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Atmospheric Environment (v.39, #29)

Editorial board (pp. i).

List for forthcoming papers (pp. i-ii).

Atmospheric ethene concentrations in Mexico City: Indications of strong diurnal and seasonal dependences by V. Altuzar; Tomas S.A. Tom�s; O. Zelaya-Angel; Sanchez-Sinencio F. S�nchez-Sinencio; J.L. Arriaga (pp. 5219-5225).

Monitoring of atmospheric ethene was carried out in Mexico City with a¹²C¹⁶O₂-laser-based photoacoustic spectrometer. We assessed the variation of the ethene content in atmospheric samples simultaneously collected in stainless-steel containers at three stations of the local government's air quality monitoring network in November 1999 and March 2000. The ethene levels in November were higher than those detected in March, reaching up to 36.9ppbV. In addition, continuous measurements of ethene were carried out in February 2001 for 1 week, with a time resolution of 1min. It allowed recording of real-time ethene levels of up to 68ppbV. A comparison between ethene profiles recorded on weekends and working days clearly shows a pronounced difference in concentration, which is almost three-fold higher on working days.

Keywords: Ethene; CO; ₂; -laser; Photoacoustic spectroscopy; Vehicle exhaust-emission

Impacts of different emission sources on air quality during March 2001 in the Pearl River Delta (PRD) region by Xuemei Wang; Gregory Carmichael; Deliang Chen; Youhua Tang; Tijian Wang (pp. 5227-5241).

This study focuses on the role played by emissions from transportation, industry and power generation on the concentrations of O₃, CO, NO, NO _y and SO₂ in Guangdong province of China. Observational data of the pollutants and numerical modeling of atmospheric chemistry, transport and removal processes with STEM-2K1 and MM5 are used for March 2001. The objective is to identify the relative importance of the three emission sources on the concentrations of the pollutants. In addition, the relative importance of NO _x and VOC emissions from the transportation sector for O₃ production is examined. The observations at a rural and an urban site in the region show distinctive characteristics, indicating the importance of local emissions. A comparison of the observed ratios of CO/NO _x and SO₂/NO _x at the two sites with those derived from the emission inventories show the usefulness of the emission inventories. A control simulation with all emissions included shows a fairly satisfactory performance of the model in terms of the comparison between the observed and modeled concentrations of CO, SO₂, NO _y and O₃ at the rural site, although the observed extremes are less well simulated. Three simulations with different emission scenarios suggest that the transportation source was the main contributor to NO _x, CO and O₃ concentrations accounting for 34.2%, 33.1% and 17.8% of their total concentrations, respectively. For SO₂ concentration, the main contributor is the power plant sources with a contribution rate of 32.9%. For SO₂, the doubling of emissions from the industry has a much larger effect on the ambient levels than the doubling from the transportation, whereas the increase in the transportation sector sources has approximately twice the effect of the increased industrial emission on O₃. The doubling of emissions from the transportation and industry sources would increase the O₃ concentration by about 26.7% and 8.5% for the whole region. Finally, the O₃/NO _z ratio of 25 was found to be a reasonable threshold to separate NO _x-limited and VOC-limited regimes concerning O₃ chemistry. It is found that the urban area is VOC-limited and the non-urban area is NO _x-limited. These results have implications for O₃ control strategy for the region.

Keywords: Air quality; Photochemical smog; Emission sources; Pearl River Delta; China

Analysis of real-time variables affecting children's exposure to diesel-related pollutants during school bus commutes in Los Angeles by Lisa D. Sabin; Kathleen Kozawa; Eduardo Behrentz; Arthur M. Winer; Dennis R. Fitz; David V. Pankratz; Steven D. Colome; Scott A. Fruin (pp. 5243-5254).

