Atmospheric Environment (v.38, #6)

Scavenging of atmospheric aerosols by falling precipitation is a major removal mechanism for airborne particles. The process can be described by a wet scavenging coefficient (WSC), denoted L, that is dependent on the rainfall rate, R, and the collision efficiency between raindrops and aerosol particles, E. We report bulk average L values for location in the Eastern United States, estimated based on sulfate mass balance in the atmospheric domain of interest. Data used are taken from several observational networks: (a) the Atmospheric Integrated Research Monitoring Network (AIRMoN) which is part of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN); (b) the Interagency Monitoring of Protected Visibility Environments (IMPROVE); and (c) the National Climatic Data Center (NCDC). The results are fitted relatively well by L values computed using a microphysical representation of the WSC process based on collision efficiency and precipitation size distribution. Such representation leads to a simple expression L=f(R) for soluble aerosols, suitable for WSC description in regional scale models. The agreement between the bulk method and the microphysical representation is due in part to the predominant widespread precipitation, well represented by Marshall and Palmer raindrop distribution, and in part due to assumptions made in the bulk model. Results indicate that high-resolution rainfall rates and realistic vertical cloud structure information are needed to improve the accuracy of aerosol wet scavenging modeling for pollution studies.
Keywords: Sulfate; Wet deposition; AIRMoN; Acid rain; Cloud;

We sampled three layers of snow at 16 sites along a 1200 km transect from Nome to Barrow across northwestern Alaska. Samples were analyzed for major element concentrations, specific conductance and pH. Samples from five of the sites were analyzed for trace element concentrations. Pb, Cd, SO4 2− and non-sea salt SO4 2− concentrations were significantly higher in layers deposited later in the winter than those deposited earlier. This is consistent with the seasonal increase in atmospheric aerosol loading (arctic haze) that develops as the Arctic polar front expands southward in March and April. Haze contaminant concentrations in the snow pack were as high south of the Brooks Range as they were to the north, suggesting the Brooks Range is not an effective orographic barrier to aerosol transport. Computed yearly non-sea salt SO4 2− loading rates at the 16 sites ranged from 12 to 281 mg/m2/yr. Elevated concentrations of Hg, Na and Cl were measured near the Arctic Ocean coast but not near the Bering Sea coast. To explain this pattern we suggest that the “effective distance from the coast,” inferred from prevailing wind directions and storm tracks, is critical in governing whether halogen emissions from the ocean are available for photochemical reactions that result in Hg deposition to the snow.
Keywords: Arctic; Snow chemistry; Trace elements; Aerosols;

Emissions from elevated point sources travel at high altitudes and contribute to regional air pollution. Emission data are required to evaluate how urban and industrial plumes traveling at high altitudes impact background plumes. Two EPA dispersion models, SCREEN and Industrial Source Complex (ISC) were considered to evaluate the importance of individual point source plumes at high altitude. Running ISC in Excel, rather than FORTRAN, facilitated data manipulation. Using the modified ISC model, plumes (mostly for unstable cases) were examined from four power plants, located in eastern Texas. Air quality data collected by the Baylor aircraft from the Baylor Sampling Project were used for calibration and validation. Emissions of SO2 were considered since SO2 acts as a non-reactant species in the atmosphere when evaluated over flight times used by the aircraft. User defined meteorological parameters were used instead of the more common annual or probabilistic meteorological data. Results demonstrated that on a typical day, using the most occurring stability class, average wind speed and average mixing height, the modified ISC accurately predicted the peak concentrations about 80% of the time. The modified ISC also correctly projected plume width within 70% of the actual spread, at least 60% of the time. However, the model was not very useful for matching concentrations at plume extremes.
Keywords: Emission modeling; Industrial Source Complex Model; SO2; Point sources; Dispersion;

