October, but the magnitude of the diurnal cycle at the Summit increases from ∼3ppb to ∼7ppb over this same time frame. This latter result is inconsistent with results from previous investigations at other alpine sites, and may indicate the presence of local, topography-influenced mixing dynamics that are unique to the White Mountains. High hourly ozone concentrations at White Mountain Summit are found to correlate with 72-hour HYSPLIT back-trajectories that reflect enhanced levels of ozone transport from polluted regions (such as the Central Valley of California) or meteorological conditions that are favorable for ozone production. Low ozone concentrations at the Summit are found to correlate with HYSPLIT back-trajectories that reflect reduced levels of ozone transport from polluted areas or meteorological conditions that are unfavorable for ozone production.► We present surface ozone data for high-elevation sites in the White Mountains. ► Our measurements are higher in elevation than any previous North American studies. ► Low O₃ at White Mountain Summit correlates with reduced transport of polluted air from inland areas. ► High O₃ at White Mountain Summit correlates with enhanced transport of polluted air from inland areas.

Keywords: Surface ozone; Portable ozone monitor; White Mountains; Elevation profile; HYSPLIT model

Spatial distribution of particle number concentration and its volume change in the planetary boundary layer over Tokyo and its suburban areas by Hiroaki Minoura; Nobuo Shimo (pp. 4603-4610).

The Japanese Ministry of the Environment established a new PM_2.5 air quality standard in September 2009, which has lead to increased public interest in air quality improvement. Concentrations of primary particles such as elemental carbon have decreased during the last decade, but secondary particles have become a major concern. When ground-based observations are carried out, the influence of neighboring emissions must be considered. At the same time, as much as possible, particles must be collected in places that will allow the average behavior of secondary particles to be understood. Making observations using aircraft in city skies is advantageous in that samples taken there are not influenced by local exhaust, such observations can cover a wide range at one time, and they are useful in understanding the spatial character of secondary particles.Using a helicopter for sampling, we measured the spatial variation of particle size distribution (0.5–10 μm) on the coastline and over inland areas in the South Kanto region, including the Tokyo metropolitan area, nine times in three days starting on July 30, 2008. The vertical profile of pollutant concentrations was approximately constant between the altitudes of 300 m and 600 m, but concentrations varied both temporally and spatially. Data gathered in level flights at an altitude of 600 m showed that the particle number concentration increased while the mean particle diameter decreased as we flew north (inland). The particle number concentration and its relationship with the mean particle diameter showed a well-organized distribution. A volumetric change of more than 3-fold was seen in the distribution of particles sampled in the inland area, whereas no clear volumetric change was seen in the distribution of particles sampled along the coast. The particle number concentration, mean particle diameter, and particle volume changed at rates of 0.74 cm⁻³ km⁻¹, −0.48 nm km⁻¹, and 0.057 μm³ cm⁻³ km⁻¹, respectively, with increasing distance from the coastline. The incremental volumetric growth rates were seen to relate positively in proportion to the NO₂ concentration in particular.► Using an aircraft, secondary PM behavior was measured in South Kanto, Japan. ► The PM number and mean diameter showed well-organized distribution. ► A volumetric change in PM was seen in the inland area, but not in the coast area. ► The increment tendency of the volume growth rates was seen with NO₂ concentration. ► The increase ratio of PM number and volume was obtained as a function of distance.

Keywords: Japan; Tokyo; Spatial distribution; Particulate matter; Particle number concentration; Secondary particle

Recent increasing trend in dust frequency over Mongolia and Inner Mongolia regions and its association with climate and surface condition change by Eun-Hee Lee; Byung-Ju Sohn (pp. 4611-4616).

Trends in dust events over China and Mongolia were examined using 34 years (1974–2007) of visibility data over dust source regions of China and 10 years (1998–2007) of dust reports from Synoptic Observations (SYNOP) stations over China and Mongolia. Dust occurrences in Mongolia and northern Inner Mongolia increased over the 1998–2007 SYNOP data period while most dust source regions of China experienced a continuous decrease over the 34-year data period. Increased dust occurrences in Mongolia as well as in Inner Mongolia appear to be caused by degraded surface vegetation and reduced soil moisture associated with intensified drought conditions after the mid-1990s. Results suggest that recent increases in dust events over Korea and Japan are linked to increased dust occurrences over Mongolia and Inner Mongolia.► SYNOP Dust occurrences in Mongolia increased over the 1998–2007. ► Increased dust in Mongolia is related with recently intensified dry conditions. ► Recent dust increase over Korea is linked to dust increase over Mongolia.

Keywords: Visibility; Dust; Drought; Vegetation; Mongolia

Use of AERMOD to determine a hydrogen sulfide emission factor for swine operations by inverse modeling by Patrick T. O’Shaughnessy; Ralph Altmaier (pp. 4617-4625).