Variables affecting children's exposure during school bus commutes were investigated using real-time measurements of black carbon (BC), particle-bound polycyclic aromatic hydrocarbons (PB-PAH) and nitrogen dioxide (NO₂) inside 3 conventional diesel school buses, a particle trap-outfitted (TO) diesel school bus and a compressed natural gas (CNG) school bus, while traveling along an urban Los Angeles Unified School District bus route. A video camera was mounted at the front of each bus to record roadway conditions ahead of the bus during each commute. The videotapes from 12 commutes, in conjunction with pollutant concentration time series, were used to determine the influence of variables such as vehicles being followed, bus type and roadway type on pollutant concentrations inside the bus. For all buses tested, the highest concentrations of BC, PB-PAH and NO₂ were observed when following a diesel school bus, especially if that bus was emitting visible exhaust. This result was important because other diesel school buses were responsible for the majority of the diesel vehicle encounters, primarily due to caravanning with each other when leaving a school at the same time. Compared with following a gasoline vehicle or no target, following a smoky diesel school bus yielded BC and PB-PAH concentrations inside the cabin 8 and 11 times higher, respectively, with windows open, and ∼1.8 times higher for both pollutants with windows closed. When other diesel vehicles were not present, pollutant concentrations were highest inside the conventional diesel buses and lowest inside the CNG bus, while the TO diesel bus exhibited intermediate concentrations. Differences in pollutant concentrations between buses were most pronounced with the bus windows closed, and were attributed to a combination of higher concentrations in the exhaust and higher exhaust gas intrusion rates for the conventional diesel buses. Conventional diesel school buses can have a double exposure impact on commuting children: first, exposures to the exhaust from other nearby diesel school buses and, second, exposure to the bus's own exhaust through “self-pollution�?.

Keywords: Black carbon; PAH; Nitrogen dioxide; Videotape

The potential importance of grazing to the fluxes of carbon dioxide and methane in an alpine wetland on the Qinghai-Tibetan Plateau by Mitsuru Hirota; Yanhong Tang; Qiwu Hu; Tomomichi Kato; Shigeki Hirata; Wenhong Mo; Guangmin Cao; Shigeru Mariko (pp. 5255-5259).

To assess the impact of livestock grazing on the emission of greenhouse gases from grazed wetlands, we examined biomass growth of plants, CO₂ and CH₄ fluxes under grazing and non-grazing conditions on the Qinghai-Tibetan Plateau wetland. After the grazing treatment for a period of about 3months, net ecosystem CO₂ uptake and aboveground biomass were significantly smaller, but ecosystem CH₄ emissions were remarkably greater, under grazing conditions than under non-grazing conditions. Examination of the gas-transport system showed that the increased CH₄ emissions resulted from mainly the increase of conductance in the gas-transport system of the grazed plants. The sum of global warming potential, which was estimated from the measured CO₂ and CH₄ fluxes, was 5.6- to 11.3-fold higher under grazing conditions than under non-grazing conditions. The results suggest that livestock grazing may increase the global warming potential of the alpine wetlands.

Keywords: Aquatic plants; Diffusive conductivity; Greenhouse gases; Global warming potential; Livestock grazing

Non-methane hydrocarbons and carbonyls in the Lower Fraser Valley during PACIFIC 2001 by D. Wang; J.D. Fuentes; D. Travers; T. Dann; T. Connolly (pp. 5261-5272).

During August 2001, a field investigation was undertaken to determine the hydrocarbon and carbonyl loadings in the Lower Fraser Valley of British Columbia, Canada. Intensive ambient air sampling was carried out at sites representing urban and rural locales to identify and quantify atmospheric levels of non-methane hydrocarbon and carbonyl species. Results indicated that in the urban atmosphere (3-h) non-methane hydrocarbon levels exceeded 20 parts per billion (ppbv). Rural sites had lower levels of non-methane hydrocarbons. The dominant anthropogenic hydrocarbon species were propane, butane, 2-methylbutane and toluene. The most common biogenic hydrocarbons included isoprene, α-pinene, β-pinene and limonene. On average, biogenic hydrocarbons represented approximately 23% of the total hydrocarbon reactivity estimated for the airshed in the Lower Fraser Valley. Based on the hydroxyl radical-reactivity scale, limonene was determined to be the most photochemically relevant species at the urban site in the Lower Fraser Valley of British Columbia. Formaldehyde, acetaldehyde, and acetone were the dominant carbonyl compounds found in the Lower Fraser Valley. Three-hour averaged carbonyls levels reached 10ppbv (3-h) and resulted from both direct emissions and hydrocarbon photooxidation. Atmospheric conditions strongly modulated the abundance of hydrocarbon and carbonyl compounds.

