Atmospheric Environment (v.34, #20)

A method is developed to estimate wet deposition of sulfate using routinely available meteorological data and air-monitored data. This approach takes into account different mechanisms of precipitation formation that determines sulfate concentration in precipitation water. Four different precipitating cloud types, including cold cloud, warm cloud, stratified layered cloud and convective cloud, according to their precipitation formations are incorporated differently to estimate sulfate concentration in precipitation water with the airborne sulfate concentration estimated by routinely available monitored data by regression equations. Two different regression equations for the estimation of airborne sulfate concentration are developed: one is applicable in winter, and the other in other seasons except for winter. The presently developed algorithm has been implemented in Seoul to estimate wet deposition of sulfate in 1997 when the measured wet deposition of sulfate is available. The estimated annual total wet deposition of sulfate in Seoul is found to be 3450  and 3770 kg km−2  yr−1 without and with consideration of the oxidation of SO2 in clouds, respectively, which is quite comparable with the measured value of 3510 kg km−2  yr−1. This result suggests that the present model can be used more practically in the field of acid rain study, especially in cases of the lack of monitoring sites for wet deposition.
Keywords: Wet deposition of sulfate; Airborne sulfate concentration; Cloud types; Sulfate concentration in precipitation; Regression model;

Estimation of sulfur deposition in South Korea by Soon-Ung Park; Hee-Jin In; Si-Wan Kim; Young-Hee Lee (3259-3269).
Dry and wet depositions of sulfur have been estimated using the routinely available meteorological data at 70 sites, air-monitored data at 27 sites and upper air soundings made daily twice (0000 and 1200 UTC) at three stations over South Korea for 4 yr from 1994 to 1997. The average annual total dry deposition and wet deposition of sulfur in South Korea are found to be 3 and 1.7 t km−2  yr−1, respectively. The ratio of dry-to-wet deposition of sulfur is found to vary with season with a maximum value of 6.6 in winter to a minimum value of 0.5 in summer in association with both precipitation amount and SO2 concentration. The maximum of annual total sulfur deposition exceeding 10 t km−2  yr−1 occurs in the southeastern part of Korea where big cities and industrial complexes are located, suggesting the importance of SO2 emission for deposition of sulfur.
Keywords: Dry deposition of sulfur; Wet deposition of sulfur; Total sulfur deposition; Precipitation mechanism; Inferential method;

NO x emissions from biomass burning of shifting cultivation areas from tropical deciduous forests of India – estimates from ground-based measurements by V. Krishna Prasad; Prabhat K. Gupta; C. Sharma; A.K. Sarkar; Yogesh Kant; K.V.S. Badarinath; T. Rajagopal; A.P. Mitra (3271-3280).
Biomass burning in the tropics is an important source of trace gases. Shifting cultivation in India is one of the major sources of biomass burning activities, during which due to combustion, several trace gases are emitted into the atmosphere. NO x emissions are relatively important as they affect the tropospheric photochemistry. In the present study, an attempt has been made to quantify the NO x emissions from shifting cultivation areas of tropical dry deciduous forests of India, at two sites. Fuel load, fuel characteristics, and nutrients with respect to carbon and nitrogen have been analyzed. Emission ratios have been calculated relative to CO2. Results of the study suggested that fuel load ranged from 12–15.3 t ha−1. Fire intensity at the first site was 3207 kcal s−1  m−1 and 2882 kcal s−1  m−1 at the second site. The differences in the fuel moisture were found to be less than 10% at both of the sites. The CO/CO2 ratio at the two sites did not vary much due to the low variation in moisture content. The carbon and nitrogen loads for the two sites ranged from 5603 t C ha−1 to 286 t N ha−1 and 6364 t C ha−1 and 239 N ha−1, respectively. Using the relation of estimating the emission factor (EF) for estimating the NO x emissions (EF (NO x )=−1.5+3.9nf, where, (r=0.95) EF(NO x ) is the emission factor for NO x in grams of nitrogen per kilogram of dry matter (g N kg−1  dm) burned and `nf’ is the percent of nitrogen bound within the plant matter, for tropical deciduous forests, suggests an emission rate of 7.0 (g N kg−1  dm) for the first site, and 4.7 (g N kg−1  dm) for the second site when compared to actual measured emission rates of 4.8 (g N kg−1  dm) for the first site and 3.5 (g N kg−1  dm) for the second site, indicating a higher estimate for the global regression relation used by Dignon and Penner. The emission factors computed from nitrogen content of the fuel material for tropical dry deciduous forests are nearer to the relation obtained for African Savanna fires. Comparison of the measured emission factors for the tropical deciduous forests with other ecosystems has been made in the paper.
Keywords: Biomass burning; Shifting cultivation areas; Tropical mixed deciduous forests;

