Atmospheric Environment (v.37, #6)

Diurnal characteristics of volatile organic compounds in the Seoul atmosphere by Kwangsam Na; Yong Pyo Kim; Kil Choo Moon (733-742).
Concentrations of volatile organic compounds (VOCs) were measured at a site in central Seoul from 8 to 13 September 1998. On each sampling day, three 2-h-integrated canister samples were collected in the morning, afternoon and evening, respectively, to observe diural variations of VOCs. Most of the VOCs species showed diurnal variations with higher concentrations during the morning and evening, and lower concentrations during the afternoon. However, in the afternoon, the concentrations of aromatic compounds, closely correlated with solvent usage such as toluene, ethylbenzene, m-/p-xylene, and o-xylene, were slightly higher than or comparable to those in the morning. This may be due to the increase of evaporative emissions derived from the rise in ambient temperature and additional sources such as the use of solvents in painting, printing and dry cleaning. To estimate the participation of individual VOCs in ozone formation, propylene equivalent concentrations were calculated. The results showed that toluene was the most dominant contributor to ozone formation as well as ambient VOC concentrations. Toluene/benzene and m-/p-xylene/benzene ratios showed a high observed in the afternoon and a low observed in the morning and evening. This may be because the contribution of evaporative emissions by solvent usage on the ambient VOC concentrations is more dominant than those of vehicle-related emissions and photochemical loss.
Keywords: VOCs; Diurnal variations; Propylene equivalent concentration; Evaporative emission; Solvent usage;

The size dependence of chloride depletion in fine and coarse sea-salt particles by Xiaohong Yao; Ming Fang; Chak K Chan (743-751).
The size dependence of the percentage of chloride depletion (%Cldep) in sea-salt aerosols was investigated using 16 sets of 48–96 h size distribution measurements of atmospheric aerosols, collected from June 2000 to May 2001 in Hong Kong. Chloride and sodium in HK are predominantly from sea-salt aerosols because the prevailing wind is from the sea. In the particles larger than 1.8 μm (coarse particles), the %Cldep generally decreased with increasing particle size, consistent with the literature. In the particles smaller than 1.8 μm (fine particles), the mode diameter of %Cldep was associated with the mode diameter of sulfate in the droplet mode. When the sulfate peak appeared at 0.32–0.54 μm, the %Cldep peaked at 0.54–1.0 μm. Heterogeneous reactions between sea-salt particles and acidic gases are the major routes of the sulfate formation and chloride depletion, leading to a decrease in the %Cldep with increasing particle size. When the sulfate peak appeared at 0.54–1.0 μm, the maximum %Cldep appeared at 1.0–1.8 μm and the %Cldep increased with increasing particle size in the fine mode. Both heterogeneous reactions and in-cloud processes occurred to form sulfate and to deplete chloride. However, heterogeneous reactions, sampling artifacts, and anthropogenic emissions of chloride cannot explain this observed size dependence. Cloud processing including the activation of sea-salt particles with subsequent SO4 2− formation, the neutralization by NH3 and the evaporation of HCl in conjunction with NH3 during water evaporation from cloud droplets can cause chloride depletion in the droplet mode. The smaller amount of evaporation of NH3 and HCl from cloud droplets in forming the 0.54–1.0 μm particles than the 1.0–1.8 μm particles can account for the observed size dependence of %Cldep although direct evidence based on cloud measurements is not available.
Keywords: Marine aerosols; In-cloud processes; Heterogeneous reactions; Sulfate; Nitrate; Size distributions;

The chemical composition of fine and coarse particles in relation with the Asian Dust events by Ki-Hyun Kim; Gyoo-Hoon Choi; Chang-Hee Kang; Jin-Hong Lee; J.Y Kim; Y.H Youn; S.R Lee (753-765).
The distribution patterns of the particulate matter (PM) and the associated elements were investigated from Seoul, Korea during spring 2001. The results of our measurements were analyzed to explain the behavior of metallic components by comparing their compositions mainly in terms of between Asian Dust (AD) and non-AD (NAD) period and between fine and coarse particle fraction. The computation of enrichment factor (EF) indicated that the magnitude of EF values for most hazardous metals during the AD period were even smaller than the NAD counterpart. The existence of low EF values during the AD period may be ascribable to the excessive input of crustal components like Al accompanied by the AD event. In accordance with this finding, the effects of the AD events were also reflected in diverse manners, when assessed by the concentration ratios of a given element for both AD/NAD period and fine-to-coarse (F/C) fraction. Results of this comparative analysis generally suggest that AD events are prominent sources for major crustal components in the fine particle fraction of PM. In addition, comparison of our measurement data with those obtained within the Korean peninsula and in the near-by Asian areas indicates that the metallic distribution patterns of the study area may be affected more sensitively by anthropogenic signatures. The results of our analysis, if investigated in relation with air mass movement patterns by means of the back-trajectory analysis, demonstrate consistently that the PM data measured during the study period can be closely tied with the signatures of both AD events and anthropogenic processes.
Keywords: Asian Dust; Fine particulates; Coarse particulates; PM2.5; PM10; Air quality;