Keywords: Isoprene; Monoterpenes; NMHC; Carbonyls; Free radicals; Hydrocarbon reactivity

Measurement of Henry's law constants of seven polybrominated diphenyl ether (PBDE) congeners as a function of temperature by Banu Cetin; Mustafa Odabasi (pp. 5273-5280).

Polybrominated diphenyl ethers (PBDEs) are emerging as a significant class of semivolatile environmental pollutants. The Henry's law constant ( H) is an important parameter in predicting the transport, behavior and fate of semivolatile organic compounds (SOCs) in the environment and it is required to model the chemical transfer between air and water.Henry's law constants for 7 PBDE congeners (BDE 28, 47, 99, 100, 153, 154, and 209) that have been frequently detected in the environment were experimentally determined between 5 and 40°C using a gas-stripping technique. The Henry's law constants ranged between 0.04±0.01Pam³mol⁻¹ (BDE 209) and 4.83±0.67Pam³mol⁻¹ (BDE 28) at 25°C. Dimensionless Henry's law constants ( H′) of PBDEs increase with temperature more than 15 times (15.3 times for BDE 100 and 34.1 times for BDE 99) over the studied temperature range. The enthalpies of phase change (Δ H_H) ranged between 54.1kJmol⁻¹ (BDE 154) and 70.8kJmol⁻¹ (BDE 99) and they were within the ranges previously reported for other organic compounds.The gas phase overall mass transfer coefficients ( K_g) for 7 PBDE congeners calculated for a moderate wind speed (3ms⁻¹) and a temperature of 25°C ranged between 0.17 and 0.27cms⁻¹. Calculated resistances indicated that only the mass transfer of BDE 28 that has a relatively higher H value is controlled by both air- and water-side resistances. However, for the remaining congeners the mass transfer is controlled by air-side resistance.

Keywords: PBDEs; Henry's law constant; Gas stripping technique; Air–water exchange

Formation of 2-methyl tetrols and 2-methylglyceric acid in secondary organic aerosol from laboratory irradiated isoprene/NO _X/SO₂/air mixtures and their detection in ambient PM_2.5 samples collected in the eastern United States by E.O. Edney; T.E. Kleindienst; M. Jaoui; M. Lewandowski; J.H. Offenberg; W. Wang; M. Claeys (pp. 5281-5289).

Recent observations in ambient PM_2.5 of 2-methylthreitol, 2-methylerythritol and 2-methylglyceric acid, proposed isoprene oxidation products, suggest the contribution of isoprene to SOA formation, long thought to be relatively unimportant, should be reexamined. To address this issue, an isoprene/NO _X/air mixture was irradiated in a flow reactor smog chamber in both the absence and presence of SO₂ to measure the SOA yield of isoprene and to establish whether the two 2-methyl tetrols and 2-methylglyceric acid are present in isoprene SOA and could serve as SOA indicator compounds. In the absence of SO₂, the SOA yield of 0.002 was low, as expected, although uncertain because the SOA concentration was near chamber background levels. However, in the presence of SO₂, the SOA yield increased significantly to 0.028. Analysis of the trimethylsilyl derivatives of the SOA samples by gas chromatography/mass spectrometry showed chamber concentrations of the two 2-methyl tetrols totaling 0.1μgm⁻³ and a 2-methylglyceric acid concentration of 0.4μgm⁻³ in the absence of SO₂, with the levels increasing significantly to 6.3μgm⁻³ and 1.2μgm⁻³, respectively, when SO₂ was added. The laboratory data suggest that these compounds are possible indicator compounds for isoprene SOA and that the presence of SO₂ enhances significantly SOA formation from isoprene photooxidation, with acid-catalyzed reactions possibly playing a major role. The importance of these findings was supported by the detection of the two 2-methyl tetrols and 2-methylglyceric acid in summertime ambient PM_2.5 samples collected at three locations in the eastern United States. However, additional mechanistic studies are required to predict the contributions of the SO₂-assisted isoprene SOA formation to ambient PM_2.5 concentrations.

Keywords: PM; _2.5; Secondary organic aerosol; Isoprene; Acid-catalyzed reactions; 2-methylthreitol; 2-methylerythritol; 2-methylglyceric acid

Simulated sensitivity of seasonal ozone exposure in the Great Lakes region to changes in anthropogenic emissions in the presence of interannual variability by Jerome D. Fast; Warren E. Heilman (pp. 5291-5306).