Modeling of long-range transport on Taiwan's acid deposition under different weather conditions by Ken-Hui Chang; Fu-Tien Jeng; Ya-Ling Tsai; Pay-Liam Lin (3281-3295).
The long-range and transboundary transport of acid deposition precursors in East Asia has become an important issue due to the industrial development in this area in recent years. To represent an entire year's impact by long-range transport for Taiwan's acid deposition, six episodes in 1993 were selected for study using Taiwan Air Quality Model, which was developed on the basis of the Regional Acid Deposition Model system. The six episodes cover five types of weather conditions including the Northeasterly monsoon, Spring stationary front, Mei-Yu front, Summer shower, and Autumn front. Two emission conditions were simulated for each episode to quantify the long-range transport effect. One condition takes all emissions within the simulated domain into account as a base case, while another condition excludes Taiwan's emission and considers all of the other emissions as a control case. The results of the present study indicate that contributions of long-range transport to Taiwan's sulfur depositions range from 9 to 45% and nitrogen depositions from 6 to 33% for the six episodes. The most serious effect of long-range transport is the northeasterly monsoon episode for both total sulfur and total nitrogen deposition. The entire year contribution by long-range transport is estimated to be 39% for wet sulfate deposition, and 37% for wet nitrogen deposition for Taiwan.
Keywords: Air quality model; Transboundary transport; Impact ratio; Sulfur deposition; Nitrogen deposition;

The total of 28 different C2–C6 non-methane hydrocarbons including isoprene emitted from natural and anthropogenic sources in urban and rural sites of Kathmandu, capital of Nepal, were characterized for the first time in Nepal, in November 1998. Thirty-eight whole air samples were analyzed by using GC/FID. Ethene, acetylene and C4–C5 alkanes were identified as the source signature in Kathmandu urban ambient air. Hydrocarbon emissions from vehicular exhaust and gasoline evaporation sources were confirmed to outweigh natural gas and biogenic sources. Comparison of NMHCs profile normalized by acetylene between Kathmandu and Tokyo, Japan showed prominent difference in C2–C5 alkanes relative to acetylene. Significant amount of ethene at the rural site was assigned as emitted from biogenic sources because of its significant correlation with isoprene (R 2=0.52). Predominant amount of isoprene (average 278 pptv) observed at the urban site were assigned to be emitted from vehicular exhaust as it exhibited high correlation with other anthropogenically emitted hydrocarbons (R 2=0.72 with acetylene). Photochemical aging analysis showed that the mixing ratio variation of urban air transporting towards the rural site took place through its OH radical initiated oxidation and the observed alkanes in the rural site were mostly from the transported urban air.
Keywords: Non-methane hydrocarbon; Urban atmosphere; Anthropogenic source; Photochemical aging;