Atmospheric deposition of heavy metals in the Pearl River Delta, China by C.S.C. Wong; X.D. Li; G. Zhang; S.H. Qi; X.Z. Peng (767-776).
Rapid urban and industrial development in China in the last few decades has provoked some serious environmental concerns. As one of the regions with the fastest economic development in China, the Pearl River Delta (PRD) is particularly susceptible to environmental degradation. Atmospheric emissions represent a major pathway of anthropogenic inputs of heavy metals into the surface environment. Samples of atmospheric deposits were collected at urban, suburban and rural locations in the PRD (including Hong Kong) using bulk deposition samplers in the summer and winter seasons of 2001–2002. The samples were analyzed for heavy metal concentrations and Pb isotopic compositions. According to the analytical results, atmospheric deposition of Cr, Cu, Pb and Zn in the PRD (6.43±3.19, 18.6±7.88, 12.7±6.72 and 104±36.4 mg/m2/yr, respectively) was significantly elevated compared with other regions, e.g. the Great Lakes region in North America and the North Sea in Europe. It was also found that atmospheric deposition of Cu, Cr and Zn was generally higher in the summer than that in the winter, which could be caused by the washout effect of the rainy season in the subtropical region. The Pb isotopic composition of the air deposits (206Pb/207Pb 1.161–1.177) indicated that atmospheric inputs of Pb derived mainly from anthropogenic sources, e.g. vehicular exhaust and Pb ore in the PRD. However, the distribution of 206Pb/207Pb and 208Pb/207Pb ratios of some atmospheric deposits in the summer season suggested that atmospheric Pb at some locations of the PRD could be attributed to other anthropogenic source(s).
Keywords: Atmospheric deposition; Heavy metals; Pb isotopes; Pearl River Delta; China;

Ozone deposition fluxes have been measured over a wheat field in Northern Italy in May–June 2001 with the eddy-correlation method in order to evaluate the amount of ozone taken up by plants during the whole grain filling period, from anthesis to harvest. Ozone uptake by plants is due to its penetration through the stomata; for this reason stomatal ozone fluxes were determined, using the analogy with water vapour fluxes (Penman–Monteith approach), which are easily measured. The total ozone dose was obtained by integrating the stomatal ozone fluxes over time. The observational results showed that ozone fluxes decrease with time, following the maturation and the senescence of plants. On the average, the stomatal flux was found to be 50–60% of the total flux, but this fraction decreased during the senescence, emphasizing the importance of non-stomatal ozone deposition pathways. The approach consisting on evaluating the total ozone dose by integrating stomatal fluxes was then compared to the currently used procedure based on the evaluation of the exposure index AOT40, which uses ozone concentrations. Important differences between these two methods are highlighted. The integration of stomatal ozone fluxes appears more suitable for the evaluation of physiological uptake than the use of the exposure index.
Keywords: Ozone deposition; Stomatal fluxes; Wheat; AOT40; Eddy correlation;

Airborne measurements of aerosol extinction in the lower and middle troposphere over Wyoming, USA by Ziguang Han; Derek C Montague; Jefferson R Snider (789-802).
Particle size distributions, and scattering and absorption coefficients were measured over the Green River basin of Wyoming during the Southwest Wyoming Visibility Study (SWYVIS) in February and March 1996. Eleven flights were carried out, using the Wyoming King Air research aircraft. In the least polluted regions of the planetary boundary layer, particle number densities detected in the diameter range 0.13–3.0 μm were <100 cm−3. Aloft, in the stable air of the free troposphere, they were generally even lower, often falling to a few tens of particles per cm3. Analyses of bulk aerosol filter samples showed that organic carbonaceous material was the dominant chemical component, with sulfate and refractory species being the largest inorganic components. Combining the filter data with separately measured black carbon mass loading values allowed refractive indices for the aerosol to be calculated, so that PCASP measured size distributions could be revised. Characterizing size parameters were obtained by fitting particle populations to bimodal lognormal distributions. Particle size distributions were somewhat broader at higher altitudes so that larger particles made greater contributions to extinction. Optical closure was attempted by comparing scattering and total extinctions computed by Mie theory with the corresponding values derived from the observations. While the calculated and measured single scattering albedo average values were in reasonably good agreement, even though individual pairs of values sometimes differed significantly, computed scattering coefficients often exceeded those derived from the measurements, by an average of 60%. Reasons for this discrepancy are explored, including the possible modification of the size distribution by partial or total volatilization of particles within the nephelometer and its inlet.
Keywords: Scattering extinction; Absorption extinction; Optical closure; Vertical profile; Aircraft sampling;