A coupled meteorological and chemical modeling system with a 12-km horizontal grid spacing was used to simulate the evolution of ozone over the Great Lakes region between May and September of 1999 and 2001. The overall temporal and spatial variations in hourly ozone concentrations and ozone exposure from control simulations agreed reasonably well with the observations at most locations with an overall monthly bias computed across all stations ranging from −3.2 to 5.3ppb, the root mean square difference ranging from 18.2 to 22.2ppb, and an index of agreement ranging from 0.63 to 0.78. As with the observations, the simulated ozone exposure was higher during most months of the summer of 1999 than during 2001. Sensitivity simulations that increased anthropogenic trace gas emissions were performed to determine the changes in ozone exposure in the presence of meteorological interannual variability. The emission scenario simulations that employed the meteorological conditions of 1999 and increased anthropogenic emissions of NO x and VOCs produced increases in ozone exceeding 80ppb over the lower peninsula of Michigan, the eastern half of the upper peninsula of Michigan, and over Ontario just north of Lake Superior and Lake Huron. Over the agricultural regions, more ozone between 60 and 80ppb was produced. The cooler and wetter conditions with more frequent periods of northwesterly flow during the summer of 2001 were not as favorable for ozone production and did not result in increased ozone, despite the increase in anthropogenic emissions. Increases in ozone exceeding 60ppb occurred only over the lake surfaces and in central Michigan when the meteorological conditions of the summer of 2001 were applied. For both summers, increases in anthropogenic emissions decreased ozone exposure in the immediate vicinity of the largest metropolitan areas. Since anthropogenic emission rate projections depend on assumptions of population, economic development, land-use patterns, and technology, the effect of anthropogenic emission rates on the magnitude and regional-scale distribution of ozone concentrations could be much larger or smaller than indicated by this study. In a subsequent study, the simulated ozone will be used as input to biological models to assess the response of ozone-sensitive tree species in the Great Lakes region to ozone levels produced by various anthropogenic emission scenarios.

Keywords: Regional photochemical modeling; Air quality; Ozone exposure; Boundary layer; NO; x; VOC

Aerosol source apportionment around a large coal fired power plant—Thermoelectric Complex Jorge Lacerda, Santa Catarina, Brazil by Maria Luiza D.P. Godoy; J.M. Jos� Marcus Godoy; Paulo Artaxo (pp. 5307-5324).

The Thermoelectric Complex Jorge Lacerda (TCJL) is located in the Southern part of Brazil, at the community of Capivari de Baixo, in the state of Santa Catarina, 130km from Florianópolis. The TCJL is the largest coal burning thermoelectric complex of Latin America, formed by seven power plants and with a total capacity of 832MW. Two aerosol-sampling campaigns were performed, during summer and winter seasons, in 12 different sites around the TCJL with aerosol collection for 10 sampling days each site. Stacked filter units were used to collect fine and coarse aerosol particles and trace element analysis by inductively coupled plasma–mass spectrometry(ICP-MS ) was performed in both size fractions. Gravimetric analysis and reflectance measurements provided aerosol mass and black carbon concentrations. Very good detection limits for up to 42 elements were obtained. Receptors models such as principal factor analysis, cluster analysis and absolute principal factor analysis were applied in order to identify and quantify the aerosol particle sources. Emissions from the TCJL are the main source of elements such as As, Bi, Cd, Pb, Sb and Se in both aerosol fractions, ranging from 34% up to 83% in mass. Based on absolute principal component analysis, source profiles were calculated. These profiles were compared with those observed on the EPA Speciate 3.1 data bank and a good similarity was observed. The ICP-MS analysis of trace elements in aerosols has proven to be reliable and feasible for large amount of samples, and the coupling with receptor models provided an excellent method for quantitative aerosol source apportionment in industrial complexes.

Keywords: ICP-MS; Aerosol analysis; Coal power plant; Receptor model; Trace elements; Brazil

Control strategies for the reduction of airborne particulate nitrate in California's San Joaquin Valley by Michael J. Kleeman; Qi Ying; Ajith Kaduwela (pp. 5325-5341).