Organic and elemental carbon in fine particles at Kosan, Korea by Yong P. Kim; K.-C. Moon; Jong Hoon Lee (3309-3317).
Concentrations of elemental carbon (EC) and organic carbon (OC) in fine particles at Kosan, Cheju Island Korea were observed during seven intensive measurement periods between 1994 and 1997. PM2.5 were collected on a pre-fired quartz filter by a low volume aerosol sampler and EC and OC concentrations were quantified by the selective thermal oxidation method with MnO2 catalyst. The OC concentrations were higher than those at other background areas in the world but the EC concentrations were lower than or comparable to those at other background areas in the world. The OC and EC concentrations varied depending on air parcel trajectories, higher concentrations were observed when air parcels were from southern China than those from northern China. Regional marine background levels of EC and OC were observed when air parcels were from the North Pacific. Except summer, the OC and EC concentrations show good relationship, implying both OC and EC had the same emission/transport characteristics outside of Cheju Island. Their relationships were poor during August 1994 and July 1995 when air parcels were from the North Pacific. It might be due to the biogenic emissions of OC around the site during summer.
Keywords: PM2.5; Long-range transport; Background areas; Emission sources; TC/EC ratio;

Samples of fine and coarse fractions of airborne particulate matter were collected in an urban residential area of metropolitan Bangkok from June 1995 to May 1996 and in a suburban residential area in Pathumthani, Bangkok's boundary province, from September 1993 to August 1994. The samples were analyzed for elemental concentrations by instrumental neutron activation analysis. The data sets were then analyzed by positive matrix factorization followed by rotation to identify the possible sources of atmospheric aerosols in both areas. The best solutions were found to be six factors for elemental compositions of each of the fine and coarse particulate matter fractions at the urban site and five factors each for both fine and coarse fractions at the suburban location. Soil was the major source of airborne particulate matter identified for all data sets. The motor vehicle factor showed much higher concentration for Br in urban than in suburban area. A motorcycle factor with high concentrations of Zn and Mn were found at the urban site. The factor containing highest concentrations of Na and Cl was attributed to sea-salt and was clearly seen in the urban atmosphere. The site was located 35 km to the north from the Gulf of Thailand and was influenced by wind from the south and southwest for most of the year. Charcoal/wood burning and incineration factors were likely to be the local sources. A factor with high concentration of Ca was attributed to a construction near the urban residential site and from two plaster manufacturing factories close to the suburban residential site.
Keywords: Positive matrix facteorization; Receptor modeling; Airborne particulate matter; Thailand;

Exposure to volatile organic compounds while commuting in Taichung, Taiwan by Hsien-Wen Kuo; Hsien-Chi Wei; Chiu-Shong Liu; Yin-Yin Lo; Wen-Chin Wang; Jim-Shoung Lai; Chang Chuan Chan (3331-3336).
The objective of this study was to compare volatile organic compounds (VOC) concentrations from six main roads in Taiwan's third largest city (Taichung) and determine factors that affect VOC concentrations. Twenty-two VOC compounds were determined on six roads using US E.P.A method TO-17. 0.2 g of Carbopack B was used as an adsorbant collector and thermal desorption and gas chromatography/mass spectrometry (GC/MS) techniques were used to analyze VOCs. Results showed that car and motorcycle commuters were exposed to the highest VOC concentrations on Ta-Ya Road (2149 and 1343 μg m−3., respectively). Except for Ta-Ya Road and Chung-Kang Road, motorcycle commuters were exposed to higher VOC concentrations than their driving counterparts. Four sampling periods were used in the study; exposure to VOC concentrations was highest from 7:30 to 8:30 AM for motorcycle commuters (1515 μg m−3) and from 5:00 to 6:00 AM for car commuters (1071 μg m−3), while concentrations for all commuters were lowest from 2:00 to 3:00 P.M. Concentrations were negatively correlated to percentage of vacant lots along the road (PVL), yet not correlated to traffic volume. The ratios for toluene, benzene, xylene, and ethylbenzene (T : B : X : E) were similar for car commuters (5.5 : 2.1 : 2.6 : 1.0) and motorcycle commuters (5.7 : 2.0 : 2.6 : 1.0) which, combined with other published data, strongly suggest a vehicular source for hydrocarbons in Taichung. Comparing these concentrations, Taichung was slightly lower than Taipei but 2–30 times higher than cities of other countries. It is concluded that the chronic exposure to high concentrations of hydrocarbons may pose a serious health risk to commuters in Taichung
Keywords: Benzene; Toluene; Automobile emissions; Hydrocarbons;