This study was conducted to determine both optimal settings applied to the plume dispersion model, AERMOD, and a scalable emission factor for accurately determining the spatial distribution of hydrogen sulfide concentrations in the vicinity of swine concentrated animal feeding operations (CAFOs). These operations emit hydrogen sulfide from both housing structures and waste lagoons. With ambient measurements made at 4 stations within 1 km of large swine CAFOs in Iowa, an inverse-modeling approach applied to AERMOD was used to determine hydrogen sulfide emission rates. CAFO buildings were treated as volume sources whereas nearby lagoons were modeled as area sources. The robust highest concentration (RHC), calculated for both measured and modeled concentrations, was used as the metric for adjusting the emission rate until the ratio of the two RHC levels was unity. Utilizing this approach, an average emission flux rate of 0.57 μg m⁻² s⁻¹ was determined for swine CAFO lagoons. Using the average total animal weight (kg) of each CAFO, an average emission factor of 6.06 × 10⁻⁷ μg yr⁻¹ m⁻² kg⁻¹ was calculated. From studies that measured either building or lagoon emission flux rates, building fluxes, on a floor area basis, were considered equal to lagoon flux rates. The emission factor was applied to all CAFOs surrounding the original 4 sites and surrounding an additional 6 sites in Iowa, producing an average modeled-to-measured RHC ratio of 1.24. When the emission factor was applied to AERMOD to simulate the spatial distribution of hydrogen sulfide around a hypothetical large swine CAFO (1 M kg), concentrations within 0.5 km from the CAFO exceeded 25 ppb and dropped to 2 ppb within 6 km of the CAFO. These values compare to a level of 30 ppb that has been determined by the State of Iowa as a threshold level for ambient hydrogen sulfide levels.► We used inverse-modeling to determine a hydrogen sulfide emission factor for swine buildings. ► When applied to a test set of swine buildings, the emission factor accurately reproduced gas concentrations. ► Inverse modeling may result in more accurate emission rates than by direct measurement. ► Large swine operations can influence local hydrogen sulfide concentrations up to 6 km. ► For all but the largest swine operations, modeled levels were below a recommended level of 30 ppb.

Keywords: Inverse modeling; AERMOD; Confinement buildings; Hydrogen sulfide; Emission factor

Implications of changing PM10 Air Quality Standards on Pacific Northwest communities affected by windblown dust by B.S. Sharratt; R. Edgar (pp. 4626-4630).

The US Environmental Protection Agency (EPA) is currently reviewing the National Ambient Air Quality Standards (NAAQS) for Particulate Matter. EPA is considering the recommendation to change both the form and level of the PM10 (particulate matter ≤10 μm in diameter) Standard. The implication of the recommended NAAQS for PM10 on air quality is explored in this study. Daily observations of PM10 were made at Kennewick and Spokane, WA from 2000 through 2010. The number of violations of the PM10 Standard was determined for both the current (not to exceed 150 μg m⁻³ on more than one day per year) and recommended (not to exceed 65 or 85 μg m⁻³ based upon the 98th percentile) Standards. The current PM10 Standard has only been violated at Kennewick. Under the recommended PM10 Standards, Kennewick would have violated the Standard at both the 65 and 85 μg m⁻³ levels while Spokane would have violated the Standard at only the 65 μg m⁻³ level. The results of this study suggest that the recommended NAAQS for PM10 using a level of 85 μg m⁻³ will tend to result in fewer violations of the Standard and using a level of 65 μg m⁻³ will tend to result in more violations of the Standard in the Inland Pacific Northwest. The Exceptional Event Rule and research on management practices to control the emission of fugitive dust will continue to be important strategies for achieving compliance with PM10 Air Quality Standards in the Inland Pacific Northwest.► New Air Quality Standards for PM10 are being recommended for the US. ► Compliance with these Standards was examined using PM10 observations from the Pacific Northwest. ► The Inland Pacific Northwest will more often violate the new PM10 Standard at a level of 65 μg m⁻³. ► The Exceptional Events Rule will help communities comply with the recommended PM10 Standards.

Keywords: Air Quality Standards; Pacific Northwest; PM10; Air pollution; Particulate matter; DustAbbreviations; PM10; particulate matter ≤10 μm in diameter; EPA; US Environmental Protection Agency; NAAQS; National Ambient Air Quality Standards

Size-fractionated water-soluble ions, situ pH and water content in aerosol on hazy days and the influences on visibility impairment in Jinan, China by Shu-hui Cheng; Ling-xiao Yang; Xue-hua Zhou; Li-kun Xue; Xiao-mei Gao; Yang Zhou; Wen-xing Wang (pp. 4631-4640).

To study the size-fractionated characteristics of aerosol chemical compounds including major water-soluble inorganic and organic ions, situ pH and water content as well as their influences on visibility, field sample collections using a Micro Orifice Uniform Deposit Impactor (MOUDI) combined with simulations by Aerosol Inorganic Model (AIM) were conducted on hazy and clear days in Jinan, China from April, 2006 to January, 2007. Average concentrations of TSP, PM₁₀ and PM_1.8 on hazy days were found to be 1.49–5.13, 1.54–5.48 and 1.30–5.48 times those on clear days during the sampling periods, indicating that particulate pollution was very serious on hazy days. Size distributions of mass, SO₄²⁻, NO₃⁻, formate and acetate were all bimodal with fine mode predominant on hazy days, demonstrating that fine particles and secondary pollutants were more easily formed on hazy days. The average total aerosol concentration of H⁺ ([H⁺]_total), of which free H⁺ concentration ([H⁺]_ins) inside aerosol accounted for 30%, was 2.36–4.21 times that in other cities in China and US. The [H⁺]_ins on hazy days was 2.43–13.11 times that on clear days and the estimated situ pH was mainly influenced by RH and mole ratios of [NH₄⁺]/[SO₄²⁻]. Size distributions of situ pH were unimodal peaked at 0.56 μm on clear days in spring and autumn as well as on hazy days in autumn, while a trend of increasing was shown on hazy days in spring and summer. The normalized water content (NWC) was higher on hazy days in autumn and winter because of the easier uptake of water by aerosol. It was found that when NWC < 2, if [NH₄⁺]/[SO₄²⁻] < 2.0, aerosol chemical components were more sensitive to water content, while if [NH₄⁺]/[SO₄²⁻] > 2.0, relative humidity (RH) was more important. Size distributions of water content showed a trend of sharp increasing on hazy days in spring and autumn. Correlation and regression analysis indicated that visibility was significantly influenced by the concentrations of SO₄²⁻ and water content in the range of 1.0–1.8 μm.► Influences of size fractionated aerosol chemical compounds on visiblity were studied. ► Secondary ions and fine particles were more easily formed on hazy days. ► Aerosol acidity on hazy days was stronger than that on clear days. ► Aerosol absorbed water more easily on hazy days. ► SO₄²⁻ and water content in 1–1.8 μm were the major factors to reduce visibility.