The effect of NO _x, volatile organic compound (VOC), and NH₃ emissions control programs on the formation of particulate ammonium nitrate in the San Joaquin Valley (SJV) was examined under the typical winter conditions that existed on 4–6 January, 1996. The UCD/CIT photochemical transport model was used for this study so that the source origin of primary particulate matter and secondary particulate matter could be identified. When averaged across the entire SJV, the model results predict that 13–18% of the reactive nitrogen (NO _y=NO _x+reaction products of NO _x) emitted from local sources within the SJV was converted to nitrate at the ground level. Each gram of NO _x emitted locally within the SJV (expressed as NO₂) produced 0.23–0.31g of particulate ammonium nitrate (NH₄NO₃), which is much smaller than the maximum theoretical yield of 1.7g of NH₄NO₃ per gram of NO₂. The fraction of reactive nitrogen converted to nitrate varied strongly as a function of location. Urban regions with large amounts of fresh NO emissions converted little reactive nitrogen to nitrate, while remote areas had up to 70% conversion (equivalent to approximately 1.2g of NH₄NO₃ per gram of NO₂). The use of a single spatially averaged ratio of NH₄NO₃/NO _x as a predictor of how changes to NO _x emissions would affect particulate nitrate concentrations would not be accurate at all locations in the SJV under the conditions studied.The largest local sources of particulate nitrate in the SJV were predicted to be diesel engines and catalyst equipped gasoline engines under the conditions experienced on 6 January, 1996. Together, these sources accounted for less than half of the ground-level nitrate aerosol in the SJV. The remaining fraction of the aerosol nitrate originated from reactive nitrogen originally released upwind of the SJV. The majority of this upwind reactive nitrogen was already transformed to nitrate by the time it entered the SJV. The effect of local emissions controls on this upwind material was small.A 50% reduction in NO _x emissions applied to sources within the SJV reduced the predicted concentration of total nitrate by approximately 25% during the study episode. VOC emissions controls were less effective, while reasonable NH₃ emissions controls had the smallest effect on the amount of ammonium nitrate produced. A 50% reduction in VOC emissions lowered predicted concentrations of total nitrate by 17.5%, while a 50% reduction in NH₃ emissions lowered predicted concentrations of total nitrate by only 10%. This latter result is expected since the formation of ammonium nitrate aerosol is limited by the availability of gas-phase nitric acid, with large amounts of excess NH₃ available. NO _x emissions controls appear to be the most efficient method to reduce the concentration of locally generated particulate nitrate in the SJV under the conditions experienced on 4–6 January, 1996.

Keywords: PM2.5; San Joaquin Valley; IMS95; CRPAQS; Emissions controls

Spatial and temporal variability of PM levels and composition in a complex summer atmospheric scenario in Barcelona (NE Spain) by M. Viana; C. P�rez; X. Querol; A. Alastuey; S. Nickovic; J.M. Baldasano (pp. 5343-5361).

Summer atmospheric coastal dynamics exert a significant influence on the levels and composition of atmospheric particulate matter (PM) in the North-Eastern Iberian Peninsula. Summer atmospheric scenarios in this region present a high degree of complexity as they are characterised by the absence of synoptic-scale air mass advections, the development of breeze circulations, enhanced photochemistry, local mineral dust re-suspension and the occurrence of African dust outbreaks. Three sampling sites were selected in Barcelona (NE Spain), an urban coastal site surrounded by complex topography. Regional dust modelling (DREAM) and high resolution meteorological modelling (MM5) were used to interpret PM levels and composition at the three sites. The results outline the effect of breeze dynamics and thermal internal boundary layer formation as the main meteorological drivers of the hourly evolution of PM levels. Levels of crustal components, secondary inorganic and carbon species are higher during the night, and only the marine aerosol content is higher during the day. Nitrate levels are higher during the night due to the thermal stability on NH₄NO₃. Sulphate levels are higher during the night as a consequence of the drainage flows. Lidar measurements and model results signalled the occurrence of two African dust episodes during the study period which mainly affected the free troposphere over Barcelona.

Keywords: PM10; PM2.5; PM1; Sea-land breeze; Western Mediterranean basin; Mesoscale modelling; African dust; Regional transport; Thermal internal boundary layer

Fine structure of mass size distributions in an urban environment by Imre Salma; Rita Ocskay; Nico Raes; Willy Maenhaut (pp. 5363-5374).