In order to study the fundamental factors associated with anthropogenic source processes for atmospheric Hg, we analyzed the total gaseous mercury (Hg: TGM) concentration data collected during the late 1980s in Seoul, Korea. Since the consumption amount of coal in Korea, known as the major anthropogenic source of Hg, peaked during those time periods, inspection of the data distribution provided an opportunity to assess the role of coal burning on the temporal distribution characteristics of airborne Hg. The Hg data for the 1980s were characteristic of notably high concentrations typically exceeding 10 ng m−3. When the seasonal patterns of the Hg data were examined in terms of relative frequency over varying concentration ranges, the data were consistently exhibiting signatures of two different source processes. In the lower concentration range where the effect of natural sources appear to be dominant in relative sense, Hg concentrations tend to decrease across spring through winter. On the other hand, in the upper concentration range where the effects of anthropogenic sources are important, its concentrations tend to increase from spring to winter. The signature of anthropogenic sources in the late 1980s was supported not only by the unique seasonal distribution patterns but also by the consistent occurrences of high Hg levels (e.g., >20 ng m−3) throughout seasons. On the basis of our data analysis, we may conclude that the temporal distribution patterns of Hg measured at several locations of the Seoul Metropolitan city during the late 1980s are useful enough to distinguish the signature of anthropogenic sources such as the use of anthracite.
Keywords: Fossil fuel; Coal consumption; Airborne; Pollutant; Emission;

Seasonal variations in aerosol concentrations and compositions in the Nepal Himalaya by Arun B Shrestha; Cameron P. Wake; Jack E Dibb; Paul A Mayewski; Sallie I Whitlow; Gregory R Carmichael; Martin Ferm (3349-3363).
This paper summarizes a 15-month long study (September 1996–November 1997) on water soluble components (Na+, NH4 +, K+, Mg2+, Ca2+, Cl, NO3 , and SO4 2−) in atmospheric aerosols conducted at a remote Himalayan site and a rural Middle-Mountain site in Nepal. Most of the species sampled behave similarly in terms of seasonal and short-term variations. The aerosol concentrations were low during the second half of the monsoon and post-monsoon seasons and gradually increased during the winter season. Aerosol concentrations in some samples were up to 20 times higher during the pre-monsoon and early-monsoon seasons than they were in post-monsoon. The seasonal variations are clearly related to variations in atmospheric circulation. The high concentrations during the pre-monsoon season are attributed to regional scale valley wind systems and the maximum concentrations at both sites coincide with maxima in the local wind speed record. Occasional shifts in the large-scale circulation resulted in dramatic fluctuations in aerosol concentrations during the pre-monsoon and early monsoon seasons. In the middle of May the large-scale circulation changed from westerly to south-easterly monsoon in the Himalaya. The summer monsoon circulation brought considerable amounts of pollution to the sites from southeast. Precipitation played an important role in short-term (few samples, e.g., during the late winter at the Himalayan site) to longer-term (several samples, e.g., after mid-June at both sites) reduction in the aerosol concentrations, while the lack of precipitation in the pre-monsoon and early monsoon supported a gradual build-up of pollutants in the atmosphere. During the pre-monsoon and early monsoon, when concentrations were elevated at both sites, they were often higher at the remote Himalayan site than at the middle mountain site. A pollution layer in the monsoon circulation due to dry convective rise of pollutants at distant sources and horizontal upper air transport by monsoon flow could have caused these elevated concentrations.
Keywords: Aerosol; Seasonal variation; Monsoon air mass; Westerly air mass; Atmospheric circulation; Valley wind system; Water soluble major ions;