Model development for spatial variation of PM2.5 emissions from residential wood burning by Yong Q. Tian; John D. Radke; Peng Gong; Qian Yu (833-843).
This paper presents a preliminary research result of spatially quantifying and allocating the potential activity of residential wood burning (RWB) by using demographic, hypsographic, climatic and topographic information as independent variables. We also introduce the method for calculating PM2.5 emission from residential wood combustion with the potential activity as primary variable. A linear regression model was generated to describe spatial and temporal distribution of the potential activity of wood burning as primary heating source. In order to improve the estimation, the classifications of urban, suburban and rural were redefined to meet the specifications of this application. Also, a unique way of defining forest accessibility is found useful in estimating the activity potential of RWB. The results suggest that the potential activity of wood burning is mostly determined by elevation of a location, forest accessibility, urban/non-urban position, climatic conditions and several demographic variables. The analysis results were validated using survey data collected through face-to-face and telephone interviews over the study area in central California. The linear regression model can explain approximately 86% of the variation of surveyed wood burning activity potential. The total PM2.5 emitted from woodstoves and fireplaces is analyzed for the study region at county level.
Keywords: Residential wood burning; Emission; Anthropogenic fine particle pollution; 1990 census; Fire appliances; Demographic characteristics;

India is a large producer and user of chlorofluorocarbons (CFCs) in the refrigeration and air-conditioning (RAC) sector. Government of India has taken several steps to restrict the production and consumption of CFCs. Refrigerant conservation through recovery, recycling (R&R) and reclamation is one way of reducing emissions and encouraging timely phase out of CFCs in developing countries. CFC recovery, recycling and reclamation have been mandated in many developed countries. However, this practice is yet to make an impact in India although it is practiced in MAC sector to some extent. India is planning for the final phase out of CFCs in the RAC service sector, in which R&R will be one of the components.A model has been developed to assess the economics of R&R for some typical parameters in developing counties like India. The model suggests that the enterprises recycling 1500 kg/a will break even within 1 year for all scenarios. However, R&R may not be cost effective for small workshops and low volume refrigerant vendors until either the price of CFC goes up or the cost of R&R unit is subsidized. A nationwide survey on RAC service sector revealed that in India, there are very few enterprises handling more than 500 kg/a. Therefore, there is a need to provide the RAC service sector with adequate and innovative financial incentives.This paper attempts to study the issues related to R&R for various sub-sectors of RAC in developing countries with an emphasis on cost effectiveness. India is used as a model for this study.
Keywords: RAC; CFC; Recovery and recycling; Cost effectiveness; India;

A mechanism for the increase of pollution elements in dust storms in Beijing by Jinghua Guo; Kenneth A Rahn; Guoshun Zhuang (855-862).
It is well known that springtime dust storms in northern China are created by strong winds associated with cold fronts. One unexpected aspect of the dust, observed recently, is that it contains high concentrations of pollution elements. It is believed that dust particles were mixed well with pollution substances during the transport, thus the pollution elements are thought to be pollution derived. A careful examination of concentrations of elements, pollution gases, and meteorology during March 2001 and March 2002 in Beijing showed that the dust itself was an important source for some pollution elements. A more important by-product was the finding that dust storms in Beijing typically contain four stages: (1) accumulation of pollutants in the atmosphere of Beijing; (2) clear-out of pollutants; (3) addition of pure dust; and (4) clear-out of the dust. The pure dust can appear before the accumulated pollutants are cleared out, which mixes the two and makes it appear that the pollutants came from it. Pollution gases and PM10 also reveal the same four stages. When PM10 varies inversely with the pollution gases, as it does during dust storms in Beijing and occasionally in Shanghai, it includes transported dust. This pattern makes it possible to study dust storms by examining the longer records of pollution gases and PM10 that are now being generated by municipal environmental bureaus throughout China.
Keywords: Dust storm; Pollution element; Beijing aerosol; Source; Air mass;

An improved dry and wet deposition collector was built to monitor the atmospheric deposition of polycyclic aromatic hydrocarbons (PAHs) onto water in lakes or seas. In the improved collector the evaporated solution is replenished from a water supply reservoir by a tubing pump. Water vapor produced by a mini-space heater is sent up to the wet funnel by the vacuum pump (outlet) pressure and the water vapor pressure produced at a given temperature. The condensed water vapor is supplied into the wet funnel with a constant flow rate to prevent the channel formation in the wet adsorption cartridge. In a performance test of the developed deposition collector, the average recovery rate of 16 standard PAHs was 86% when using 30 ml of CH2Cl2 as an eluent for 10 g of ENVI-18 packed in the adsorption cartridge. The drawbacks, such as evaporation of surrogate solution for dry deposition, channel formation inside wet adsorption cartridge during dry periods and expensive cost for system building, which are commonly faced with measurement of atmospheric deposition of PAHs onto water surfaces have been substantially solved in this improved collector system. The total (dry and wet) atmospheric deposition of PAHs onto Ulsan Bay during the winter was much greater than that during the summer. This was mainly due to a difference in the amounts of fossil fuel used and the prevailing wind characteristics during each season. Dry deposition of PAHs was predominant during the winter, however, wet deposition was the major deposition during the summer. Most of the PAHs deposited onto Ulsan Bay had less than or equal to 4 aromatic rings. The atmospheric deposition of the PAHs with 2, 3 and 4 aromatic rings was 38.0%, 27.8% and 24.1%, respectively.
Keywords: PAH measurement; Channel formation; Water vapor; Seasonal variation; Fossil fuel uses;