Keywords: Size distribution; Water-soluble ions; AIM; Situ pH; Water content; Visibility impairment

Monitoring spatio-temporal aerosol patterns over Pakistan based on MODIS, TOMS and MISR satellite data and a HYSPLIT model by Khan Alam; Salman Qureshi; Thomas Blaschke (pp. 4641-4651).

Three different satellite-borne sensors, namely the Total Ozone Mapping Spectrometer (TOMS), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Multi-angle Imaging Spectroradiometer (MISR), were used to investigate the spatial and temporal variations of aerosols over several cities in Pakistan. A Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used for trajectory analysis in order to reconstruct the origins of air masses and understand the spatio-temporal variability of aerosol concentrations. Recent MODIS aerosol data (2002–2008) and earlier TOMS data (1979–2001) revealed increasing concentrations of aerosols over Pakistan and adjacent areas. Validation of MODIS and MISR derived aerosol optical depths (AODs) with Aerosol Robotic Network (AERONET) data for 2007 demonstrated that the MISR data was more accurate when close to the ocean, while the MODIS was more accurate over vegetated areas. The relationship between MODIS and MISR AOD data from 2002 to 2008 was analyzed, revealing a strong correlation between the two datasets. An assessment of seasonal variability in AOD for industrial, urban, semi-urban, rural, and semi-arid areas revealed maximum AOD values during the summer over all the areas investigated. Back trajectory analyses indicated that while winter air masses reaching Pakistan had travelled long distances, summer air masses had travelled only short distances. The higher aerosol concentrations during the summer are interpreted to be a result of the air masses spending more time over land during the summer than they do during the winter. While monsoonal rainfall tends to reduce aerosol concentrations by washing aerosols out of the atmosphere, this effect is mainly restricted to the eastern and south-eastern parts of Pakistan.► This study carried out the spatial and temporal variations of aerosols over several cities in Pakistan. ► TOMS (1979–2001) and MODIS aerosol data (2002–2008) revealed increasing concentrations of aerosols over Pakistan. ► The spatial correlation between MODIS and MISR AOD found to be higher in winter than in summer. ► The MISR provides better AOD estimates close to the ocean while MODIS provides better over terrestrial regions. ► The highest mean AODs were recorded during the summer and the lowest AODs in the winter.

Keywords: Aerosols; TOMS; MODIS; MISR; AERONET; HYSPLIT; AOD; Remote sensing; Pakistan

Numerical research of extreme wind-induced dust transport in a semi-arid human-impacted region of Mexico by Luis F. Pineda-Martinez; Noel Carbajal; Arturo A. Campos-Ramos; Cristina Noyola-Medrano; Antonio Aragón-Piña (pp. 4652-4660).

A numerical research is carried out to investigate the regional impact of extreme wind-induced dust transport in the central-northern part of Mexico. In boreal winter, strong wind soil erosion processes occur in the arid zones of the Mexican highlands, as a consequence of land use change and land cover change. The effect of land use change and land cover change has consequences in the atmospheric circulation, by altering the balance in solar radiation, albedo, soil moisture and texture, aerodynamic roughness and other surface properties. The anthropogenic impact of land use change from natural vegetation to agriculture lands developed into land degradation and air quality loss by large dust fluxes into the atmosphere. To investigate the magnitude of these erosion processes, we carried out the numerical modeling of a strong dust storm during the passage of a cold front on 18 March 2008. An algorithm based on the friction velocity was implemented for estimating the flux of particulate matter less than 10μm (PM₁₀) into the atmosphere. It was possible to show how the plume of high concentration of particulate matter propagates through the complex topography of the highland and of the Eastern Sierra Madre to reach large urban zones in the northern part of Mexico and Southern of Texas. In our numerical simulation we estimated roughly that the fraction of PM₁₀ emitted during this event was of the order of 9162.72ton. A direct effect of dust particles is appreciated in the reduction of measured solar radiation.► In this study we investigate the strong impact of human activities impact on semi-arid environments. ► We point out the importance of researching wind erosion and its relationship with soil loss and desertification process. ► To document dust storms in the Mexican highlands of northern Mexico, we selected a case study in the State of Zacatecas. ► We find that land cover change generates a local environmental impact affecting the weather in the Mexican highlands. ► We conclude that a relatively small human-impacted area is converted in a source of dust affecting large regions.

Keywords: Dust storm; PM; ₁₀; Numerical modeling; Dust characterization

Measurements of hygroscopicity and volatility of atmospheric ultrafine particles in the rural Pearl River Delta area of China by Jae-Seok Kim; Young J. Kim; Kihong Park (pp. 4661-4670).