Rural particulate matter in Alberta, Canada by L Cheng; H.S Sandhu; R.P Angle; K.M McDonald; R.H Myrick (3365-3372).
Characteristics of particulate matter in rural Alberta locations were examined and compared with those in cities. Mean PM2.5 and PM10 concentrations at remote rural Alberta locations are 3.2 and 8.8 μg m−3, respectively. However, PM2.5 and PM10 concentrations at rural locations within 100 km of regional industrial and urban sources are 7.9 and 16.8 μg m−3, respectively. The mean PM2.5 to PM10 ratio in remote rural Alberta is about 0.4, similar to that observed in the cities. However, the ratio is about 0.5 in the rural locations influenced by regional sources. The PM2.5 values at remote rural locations are about one-third of those observed in urban centres in Alberta. PM2.5 concentrations in remote rural areas seem to constitute a “regional background” for Alberta. Sulphate content in PM2.5 at remote rural locations varies from 0.3 to 1.4 μg m−3. A mean PM2.5 sulphate content of 22% in rural Alberta is in the same range as that observed at nonurban sites in the northwestern United States.
Keywords: Inhalable particulate; Non-urban aerosols; Fine and coarse fraction; Air quality;

Optical properties of aerosols at Grand Canyon National Park by William C. Malm; Derek E. Day (3373-3391).
Visibility in the United States is expected to improve over the next few decades because of reduced emissions, especially sulfur dioxide. In the eastern United States, sulfates make up about 60–70% of aerosol extinction, while in the inner mountain west that fraction is only about 30%. In the inner mountain west, carbon aerosols make up about 35% of extinction, while coarse mass contributes between 15 and 25% depending on how absorption is estimated. Although sulfur dioxide emissions are projected to decrease, carbon emissions due to prescribed fire activity will increase by factors of 5–10, and while optical properties of sulfates have been extensively studied, similar properties of carbon and coarse particles are less well understood.The inability to conclusively apportion about 50% of the extinction budget motivated a study to examine aerosol physio-chemical–optical properties at Grand Canyon, Arizona during the months of July and August. Coarse particle mass has usually been assumed to consist primarily of wind-blown dust, with a mass-scattering efficiency between about 0.4 and 0.6 m2  g−1. Although there were episodes where crustal material made up most of the coarse mass, on the average, organics and crustal material mass were about equal. Furthermore, about one-half of the sampling periods had coarse-mass-scattering efficiencies greater than 0.6 m2  g−1 and at times coarse-mass-scattering efficiencies were near 1.0 m2  g−1. It was shown that absorption by coarse- and fine-particle absorption were about equal and that both fine organic and sulfate mass-scattering efficiencies were substantially less than the nominal values of 4.0 and 3.0 m2  g−1 that have typically been used.
Keywords: Visibility; Aerosol particle absorption; Aerosol scattering efficiencies;

Spatiotemporal maps of particulate matter (PM) concentrations contribute considerably to the understanding of the underlying natural processes and the adequate assessment of the PM health effects. These maps should be derived using an approach that combines rigorous mathematical formulation with sound science. To achieve such a task, the PM10 distribution in the state of North Carolina is studied using the Bayesian maximum entropy (BME) mapping method. This method is based on a realistic representation of the spatiotemporal domain, which can integrate rigorously and efficiently various forms of physical knowledge and sources of uncertainty. BME offers a complete characterization of PM10 concentration patterns in terms of multi-point probability distributions and allows considerable flexibility regarding the choice of the appropriate concentration estimates. The PM10 maps show significant variability both spatially and temporally, a finding that may be associated with geographical characteristics, climatic changes, seasonal patterns, and random fluctuations. The inherently spatiotemporal nature of PM10 variation is demonstrated by means of theoretical considerations as well as in terms of the more accurate PM10 predictions of composite space/time analysis compared to spatial estimation. It is shown that the study of PM10 distributions in North Carolina can be improved by properly incorporating uncertain data into the mapping process, whereas more informative estimates are generated by considering soft data at the estimation points. Uncertainty maps illustrate the significance of stochastic PM10 characterization in space/time, and identify limitations associated with inadequate interpolation techniques. Stochastic PM10 analysis has important applications in the optimization of monitoring networks in space and time, environmental risk assessment, health management and administration, etc.
Keywords: Particulate matter; Spatiotemporal mapping; Stochastic analysis; BME;