PCDD/F emissions from forest fire simulations by Brian K Gullett; Abderrahmane Touati (803-813).
Polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran (PCDD/F) emissions from combustion of forest biomass were sampled to obtain an estimated emission factor for forest fires. An equal composition of live shoot and litter biomass from Oregon and North Carolina was burned in an enclosed monitored facility, and emissions were sampled with a modified ambient air sampling method. Seven tests resulted in an average PCDD/F toxic equivalency (TEQ) emission of 19  ng/kg burned, a value close to previous indirect estimates. Emissions from the Oregon biomass averaged 25  ng TEQ/kg burned (range: 14–47) and those from the North Carolina biomass averaged 15  ng TEQ/kg burned (range: 1–56). Source-specific distinctions in isomer patterns and homologue profiles, however, suggest that the type of biomass has a significant effect on the composition of the resulting PCDD/F emissions. The mass-specific emissions are about 20 times higher than the concentration of the extracted biomass, suggesting that PCDD/F emissions are not simply a result of vaporization of cuticle-bound PCDD/F but are formed predominantly during the biomass combustion.
Keywords: Forest fires; Polychlorinated dibenzodioxin; Polychlorinated dibenzofuran; Polychlorinated biphenyl; Emission factors; Combustion;

Estimating population exposure to power plant emissions using CALPUFF: a case study in Beijing, China by Ying Zhou; Jonathan I Levy; James K Hammitt; John S Evans (815-826).
Epidemiological studies have shown a significant association between ambient particulate matter (PM) exposures and increased mortality and morbidity risk. Power plants are significant emitters of precursor gases of fine particulate matter. To evaluate the public health risk posed by power plants, it is necessary to evaluate population exposure to different pollutants. The concept of intake fraction (the fraction of a pollutant emitted that is eventually inhaled or ingested by a population) has been proposed to provide a simple summary measure of the relationship between emissions and exposure. Currently available intake fraction estimates from developing countries used models that look only at the near field impacts, which may not capture the full impact of a pollution source. This case study demonstrated how the intake fraction of power plant emissions in China can be calculated using a detailed long-range atmospheric dispersion model—CALPUFF. We found that the intake fraction of primary fine particles is roughly on the order of 10−5, while the intake fractions of sulfur dioxide, sulfate and nitrate are on the order of 10−6. These estimates are an order of magnitude higher than the US estimates. We also tested how sensitive the results were to key assumptions within the model. The size distribution of primary particles has a large impact on the intake fraction for primary particles while the background ammonia concentration is an important factor influencing the intake fraction of nitrate. The background ozone concentration has a moderate impact on the intake fraction of sulfate and nitrate. Our analysis shows that this approach is applicable to a developing country and it provides reasonable population exposure estimates.
Keywords: Intake fraction; Atmospheric dispersion modeling; Particulate matter; Power plants; Air pollution; China;

Continuous measurement of fine and ultrafine particulate matter, criteria pollutants and meteorological conditions in urban El Paso, Texas by Christopher A Noble; Shaibal Mukerjee; Melissa Gonzales; Charles E Rodes; Philip A Lawless; Sanjay Natarajan; Eric A Myers; Gary A Norris; Luther Smith; Halûk Özkaynak; Lucas M Neas (827-840).
Continuous measurements of aerosol size distributions were made in El Paso, TX, for a 21 day period in winter 1999. Size distribution measurements were performed at two urban locations in El Paso using two pairs of the scanning mobility particle sizer and the aerodynamic particle sizer. Complementary measurements also were performed for gas phase pollutants (CO, NO, NO2, O3) and meteorological conditions. Throughout the study, the mean ultrafine particle (those smaller than 0.1 μm in diameter) number concentration was 14,400 particles cm−3. There was a significant correlation between CO and both ultrafine and accumulation mode (those between 0.1 and 1 μm in diameter) particle count, with the Pearson correlation coefficient (r) values of 0.81 and 0.87, respectively. The correlation between NO and both ultrafine and accumulation mode particle count is also significant, but not as strong as the correlation of CO and the particle concentrations. Most pollutants were found to vary on diurnal cycles and to follow one of two different trends, either vehicular traffic schedules or sunlight intensity. Wind direction was found to have an influence not only on pollutant concentrations, but also on the correlation between pollutants. With southerly winds, CO, NO and NO2 concentrations were 25–140% greater than when the wind was coming from the north. Likewise, ultrafine and accumulation mode particle concentrations were approximately 100% greater for southerly than for northerly winds.
Keywords: Aerosol; Ultrafine particles; Fine particles; Particle number concentration; Urban air quality;