A hygroscopicity and volatility tandem differential mobility analyzer (HVTDMA) technique was used to determine the time- and size-resolved properties of ultrafine particles and to infer relative volume fractions of non-volatile and non-hygroscopic (NV_NH), volatile and non-hygroscopic (V_NH), volatile and hygroscopic (V_H), and non-volatile and hygroscopic (NV_H) groups. Cluster analysis of wind direction and air mass backward trajectory have revealed that enhanced ultrafine particle concentrations were often observed when air mass was transported with high wind speed (>3 m s⁻¹) from the polluted northeast region containing a significant amount of SO₂ and experienced a strong photochemical activity. We found the photochemically-produced ultrafine particles to consist primarily of NV_H with a little V_NH and V_H. In morning traffic events, we estimated ultrafine particles to consist of 61% NV_NH, 36% V (volatile group = the sum of V_NH and V_H), and 2% NV_H, while during biomass burning events, ultrafine particles consisted of 69% NV_NH, 21% V and 10% NV_H. Further, as determined by TEM/EDS analysis, the increase in NV_H during the biomass burning event was consistent with the frequent existence of K element in ultrafine particles. Comparison of data among different geometric locations in China and Korea revealed ultrafine particle hygroscopicity and volatility during the photochemical event as being highly variable in locations affected by diverse sources and variable precursor gases (e.g., SO₂ and VOC), while during the combustion events, less hygroscopicity variation across different locations was observed.► Time-resolved measurements of hygroscopicity and volatility of ultrafine particles were done. ► Volume fractions of similarly-propertied groups were determined. ► Photochemically-produced ultrafine particles were affected by diverse sources. ► Less hygroscopicity variation was found for combustion-related ultrafine particles.

Keywords: Pearl River Delta (PRD); Hygroscopicity; Volatility; Photochemical event; Combustion event

Ozone in the marine boundary layer of Bay of Bengal during post-winter period: Spatial pattern and role of meteorology by Prabha R. Nair; Liji Mary David; I.A. Girach; K. Susan George (pp. 4671-4681).

Ozone measurements were carried out in the marine environment of the Bay of Bengal during the post-winter months of March–April 2006, as part of the Integrated Campaign for Aerosols, gases and Radiation Budget. The ozone mixing ratio was found to be high over the northern/head BoB with a mean value of 27 ± 3 ppb and minimum over the mid-BoB with 12 ± 3 ppb. The spatial distribution was closely associated with the airflow pattern, airmass back trajectories and the boundary layer height. In this marine environment, meteorology, in particular, the water vapour content also played a significant role in governing the diurnal pattern of ozone in addition to photochemistry. Vertical transport is also partially responsible for the high ozone over the head BoB. The diurnal patterns were simulated using the chemical box model. The spatial map of marine boundary layer ozone was compared with that of tropospheric column ozone, NO₂ and CO.► Ozone in the marine boundary layer of Bay of Bengal. ► Long range transport of ozone and precursors. ► Role of water vapour content in the evolution of diurnal variation of ozone.

Keywords: Surface ozone; Marine boundary layer; Long range transport; Bay of Bengal; ICARB

A study of regional-scale variability of in situ and model-generated tropospheric trace gases: Insights into observational requirements for a satellite in geostationary orbit by Jack Fishman; Morgan L. Silverman; James H. Crawford; John K. Creilson (pp. 4682-4694).

We examine the results from a regional-scale chemical-transport model with 4-km resolution to determine the spatial variability of trace gases on this scale. Model-derived variability statistics are generated using 1st-order structure functions and then compared with in situ trace gas measurements from a series of aircraft campaigns. The variability of the observations and the model-derived concentrations are found to be in reasonable agreement for O₃ and CO, but the model underestimates the observed variability of NO₂. Variability statistics are then calculated for model-derived tropospheric column integrals. These integrals are calculated for 0–10 km (representative of the entire tropospheric column), 0–2 km (representative of the planetary boundary layer, PBL) and 2–10 km (representative of the free troposphere, FT). For each of the species examined, the variability of the tropospheric column is generally controlled by the variability in the lowest 2 km. The degree of control for each trace gas, however, is different. Whereas NO₂ is completely dominated by PBL processes, CO variability in the FT contributes appreciably to the variability of the entire tropospheric column, suggesting that two independent pieces of information for CO would be most helpful for describing the variability of the entire tropospheric column. Likewise, the variability of an independent free tropospheric measurement of O₃ would provide additional insight into the observed variability of the entire column, but the amount of additional information provided by a separate FT measurement is not as beneficial to what was found for CO. We provide additional analyses to quantify relationships that can be used to better understand the model-derived structure functions and their dependence on grid size and time of day. Lastly we present a practical example of how this information may be used for guidance in the development of science requirements for future satellite instruments since measurements from these instruments must be able to resolve smaller scale gradients to be used successfully for air quality applications.► Characterize regional-scale variability of trace gases using a regional-scale model. ► Favorable statistical comparison between model and observations for CO and O₃. ► Surface concentrations mirrored by integrated column amounts. ► Variability in lowest two km controls tropospheric column variability.

Keywords: Tropospheric chemistry; Regional modeling; Trace gas variability; Tropospheric trace gas satellite measurements

Spatial heterogeneities in aerosol size distribution over Bay of Bengal during Winter-ICARB Experiment by P.R. Sinha; R.K. Manchanda; D.G. Kaskaoutis; S. Sreenivasan; K. Krishna Moorthy; S. Suresh Babu (pp. 4695-4706).