As part of an urban aerosol research project, aerosol samples were collected by a small deposit area low-pressure impactor and a micro-orifice uniform deposit impactor in downtown Budapest in spring 2002. A total number of 23 samples were obtained with each device for separate daytime periods and nights. The samples were analysed by particle-induced X-ray emission spectrometry for 29 elements, or by gravimetry for particulate mass. The raw size distribution data were processed by the inversion program MICRON utilising the calibrated collection efficiency curve for each impactor stage in order to study the mass size distributions in the size range of about 50nm to 10μm in detail. Concentration, geometric mean aerodynamic diameter, and geometric standard deviation for each contributing mode were determined and further evaluated. For the crustal elements, two modes were identified in the mass size distributions: a major coarse mode and a (so-called) intermediate mode, which contained about 4% of the elemental mass. The coarse mode was associated with suspension, resuspension, and abrasion processes, whereby the major contribution likely came from road dust, while the particles of the intermediate mode may have originated from the same but also from the other sources. The typical anthropogenic elements exhibited usually trimodal size distributions including a coarse mode and two submicrometer modes instead of a single accumulation mode. The mode diameter of the upper submicrometer mode was somewhat lower for the particulate mass (PM) and S than for the anthropogenic metals, suggesting different sources and/or source processes. The different relative intensities of the two submicrometer modes for the anthropogenic elements and the PM indicate that the elements and PM have multiple sources. An Aitken mode was unambiguously observed for S, Zn, and K, but in a few cases only. The relatively large coarse mode of Cu and Zn, and the small night-to-daytime period concentration ratio pointed to anthropogenic sources by disintegration, which were related to vehicular traffic, i.e., mechanical wear of brake linings and tires, respectively.

Keywords: Cascade impactor; Inversion; Size distribution; Modal parameters; Element; PM

Fog water flux at a canopy top: Direct measurement versus one-dimensional model by Otto Klemm; Thomas Wrzesinsky; Clemens Scheer (pp. 5375-5386).

A one-dimensional model [Lovett, 1984. Rates and mechanisms of cloud water deposition to a subalpine balsam fir forest. Atmospheric Environment 18, 361–371] to quantify fog water deposition was compared with results of long term (13 months) measurements of turbulent exchange with the eddy covariance method at a mountainous site in Central Europe. Turbulent exchange is mainly deposition and dominates over sedimentation at that site, therefore eddy covariance is a suitable tool in quantifying fog water deposition. The model can be operated with use of the measured droplet size distribution (DSD), with a DSD as parameterized from liquid water content (LWC) data, or with the measured visibility (VIS) as a quantitative indicator for fog. The latter is the easier measurement and therefore preferable for long-term applications. We compared the fog water deposition on a monthly basis. If VIS data are used as model input, the overall underestimate of the measurement is −23% as compared to the measurements. Using LWC and the parameterized DSD as input, the deviation is +37%. All deviations are highly significant.

Keywords: Eddy covariance; Fog deposition; Liquid water content; Lovett model; Norway spruce

Odour emission factors for assessment and prediction of Italian MSW landfills odour impact by Selena Sironi; Laura Capelli; Centola Paolo C�ntola; Renato Del Rosso; Massimiliano Il Grande (pp. 5387-5394).

Among the human activities that may generate problems related to unpleasant odour emissions, landfills represent one of the major causes of odour complaints. In this study, the results of odour concentration measurements sampled from the principal odour sources of seven different and dimensionally representative Italian Municipal Solid Waste (MSW) landfills are presented. Experimental data are then used to estimate an Odour Emission Factor (OEF). The average OEF is calculated to be 5.5±3.4ou_Es⁻¹m⁻². This value is consistent with the OEFs that can be calculated based on existing literature data, such as surface flux rates relevant to the Landfill Gas (LFG) emissions through the landfill surface.

Keywords: Landfill gas; Odour emission rate; Specific odour emission rate; Dynamic olfactometry; Emission factors

Impact of an industrial complex on the ambient air quality: Case study using a dispersion model by T.V.B.P.S. Rama Krishna; M.K. Reddy; R.C. Reddy; R.N. Singh (pp. 5395-5407).