Source characterisation of fine urban particles by multivariate analysis of trace metals speciation by Antonio José Fernández Espinosa; Miguel Ternero Rodrı́guez; Félix Fernández Álvarez (873-886).
A series of 41 samples of atmospheric particles below 0.61 μm were collected at 12 sites in a Mediterranean city. Meteorological data were related to analytical data. Direct observation technique was used to support the results. Total content and chemical form concentrations for 11 metals were determined, the chemical forms being determined through a specific speciation scheme for fine particles. All sampling sites were differentiated according to the level of metallic pollution. These levels were 40% higher on weekdays than at weekends. Organic-sulphidic chemical form levels of Cd, Pb, Mn and Ti were highest on weekdays. Sources from the sampling sites were influenced by local emissions more than by transferred pollution. Various sources were identified by a sequential principal component analysis as the resuspended soil particles, oil combustion, emissions from foundries and traffic.
Keywords: Chemical speciation; Toxic metals; Bioavailability; Fine particles; Sources identification; Principal component analysis;

The paper presents the trends in ambient SO2, NO x and O3 concentrations, SO4 2− and NO3 concentrations in precipitation and S/SO4 2− and N/NO3 wet-only atmospheric deposition over the period 1993–2001. The trends are presented for selected rural sites operated by the Czech Hydrometeorological Institute. The results show that the remarkable decrease of SO2 emissions during the 1990s was followed by a statistically significant downward trend in SO2 ambient levels, SO4 2− concentration in precipitation and S/SO4 2− wet-only deposition. S/SO4 2− wet-only deposition decreased by ca. 30% while ambient SO2 concentrations decreased by ca. 80%, in line with total Czech SO2 emissions over the period 1993–2001. In contrast, the ambient NO x concentrations show a significantly downward trend at only five out of 14 stations, while other stations do not have any trend. N/NO3 wet deposition does not show any trend between 1993 and 2001. This corresponds to a less pronounced decrease in NO x emissions. N/NO3 wet deposition decreased at some stations by ca. 10% (the trend is not statistically significant), while at some stations it had not decreased at all. Ambient NO x concentrations decreased by approximately 35%, NO x emissions decreased by ca. 30%. The SO2 limit value of 20 μg m−3 for vegetation protection, though vastly exceeded in the past, has not been exceeded since 1999, while the NO x limit value of 30 μg m−3 for vegetation protection has never been exceeded at the 14 stations analysed. Elevated O3 concentrations and, consequently, AOT40 limit exceedances were recorded at most of the stations under review.
Keywords: Ecosystems; Nitrogen oxides; Ozone; Sulphur dioxide; Czech Republic;

Seasonal variations in the atmospheric concentrations of non-filterable metals (NFM) have been observed using a newly developed Mist-UV sampling system in Auckland, New Zealand. We define NFM as volatile metal compounds and metal compounds in the nucleation and accumulation modes (<0.2 μm). Fifty-seven NFM samples were collected between September 2000 and August 2001, and five metals (Mg, Al, Cu, Sr and Ba) were detected in most samples during the campaign. Elevated atmospheric concentrations of Mg, Cu, Sr and Ba were detected in midsummer. Since the UV-oxidisable fraction was dominant, and high enrichment factors (EFcrust) were found for these metals, it is suggested that natural emission of organometals or reduced metals, were responsible for the high concentrations observed. Several elevated concentrations of Al and Sr as NFM were discovered in their annual profiles and these are attributed mainly to inorganic or oxidised metals associated with ultra-fine particulates. In summer Mg, Cu, Sr and Ba as NFM concentrations were 3–15 times higher than when associated with particulate matter (PM10). The emissions from automobile and seawater samples do not appear to be NFM emission sources, however, wet soils and dry soils were probably the sources of Mg+Ba and Al+Sr, respectively. Overall it is believed that metals released by natural sources were responsible for at least some of the metals detected, although a contribution from urban sources is possible. Our observations imply that atmospheric volatile and ultra-fine particulate metals cannot be ignored in the estimation of global metal circulation and budgets.
Keywords: Volatile metal compound; Metal circulation; Nanoparticles; Biological activities;