This work examines the aerosol physical properties and size distribution measured in the Marine Atmospheric Boundary Layer (MABL) over entire Bay of Bengal (BoB) and Northern Indian Ocean (NIO) during the Winter Integrated Campaign on Aerosols, Gases and Radiation Budget (W-ICARB). The measurements were taken using the GRIMM optical particle counter from 27th December 2008 to 30th January 2009. The results show large spatial heterogeneities regarding both the total aerosol number concentrations (N _T) and the size distributions over BoB, which in turn indicates the variations in the source strength or advection from different regions. The aerosol number size distribution seems to be bi-modal in the 72% of the cases and can also be parameterized by uni-modal or by a combination of power-law and uni-modal distributions for the rest of the cases. The mode radius for accumulation and coarse-mode particles ranges from ∼0.1–0.2 μm and ∼0.6–0.8 μm, respectively. In the northern BoB and along the Indian coast, the aerosols are mainly of sub-micron size with effective radius ( R_eff) ranging between 0.25 and 0.3 μm highlighting the strong anthropogenic influence, while in the open oceanic areas they are much higher (0.4–0.6 μm). It was also found that the sea-surface wind plays a considerable role in the super-micron number concentration, R_eff and mode radius for coarse-mode aerosols. Using the relation betweenN _T and columnar AOD from Terra and Aqua-MODIS we found that the majority of the aerosols are within the lower MABL, while in some areas vertical heterogeneities also exist.► Data represent in-situ measurements of aerosol size distribution over Bay of Bengal. ► Aerosol of anthropogenic origin are prevalent in Northern BoB and on Indian coast. ► Open oceanic aerosols originate mainly from sea-surface and wind interaction. ► During winter period the aerosol are mainly confined within the MABL region.

Keywords: Aerosol concentration; Size distribution; MABL; Bay of Bengal; W-ICARB

Ozone pollution regimes modeled for a summer season in California’s San Joaquin Valley: A cluster analysis by Ling Jin; Robert A. Harley; Nancy J. Brown (pp. 4707-4718).

This study demonstrates an application of cluster analysis to model simulation data for California’s San Joaquin Valley (SJV) for the purpose of identifying meteorologically representative pollution regimes. Principal component analysis is employed to facilitate exploring and visualizing temporal variations in highly resolved gridded model data. Six regimes are clustered according to the spatial distribution of SJV 8 h ozone maxima. Meteorological effects (temperature and winds) are shown to explain the observed ozone spatial distributions in the SJV, and their relationship to those in upwind San Francisco Bay Area air basin (SFB) under certain prevailing wind flow patterns. In general, average ozone levels in the SJV increase with temperature, while their spatial distributions depend on flow regimes, especially the strength of sea breezes and upslope flows. More ventilated flow regimes, associated with stronger sea breeze and upslope flows, cause eastward transport of pollutants, increasing ozone in the southeastern SJV and decreasing it in the northwest SJV. The opposite occurs during the most stagnant conditions associated with the weakest sea breeze and upslope flows. The two most prominent relationships between the SFB and SJV were found to be associated with the most ventilated and the most stagnant conditions, respectively, indicating a strong inter-basin transport (or the lack thereof) event. Spatial representativeness of existing measurement sites and the confounding influences of emission changes on clustering results are also investigated. Existing measurement sites are able to capture ozone spatial patterns in the SFB and Sacramento Valley (SV), whereas those along the western side of the SJV are under-represented. Differences in day-of-week emissions produce minor effects on spatial ozone distributions and the clusters are largely stable under these changes.► Cluster analysis and principal component analysis applied to modeled results. ► Pollution regimes characterized by spatial ozone and underlying meteorology. ► Visualized variability in air quality model results with high-dimensionality. ► Determined spatial representativeness of measurement locations. ► Determined influences of emission changes on clustering stability.

Keywords: Seasonal ozone; Central California; Cluster analysis; Photochemical modeling

Prediction of traffic-related nitrogen oxides concentrations using Structural Time-Series models by Anneka Ruth Lawson; Bidisha Ghosh; Brian Broderick (pp. 4719-4727).

Ambient air quality monitoring, modeling and compliance to the standards set by European Union (EU) directives and World Health Organization (WHO) guidelines are required to ensure the protection of human and environmental health. Congested urban areas are most susceptible to traffic-related air pollution which is the most problematic source of air pollution in Ireland. Long-term continuous real-time monitoring of ambient air quality at such urban centers is essential but often not realistic due to financial and operational constraints. Hence, the development of a resource-conservative ambient air quality monitoring technique is essential to ensure compliance with the threshold values set by the standards. As an intelligent and advanced statistical methodology, a Structural Time Series (STS) based approach has been introduced in this paper to develop a parsimonious and computationally simple air quality model. In STS methodology, the different components of a time-series dataset such as the trend, seasonal, cyclical and calendar variations can be modeled separately. To test the effectiveness of the proposed modeling strategy, average hourly concentrations of nitrogen dioxide and nitrogen oxides from a congested urban arterial in Dublin city center were modeled using STS methodology. The prediction error estimates from the developed air quality model indicate that the STS model can be a useful tool in predicting nitrogen dioxide and nitrogen oxides concentrations in urban areas and will be particularly useful in situations where the information on external variables such as meteorology or traffic volume is not available.► First successful application of Structural Time Series predicting ambient NO_x and NO₂ levels. ► Road-side measurement of pollutants from the city center of Dublin used for validation. ► One-step and multi-step ahead forecasts with high accuracy achieved. ► Individual tracking of temporal evolution of pollutant concentrations achieved. ► Particularly useful when meteorological or emission data is unavailable.

Keywords: Air quality forecast; Structural Time Series; Nitrogen oxides; Vehicular emission; Dublin city

Can mortality displacement mask thresholds in the concentration-response relation between particulate matter air pollution and mortality? by Steven Roberts (pp. 4728-4734).