Atmospheric transport and wet deposition of ammonium in North Carolina by John T. Walker; Viney P. Aneja; David A. Dickey (3407-3418).
Wet deposition and transport analysis has been performed for ammonium (NH4 +) in North Carolina, USA. Multiple regression analysis is employed to model the temporal trend and seasonality in monthly volume-weighted mean NH4 + concentrations in precipitation from 1983 to 1996 at six National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites. A significant (p<0.01) increasing trend beginning in 1990, which corresponds to an annual concentration increase of approximately 9.5%, is detected at the rural Sampson County site (NC35), which is located within a densely populated network of swine and poultry operations. This trend is positively correlated with increasing ammonia (NH3) emissions related to the vigorous growth of North Carolina's swine population since 1990, particularly in the state's Coastal Plain region. A source–receptor regression model, which utilizes weekly NH4 + concentrations in precipitation in conjunction with boundary layer air mass back trajectories, is developed to statistically test for the influence of a particular NH3 source region on NH4 + concentrations at surrounding NADP/NTN sites for the years 1995–1996. NH3 emissions from this source region, primarily evolving from swine and poultry operations, are found to increase NH4 + concentration in precipitation at sites up to ≈80 km away. At the Scotland County (NC36) and Wake County (NC41) sites, mean NH4 + concentrations show increases of at least 44% for weeks during which 25% or more back trajectories are influenced by this source region.
Keywords: Ammonium; Ammonia; Multiple regression; Wet deposition; Back trajectories; Source–receptor;

Biogenic hydrocarbon emission estimates for North Central Texas by Christine Wiedinmyer; I. Wade Strange; Mark Estes; Greg Yarwood; David T. Allen (3419-3435).
Biogenic hydrocarbon emissions were estimated for a 37 county region in North Central Texas. The estimates were based on several sources of land use/land cover data that were combined using geographical information systems. Field studies were performed to collect species and tree diameter distribution data. These data were used to estimate biomass densities and species distributions for each of the land use/land cover classifications. VOC emissions estimates for the domain were produced using the new land use/land cover data and a biogenic emissions model. These emissions were more spatially resolved and a factor of 2 greater in magnitude than those calculated based on the biogenic emissions landuse database (BELD) commonly used in biogenic emissions models.
Keywords: Biogenic emissions; Isoprene; Land cover; GIS; Biogenic emissions modeling;

Atmospheric wet deposition of trace elements to Chesapeake and Delaware Bays by Guebuem Kim; Joseph R. Scudlark; Thomas M. Church (3437-3444).
The atmospheric wet depositional fluxes of selected trace elements (Al, Fe, Mn, Pb, Cd, Zn, Cu, Ni and Cr) were examined at two mid-Atlantic sites, which were located on the shore of the Delaware (Lewes) and Chesapeake (Stillpond) Bays from September 1995 to August 1996. Sampling was carried out on a weekly integrated basis, using “clean” techniques developed especially for trace elements. Monthly integrated depositional fluxes showed large variability in space and time for most of the measured elements, with similar ranges of variation for both sites. A good correlation (r2=0.80, Fe/Al=0.59) between Al and Fe fluxes at both sites suggests their major origin from the earth's crust (resuspended regional soils). Based on the enrichment factor calculation, it is suggested that at least half of the Cr and Mn and more than 90% of the Cd, Zn, Pb, Cu, and Ni are from non-crustal (presumably anthropogenic) sources. Seasonal variations of Al, Fe and Mn appear to be mainly controlled by local/regional agricultural activities, whereas Pb and Cd fluxes are largely dictated by precipitation amounts. Factors controlling fluxes of other elements (Zn, Cu, Ni and Cr) are not clearly distinguished due to large episodic inputs of local/regional contamination sources. A comparison of annual trace element fluxes during 1991–1996 reveals a factor of 2–4 higher fluxes of Zn, Cu and Ni during 1995–1996 at both sites, indicating regional (i.e., mid-Atlantic region) increase of the fluxes during the sampling period. The similar annual fluxes and trends at both sites suggest that these data can be employed to estimate atmospheric wet flux of trace elements to mid-Atlantic coastal waters (including Chesapeake/Delaware Bays and mid-Atlantic Bight).
Keywords: Precipitation; Mid-Atlantic bight; Enrichment factor; Pollution; Trace metals;