The Industrial Source Complex Short Term (ISCST-3) model has been used to study the impact of an industrial complex, located at Jeedimetla in the outskirts of Hyderabad city, India, on the ambient air quality. The emissions of SO₂ from 38 elevated point sources and 11 area sources along with the meteorological data for 2 months (April and May 2000) representing the summer season and for 1 month (January 2001) representing the winter season have been used for computing the ground level concentrations of SO₂. The 8- and 24-h averaged model-predicted concentrations have been compared with corresponding observed concentrations at three receptors in April 2000 and at three receptors in May 2000 where ambient air quality is monitored during the study period. A total of 90 pairs of the predicted and observed concentrations have been used for model validation by computing different statistical errors and through Quantile–Quantile (Q–Q) plot. The results show that the model-predicted concentrations are in good agreement with observed values and the model performance is found to be satisfactory. The spatial distribution of SO₂ concentrations over the study area is examined in the summer and winter months and found that the levels of SO₂ are within the limits in comparison to the National Ambient Air Quality Standards except near the industrial area.

Keywords: Air quality; Industrial sources; Observed concentrations; Statistical errors; Spatial distribution; Model evaluation

Impact of vertical transport processes on the tropospheric ozone layering above Europe. Part I: Study of air mass origin using multivariate analysis, clustering and trajectories by Augustin Colette; G�rard Ancellet; Fran�ois Borchi (pp. 5409-5422).

A new method is proposed to classify ozone-rich layers observed in tropospheric profiles in terms of their origin using multivariate analysis. We combine principal component and discriminant analyses to quantify the respective ability of 21 measured physical parameters to describe the layers. Agglomerative hierarchical clustering shows the existence of clusters of air masses with specific physical characteristics. Quadratic discriminant analysis allows the definition of multidimensional borders between these clusters. The geophysical characteristics of the clusters are discussed and related to the origins of the layers: recently transported from the stratosphere (ST) or from the boundary layer (BL) or transported over long distances in the free troposphere.This clustering is compared to the results of a Lagrangian particle dispersion model for a 2-year period. The proportions of layers originating either from the BL or from the ST are highly consistent using both methods as well as the respective contribution of each reservoir to the total ozone mass. About 10% of the ozone measured in the tropospheric layers was exported recently from the BL and one-fifth has a recent stratospheric origin. The remaining proportion could not be attributed to any recent transport pathway. Season-dependent criteria allow very satisfactory reproduction of the seasonal variability of the layering as seen by the Lagrangian model. Analysis of the geographical origin of BL air masses suggests that the statistical clustering underestimates long-range transport, especially in fall.

Keywords: Tropospheric ozone; Ozone profiles; Air mass classification; Multivariate analysis; Trajectories

Impact of vertical transport processes on the tropospheric ozone layering above Europe. by Augustin Colette; G. Gerard G�rard Ancellet (pp. 5423-5435).

Using the set of multivariate criteria described in a companion paper, ozone-rich layers detected in tropospheric soundings are clustered according to their stratospheric or boundary layer origin. An additional class for aged tropospheric air masses is also considered. This analysis is exclusively based on the measured physical properties of the layers. The database includes 27,000 ozone profiles collected above 11 European stations—two of which provide measurements since 1970. The seasonal cycle of the tropospheric ozone stratification exhibits a clear summer maximum. This increase is due to aged tropospheric air masses that are more frequently detected, suggesting an enhanced lifetime of layers in summer. In terms of ozone content, the relative impact of stratospheric ozone compared to the other sources is highest in winter while export from the boundary layer presents a uniform seasonal cycle. Altitude and thickness distributions of the layers are consistent with the dynamical processes involved in the layering. Northernmost and southernmost stations are more exposed to stratospheric air intrusions into the free troposphere. Long-term trends show that transport from the tropopause region has increased since the mid 1980s. This trend being concomitant with lower ozone content of such layers, a moderate trend of the transport efficiency from the stratosphere on total tropospheric ozone is observed. The increase of ozone detected in tropospheric layers since the mid 1980s cannot be attributed to any recent export process from either the stratosphere or the boundary layer but rather to enhanced photochemical production in aged air masses or to an increase in the lifetime of the layers.

Keywords: Tropospheric ozone climatology; Ozonesondes; Transport pathways; Stratosphere to troposphere exchange; Boundary layer venting

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Atmospheric Environment (v.39, #29)