Two critical issues for public health assessment and regulation of exposure to particulate matter air pollution (PM) are whether there is a threshold in the concentration-response relation between PM and mortality and whether there is mortality displacement. In this vein, a number of time-series studies have concluded that a linear relation without a threshold is appropriate for describing the concentration-response relation, and a number of other studies have concluded that the mortality effects of PM exposure cannot be attributed to mortality displacement alone. Using three-state (healthy, frail, dead) population models that incorporate actual time series data from Cook County, Illinois for the period 1987–2000, the author investigates the shape of the concentration-response relation between PM and mortality, as observable from time-series data, in the presence of mortality displacement. It is found that thresholds in the concentration-response relation can be masked, or hidden, by linear concentration-response relations if some of the effect of PM exposure is attributable to mortality displacement. This is an important finding that has implications for studies that have found a linear concentration-response relation appropriate, particularly given the markedly different implications for public health assessment and regulation of a linear versus threshold concentration-response relation.► We use a three-state (healthy, frail, dead) population model to simulate mortality that exhibit mortality displacement. ► We are interested in the relation between particulate matter (PM) air pollution and entry into the frail population. ► We find that mortality displacement can mask the relation between PM and entry into the frail population.

Keywords: Air pollution; Concentration-response; Harvesting; Mortality; Mortality displacement; Particulate matter; Time-series

Impacts of HONO sources on the air quality in Beijing, Tianjin and Hebei Province of China by Ying Li; Junling An; Min Min; Wei Zhang; Feng Wang; Pinhua Xie (pp. 4735-4744).

Incorporated into the WRF–Chem model are the three HONO sources, i.e., the reaction of photo-excited NO₂ with water (NO₂^* chemistry), heterogeneous reactions on aerosol surfaces, and HONO emissions. Four case simulations were performed in Beijing, Tianjin, and Hebei Province (BTH region) in August of 2007. Results indicate that the NO₂^* chemistry yields 30–50 ppb enhancements in daily maximum 1-h surface O₃ concentrations in major cities and 3–10 ppb increases in monthly mean daily maximum 8-h surface O₃ concentrations in most areas of the BTH region. Heterogeneous reactions on aerosol surfaces lead to 5–20 ppb decreases in monthly mean NO_y concentrations in major cities over the BTH region. Heterogeneous reactions and HONO emissions are the largest, and the second largest contributor, accounting for ∼59% and 26% of simulated HONO concentrations, respectively. The three HONO sources produce enhancements in monthly mean daytime (10:00–17:00) concentrations of NO_y (total reactive N-containing compounds), NO₂, HNO₃, HONO, and HO_x being −1 to −5 ppb, −1 to −3 ppb, −0.3 to 0.5 ppb, 20–50 ppt, and 1–3 ppt, respectively. Comparison with observations shows that inclusion of the three HONO sources into the WRF–Chem model can considerably improve HONO and O₃ simulations in the BTH region. For HONO the mean, the normal mean bias, the root mean square error, and the correlation coefficient is greatly improved from 0.04 to 0.56 ppb, from −97% to −39%, from 1.10 ppb to 0.51 ppb, and from −0.56 to 0.86, respectively, by comparison with the gas chemical mechanism only considered in the WRF–Chem model. This suggests that the three important HONO sources be included in air quality models, particularly in the areas with high emissions of NO_x and particulate matter.► Three HONO sources are incorporated into the WRF–Chem model. ► Heterogeneous reactions are the most important source of HONO. ► The reaction of NO₂^* with water contributes much to hourly maximum O₃. ► The three key sources of HONO should be considered in modeling studies.

Keywords: Nitrous acid; Heterogeneous reaction; WRF–Chem; DOAS

Modeling of in-vehicle human exposure to ambient fine particulate matter by Xiaozhen Liu; H. Christopher Frey (pp. 4745-4752).

A method for estimating in-vehicle PM_2.5 exposure as part of a scenario-based population simulation model is developed and assessed. In existing models, such as the Stochastic Exposure and Dose Simulation model for Particulate Matter (SHEDS-PM), in-vehicle exposure is estimated using linear regression based on area-wide ambient PM_2.5 concentration. An alternative modeling approach is explored based on estimation of near-road PM_2.5 concentration and an in-vehicle mass balance model. Near-road PM_2.5 concentration is estimated using a dispersion model and fixed site monitor (FSMs) data. In-vehicle concentration is estimated based on air exchange rate and filter efficiency. In-vehicle concentration varies with road type, traffic flow, windspeed, stability class, and ventilation. Average in-vehicle exposure is estimated to contribute 10–20 percent of average daily exposure. The contribution of in-vehicle exposure to total daily exposure can be higher for some individuals. Recommendations are made for updating exposure models and implementation of the alternative approach.► Fine particulate matter (PM_2.5) infiltrates into vehicle cabins. ► The magnitude of human exposures to fine particles inside vehicles are estimated. ► The in-vehicle microenvironment is a significant contributor to daily average total exposure. ► High end in-vehicle exposures can be a factor of three greater than mean exposures.

Keywords: Particulate matter; Vehicles; Exposure; Variability; Sensitivity; Simulation

Impact of European emission control strategies on urban and local air quality by Myrsini Giannouli; Evangelia-Anna Kalognomou; Giorgos Mellios; Nicolas Moussiopoulos; Zissis Samaras; Jaroslav Fiala (pp. 4753-4762).