This paper examines differences between daily maximum weekday and weekend ambient ozone concentrations in the Tucson, AZ metropolitan area. Temporal variations in the Weekend Effect (i.e. weekend ozone concentrations are larger than weekday concentrations) are not explained entirely by changes in anthropogenic emissions of ozone precursor chemicals (i.e. nitrogen oxides and volatile organic compounds). A dramatic change from the Weekend Effect in June to the Weekday Effect (i.e. weekday ozone concentrations are larger than weekend concentrations) in July is associated with the onset of the North American Monsoon. A transition from a relatively dry atmosphere during the arid foresummer months of May and June to a relatively moist atmosphere during the monsoon months of July and August seems to explain the changes in ozone concentrations. Moist conditions are associated with increases in biogenic volatile organic compound (BVOC) emissions in the urban forest and surrounding desert areas. BVOC emissions appear to be an important source of VOCs, especially during the monsoon months. Therefore, an increase in ambient BVOC concentrations from June to July presumably reverses the sensitivity of ozone production in the Tucson area from VOC- to NO x -sensitive.
Keywords: Air pollution; Ozone; Climatology; Biogenic emissions;

Anomalous high sulphate levels during summer in the atmosphere over Israel and the Eastern Mediterranean sea have recently been reported. The usual explanation for this phenomenon has been long-range transport of sulphates emanating from industrial areas of eastern and central Europe. However, another possible source of the anomaly is marine biogenic production of sulphate from the oxidation of dimethylsulphide. Aerosols and gases were sampled on land, sea and air in central and northern Israel and analysed to determine inorganic ions, dimethylsulphide (DMS) and methanesulphonic acid (MSA). The results show that there is a definite, biogenic generation of sulphate from the Mediterranean sea and that this contributes to the sulphate content of the aerosol over Israel during the summer season. Using MSA as a tracer for DMS-derived sulphate, an attempt is made to assess the amount of this contribution. Based on MSA determinations and various reasonable assumptions about the behaviour of DMS, the biogenic contribution to the atmosphere is calculated to be between 6 and 22% (mean=11%) of the non-sea salt sulphate. These figures are supported by alternative calculations based on DMS data from independent sources. Lake Kinneret is possibly an additional minor source of atmospheric DMS in the region.
Keywords: Atmospheric sulphate; Sulphate biogenesis; Dimethylsulphide; Methanesulphonic acid; Sulphate anomaly;

Impact of traffic on black carbon aerosol concentration at la Réunion Island (Southern Indian Ocean) by Chatrapatty Bhugwant; Hélène Cachier; Miloud Bessafi; Jean Leveau (3463-3473).
To gain information on particle pollution by mobile sources, 3 experiments were conducted during the 1996–1998 period at Saint-Denis, the biggest urban site of La Réunion island (21.5°S; 55.5°E), situated in the Indian Ocean. Black Carbon (BC) concentrations were recorded with an Aethalometer which show high levels whatever the season (daily average: 270–650 ng m−3). At this site, a marked diurnal BC concentration variation is also evidenced in accordance with the observed traffic pattern. Measured daytime BC concentrations are 2–4 times greater than nighttime values. We checked that neither MBL height obtained by radio soundings nor wind speed or direction could explain satisfactorily the BC variations. A comparison with BC concentrations measured at other more remote sites of the island (Sainte-Rose and the altitude site Piton Textor) suggests that the background concentrations of the island are of the order 50 ngC m−3. These background values are almost never encountered in the main city (range: 80–2800 ngC m−3). We show that due to a singular convergence of parameters (topography of the island, road network, movement of population, quality of fuel), the city of Saint-Denis appears as polluted as continental European big cities.
Keywords: Black carbon; Tropical atmosphere; Urban site; Traffic pollution; Diesel;