Eighteen PM2.5 samples that included three episodes were collected in Qingdao, China in spring 2002. Simultaneous total suspended particulate samples were also collected during two of the episodes for comparisons. Meteorological data along the back trajectories suggested that two of the episodes may have been the consequence of Asian dust storms. The chemical characteristics of selected elements and solvent-extractable organic compounds (SEOC) were determined to identify the origin of these materials. The elements Fe, Mg, Ca, Al, Ti, Mn and V in the dust samples were found to be of crustal origin and were transported to Qingdao from outside the area, while Pb and Zn were attributed to local (regional) emissions. Small variations in the distribution patterns and compositions were observed in the SEOC from the PM2.5 samples collected during the two dust episodes although there was a significant increase in total loading. The results suggest that the dust episodic aerosols possess strong local characteristics superimposed with a heavy crustal and possibly pollution influx transported with the dust particles during the storms. Because the sources of the aerosols were different, high pollutant concentrations were observed at Qingdao in two waves: the locally emitted aerosols such as vehicular exhaust rose first before the long-distance transported materials arrived and the locally emitted pollutants tended to stay in the atmosphere for a longer time.
Keywords: Aerosol; Solvent-extractable organic compounds; Metals; Asian dust storm;

Laboratory experiments were performed to study the “Fenton-like” reduction of methylhydroperoxide and ethylhydroperoxide by Fe2+ ions in aqueous solutions, under tropospheric conditions. The rate constants obtained at 6°C and pH=2 (16±5 and 24±9 M−1  s−1, respectively) are of the same order as the rate constant of H2O2 reduction by Fe2+. The major reaction products identified were the corresponding aldehydes (formaldehyde and acetaldehyde). The other identified organic products (the corresponding acids and hydroxy hydroperoxides) accounted for <10% of the total organic reaction products. Aldehydes and organic acids total production represented >87% of the consumed hydroperoxides. Between the two possible mechanisms (i.e. Fe2++ROOH→Fe3++OH+RO (M1) and Fe2++ROOH→Fe3++OH+RO (M2)), the experiments showed that M2 is negligible. The alkoxy (RO) radicals formed in (M1) further rearrange in water solution, and react with oxygen to form peroxy radicals which decompose, leading to the formation of aldehydes. This decomposition is in competition with (i) the self-reaction forming the corresponding acids and H2O2, and (ii) the reaction with HO2 radicals forming the corresponding hydroxy hydroperoxides. The atmospheric implications of these reactions are discussed.
Keywords: Laboratory experiments; Fenton reaction; Organic hydroperoxides; Methylhydroperoxide; Ethylhydroperoxide; Kinetic rate constant; Chemical mechanism;

Texture characterization of atmospheric fine particles by fractional Brownian motion analysis by Chin-Hsiang Luo; Che-Yen Wen; Jiun-Jian Liaw; Shih-Hsuan Chiu; Whei-May Grace Lee (935-940).
This study describes some preliminary results of a new approach which seeks to develop a method by which the surface structure of individual airborne fine particles can be defined and classified by a specific parameter, Hurst coefficient (H), which is directly related to the fractal dimension. The proposed method uses the fractional Brownian motion analysis to parameterize the microscopic images recorded from five particles with various components (Mugica et al., J. Aerosol. Sci., 33 (2002) 91–102). On the microscopic scale, the particle was segmented from its background via detection of H values of the selected regions of images. Furthermore, the central texture of the above five particles was parameterized by a set of H values. The range of H values was between 0.27 and 0.56, according to the fluctuation of the particle surface. Finally, some future applications of texture characterization of fine particles, which are currently under development, are briefly discussed.
Keywords: Fine particles; Texture; Microscopy; Fractional Brownian motion;