The aim of this study is to investigate the effects of specific emission control measures on the air quality of urban centres and local area hotspots. In order to achieve this, a sequence of regional, urban and local scale models was applied to assess the impact of European emission control strategies on urban and local scale air quality. First, vehicle fleet and activity data were estimated using appropriate models. The results of these models were used as input to the COPERT model in order to estimate vehicle emissions (NO₂ and PM₁₀) at country level up to the year 2030. Vehicle emissions were calculated according to two different scenarios: A baseline scenario, CLE (CurrentLEgislation) and an optimistic alternative, MFR (MaximumFeasibleReductions). Urban background and traffic hotspot concentrations of air pollutants were then calculated using the OFIS (OzoneFIneStructure) and OSPM (OperationalStreetPollutionModel) models respectively for 20 cities in Europe and for particular types of streets. Air quality was found to improve in 2030 compared to the reference year 2000, in line with the stricter NO_x and PM vehicle emission limits imposed. The NO₂ street increments for narrow canyons estimated for the reference year were found to be in the range of 16–53 μg m⁻³, depending on the city considered. These were reduced to 14–36 μg m⁻³ in the CLE scenario and 7–24 μg m⁻³ in the MFR scenario. The corresponding range for PM₁₀ was estimated to be 5–15 μg m⁻³ for the reference year and was reduced to 2–8 μg m⁻³ and 0.2–2.4 μg m⁻³ for the CLE and the MFR scenarios respectively.

Keywords: Air quality modelling; Street canyons; Vehicle emissions; Traffic

On the mechanism of air pollutant re-entrainment in two-dimensional idealized street canyons by Chun-Ho Liu; W.C. Cheng; Thomas C.Y. Leung; Dennis Y.C. Leung (pp. 4763-4769).

The two-dimensional (2D) idealized street canyon, which is the generic unit of a city, is the platform for our fundamental understanding of ventilation and pollutant removal at the neighborhood scale. The building-height-to-street-width aspect ratio h/ b is the key geometric parameters affecting the flow structures in a street canyon. In this study, a series of computational fluid dynamics (CFD) sensitivity tests were performed to examine how the air pollutant concentration in a street canyon is related to the aspect ratio. The Reynolds-averaged Navier–Stokes (RANS) equations and the Renormalization Group (RNG)k−ε turbulence model were used in the mathematical model. The spatial behaviors of air pollutant transport from the facades, streets, and roofs to the shear layer were depicted by the local pollutant exchange rate ω. Besides, the bulk quantities, air exchange rate ACH, pollutant exchange rate PCH, and volume average pollutant concentration Θ, were used to elucidate the ventilation and pollutant removal mechanisms of the street canyon. The aspect ratios tested were in the range 0.067 ≤ h/ b ≤ 2.5 that covered the isolated roughness, wake interference and skimming flow regimes in 2D street canyons. A local maximum Θ was determined in 0.2 ≤ h/ b ≤ 0.5 that is different from the monotonic ACH or PCH. The CFD results showed that the mildly elevated air pollutant concentration is not caused by poor pollutant removal but the pollutant re-entrainment from the shear layer aloft back into the street canyon. It is thus suggested that ACH, PCH, and Θ should be used as complementary indicators.

Keywords: Air exchange rate ACH; Volume average pollutant concentration Θ; Pollutant exchange rate PCH; Local pollutant exchange rate; ω; Street canyons; Urban canopy layer

Study on wet scavenging of atmospheric pollutants in south Brazil by Flavio Wiegand; Felipe Norte Pereira; Elba Calesso Teixeira (pp. 4770-4776).

The present paper presents the study of in-cloud and below-cloud SO₂ and SO₄²-scavenging processes by applying numerical models in the Candiota region, located in the state of Rio Grande do Sul, South Brazil. The BRAMS (Brazilian Regional Atmospheric Modeling System) model was applied to simulate the vertical structure of the clouds, and the B.V.2 (Below-Cloud Beheng Version 2) scavenging model was applied to simulate in-cloud and below-cloud scavenging processes of the pollutants SO₂ and SO₄²⁻. Five events in 2004 were selected for this study and were sampled at the Candiota Airport station. The concentrations of SO₂ and SO₄²⁻ sampled in the air and the simulated meteorological parameters of rainfall episodes were used as input data in the B.V.2, which simulates raindrop interactions associated with the scavenging process. Results for the Candiota region showed that in-cloud scavenging processes were more significant than below-cloud scavenging processes for two of the five events studied, with a contribution of approximately 90–100% of SO₂ and SO₄²⁻ concentrations in rainwater. A few adjustments to the original version of B.V.2 were made to allow simulation of scavenging processes in several types of clouds, not only cumulus humilis and cumulus congestus.

Keywords: Numerical models; Wet precipitation

Analysis of the relationship between MODIS aerosol optical depth and particulate matter from 2006 to 2008 by Tzu-Chin Tsai; Yung-Jyh Jeng; D. Allen Chu; Jen-Ping Chen; Shuenn-Chin Chang (pp. 4777-4788).

This study used sunphotometer, lidar, and surface particulate matter measurements to assess MODIS AOD products for air quality monitoring in Taiwan. The MODIS AOD retrievals revealed a satisfactory validation against AERONET measurements with correlation coefficient ∼0.91 during Terra and ∼0.83 during Aqua overpasses in the period of 2006–2008. The correlations in cold season (September–February) ∼0.85–0.96 appear to be slightly higher than those in warm season (March–August) ∼0.78–0.87. The relationships derived between PM_2.5 and AOD from both MODIS and AEROENT show a strong seasonality as a result of aerosol vertical distribution. The high correlations (∼0.88–0.93) obtained in autumn between PM_2.5 and AOD normalized by boundary layer height (or equivalent haze layer height) are attributed to stable and well-mixed boundary layers as opposed to the summer lows (∼0.12–0.67) resulted from strong convection associated with unstable weather systems. With the long-range transport of Asian dust and pollution in winter and spring under prevalent northeasterly and biomass burning from Southeast Asia in spring under prevalent southwesterly flows, better correlation is derived from the normalization by haze layer height than boundary layer height owing to abundance of aerosols aloft above boundary layer. The former is shown with correlation coefficients in the range of ∼0.76–0.87 and ∼0.77–0.80 and the latter ∼0.56–0.79 and ∼0.39–0.54 for winter and spring, respectively. The results of MODIS that uphold the relationships derived from AERONET in autumn, winter, and spring suggest MODIS AOD products have the level of quality as sunphotometer measurements for monitoring local PM_2.5 in Taiwan.