This work investigates the interactions between gas-phase carbonyl compounds and sulfuric acid aerosols. It focuses on understanding the chemical processes, giving a first estimate of their importance in the atmosphere, and suggesting directions for further investigations. The solubility and reactivity of a compound with a large enolization constant, 2,4-pentanedione, in water/sulfuric acid solutions 0–96 wt% have been investigated at room temperature using the bubble column/GC-FID technique. 2,4-pentanedione was found to undergo aldol condensation at acidities as low as 20 wt% H2SO4, that is, well in the tropospheric range of aerosol composition. In agreement with well-established organic chemical knowledge, this reaction resulted in changes of color of the solutions of potential importance for the optical properties of the aerosols. 2,4-pentanedione was also found to undergo retroaldol reaction, specific to dicarbonyl compounds, producing acetone and acetaldehyde. The Henry's law coefficient for 2,4-pentanedione was found to be a factor 5 larger than the one of acetone over the whole range of acidity, with a value in water of H (297 K)=(155±27) M atm−1. A chemical system is proposed to describe the transformations of carbonyl compounds in sulfuric acid aerosols. Aldol condensation is likely to be the most common reaction for these compounds, probably involving a large number of the ones present in the atmosphere and a wide range of aerosol compositions. The enolization constant contributes as a proportional factor to the rate constant for aldol condensation, and is shown in this work to contribute as an additive constant to the Henry's law coefficient. In addition to the many important aspects of these reactions illustrated in this work, the rate of aldol condensation was estimated to be potentially fast enough for the losses of some compounds in acidic aerosols to compete with their gas-phase chemistry in the atmosphere.
Keywords: Acidic aerosol; Carbonyl compounds; Heterogeneous chemistry; Henry's law constant; Aldol condensation;

The partitioning of a diverse set of semivolatile organic compounds (SOCs) on a variety of organic aerosols was studied using smog chamber experimental data. Existing data on the partitioning of SOCs on aerosols from wood combustion, diesel combustion, and the α-pinene-O3 reaction was augmented by carrying out smog chamber partitioning experiments on aerosols from meat cooking, and catalyzed and uncatalyzed gasoline engine exhaust. Model compositions for aerosols from meat cooking and gasoline combustion emissions were used to calculate activity coefficients for the SOCs in the organic aerosols and the Pankow absorptive gas/particle partitioning model was used to calculate the partitioning coefficient K p and quantitate the predictive improvements of using the activity coefficient. The slope of the log  K p vs. log  p L 0 correlation for partitioning on aerosols from meat cooking improved from −0.81 to −0.94 after incorporation of activity coefficients iγ om . A stepwise regression analysis of the partitioning model revealed that for the data set used in this study, partitioning predictions on α-pinene-O3 secondary aerosol and wood combustion aerosol showed statistically significant improvement after incorporation of iγ om , which can be attributed to their overall polarity. The partitioning model was sensitive to changes in aerosol composition when updated compositions for α-pinene-O3 aerosol and wood combustion aerosol were used. The octanol–air partitioning coefficient's (K OA) effectiveness as a partitioning correlator over a variety of aerosol types was evaluated. The slope of the log  K p −log  K OA correlation was not constant over the aerosol types and SOCs used in the study and the use of K OA for partitioning correlations can potentially lead to significant deviations, especially for polar aerosols.
Keywords: Gas–particle partitioning; Semivolatile organic compounds; Activity coefficient estimation; Octanol–air partitioning coefficient;

In this work, we have compared two methods for the determination of water-soluble organic carbon (WSOC) in ambient aerosols, one based on a total organic carbon analyzer (TOC) and the other based on an aerosol carbon analyzer (ACA). The two instruments entail different pre-analysis treatment of aerosol water extracts. Standard compounds spiked onto filters showed satisfactory recovery ranging from 75% to 105% by the two methods, demonstrating that the extraction procedures and subsequent pre-analysis pre-treatment are quantitative in both methods. Measurements of standard compounds and aerosol samples show that the two methods give equivalent results by paired t-test. The TOC method offers a limit of detection (LOD) of 2.6 μg/sample, whereas the ACA method gives a LOD of 4.8 μg/sample. Acid treatment to remove carbonate carbon was found unnecessary for the determination of WSOC in PM2.5 aerosols. Although the sample treatment procedures and the detection limits of these two methods are different to some extent, both are suitable for the determination of WSOC in aerosol samples. The TOC method has the advantages of requiring less time in sample pre-treatment and offering slightly lower detection limit. The TOC instruments also have the additional advantage of automated sample analysis while the ACA instruments require manual operation.
Keywords: Water-soluble organic carbon (WSOC); Total organic carbon analyzer (TOC); Aerosol carbon analyzer (ACA); Atmospheric particles; Determination;