Keywords: MODIS; Aerosol optical depth; Boundary layer height; Haze layer height; Particulate matter; Taiwan

Urban morphology and air quality in dense residential environments in Hong Kong. Part I: District-level analysis by P. Edussuriya; A. Chan; A. Ye (pp. 4789-4803).

The link between urban morphology and air quality in various dense residential environments of Hong Kong are investigated through field measurements and statistical analysis. With regard to the myriads of parameters available in the literature, this study aims to identify the most important urban morphological parameters in the street-level and district levels that affect air quality dispersion in Hong Kong. The study postulates that there is an association between urban morphology and urban air quality and a statistical model is developed to explain the relationship amongst urban air quality, micro-meteorology and urban morphological dimension. The study considered 20 different urban residential areas in five major districts of Hong Kong and real-time street-level air pollutant and microclimatic data are collected. 21 morphological variables are identified and calculated based on the geometry of the urban fabric.This study is the first part of a series which focuses to identify whether district demarcation is correlated with air quality. According to the findings of statistical analysis, only 6 out of 21 morphological variables (complete aspect ratio, occlusivity, roughness height, zero-plane displacement height, total building volume/number of buildings, and standard deviation of height) report distinguishable variability at the district level. Their R² (correlation coefficient) values vary from 0.2937 (plan area index) to 0.5457 (occlusivity) which is considered moderate to high. All these parameters describe the overall feature of the site rather than the specific geometric features of the street. It translates to the fact that in spite of the large in-district variations in morphological variables amongst the five districts, air pollution variations are insignificant for most of the variables and it means that air quality in Hong Kong are distinguishable not by district-level variable but rather in-site fabrics.

Keywords: Urban morphology; Street district analysis; Statistical analyses; Microclimate; Real-time measurements; Street-level air quality

First detailed observations of long-range transported dust over the northern South China Sea by Sheng-Hsiang Wang; Si-Chee Tsay; Neng-Huei Lin; N. Christina Hsu; Shaun W. Bell; Can Li; Qiang Ji; Myeong-Jae Jeong; Richard A. Hansell; Ellsworth J. Welton; Brent N. Holben; Guey-Rong Sheu; Yu-Chi Chu; Shuenn-Chin Chang; Jyh-Jian Liu; Wei-Li Chiang (pp. 4804-4808).

Trans-Pacific Asian dust transport has been well documented, but little is known about dust invasion to the South China Sea (SCS). This study presents the first detailed characterization of dust aerosols transported to the northern SCS. On 21 March 2010, a strong Asian dust storm affected large areas from the Gobi deserts to the West Pacific, including Taiwan and Hong Kong, and was also observed by a comprehensive set of instruments at Dongsha Island, a small island (about 2 km², 20°42′52" N, 116°43′51" E) in the northern SCS. Aerosol measurements including particle mass concentrations, size distribution, optical properties, hygroscopicity, and vertical profiles help depict the evolution of this dust event. Our results indicate that the dust particles were mixed with anthropogenic and marine aerosols, and transported within 250 m above ground level. The long-range transport of Asian dust to the northern SCS could significantly impact the ecosystems in the region.► First detailed in-situ measurements of Asian dust transported to northern South China Sea. ► A super Asian dust storm (21 March 2010) was observed at Donsgha Island. ► Detailed aerosol properties are characterized by in-situ and lidar measurements. ► Dust particles were mixed with anthropogenic and marine aerosols. ► Dust-laden air mass transported near the surface.

Keywords: Asian dust; South China Sea; Aerosol optical thickness (AOT); 7SEAS; Lidar; Dongsha

Determination of²¹⁰Po and²¹⁰Pb concentrations in atmospheric deposition in İzmir (Aegean sea-Turkey) by Aysun Uğur; Banu Özden; Işık Filizok (pp. 4809-4813).

The natural radionuclide²¹⁰Pb was analysed in rainwater samples in İzmir by radiometric methods. The samples were collected continuously from January 2000 through December 2003 depending on the frequency of rain. The levels of²¹⁰Pb in the samples were found to vary between 9 ± 1 and 198 ± 6 mBq l⁻¹ with an average value of 51 ± 0.5 mBq l⁻¹.²¹⁰Po activity concentration in total (wet and dry) deposition has also been investigated in the study from November 2001 to April 2003 and the results were found to vary between 2 ± 0.4 and 35 ± 3 mBq l⁻¹. The average value of²¹⁰Po activity concentration is found as 8 ± 0.5 mBq l⁻¹.²¹⁰Po/²¹⁰Pb activity ratios were derived as between 0.03 and 1.09. The annual²¹⁰Po and²¹⁰Pb fluxes were 12 and 48 Bq m⁻² y⁻¹, respectively.

Keywords: Rainwater; Precipitation; ²¹⁰; Pb; ²¹⁰; Po; Aegean region

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Atmospheric Environment (v.45, #27)