Arctic air pollution: An overview of current knowledge

From December to April, the Arctic air mass is polluted by man-made mid-latitudinal emissions from fossil fuel combustion, smelting and industrial processes. In the rest of the year, pollution levels are much lower. This is the outcome of less efficient pollutant removal processes and better south (S) to north (N) transport during winter. In winter, the Arctic air mass covers much of Eurasia and N. America. Meteorological flow fields and the distribution of anthropogenic SO₂ emissions in the northern hemisphere favor northern Eurasia as the main source of visibility reducing haze. Observations of SO₄²⁻ concentrations in the atmosphere throughout the Arctic yield, depending on location and year, a January–April mean of 1.5–3.9 μg m⁻³ in the Norwegian Arctic to 1.2–2.2 μg m⁻³ in the N. American Arctic. An estimate of the mean vertical profile of fine particle aerosol mass during March and April shows that, on average, pollution is concentrated in the lower 5 km of the atmosphere. Not only are anthropogenic particles present in the Arctic atmosphere but also gases such as SO₂, perfluorocarbons and pesticides. The acidic nature and seasonal variation of Arctic pollution is reflected in precipitation, the snowpack and glacier snow in the Arctic. A pH of 4.9–5.2 in winter and ~ 5.6 in summer is expected in the absence of calcareous wind blown soil. Glacial records indicate that Arctic air pollution has undergone a marked increase since the mid 1950s paralleling a marked increase in SO₂ and NO_x emissions in Europe. Effects of Arctic pollution include a reduction in visibility and perturbation of the solar radiation budget in April–June. Potential effects are the acidification and toxification of sensitive ecosystems.     

A simulation of sulfur wet deposition and its dependence on the inflow of sulfur species to storms

Numerical precipitation scavenging models are used to investigate the relationship between the inflow concentrations of sulfur species to precipitation systems and the resulting sulfur wet deposition. Simulations have been made for summer and winter seasons using concentration ranges of SO₂, aerosol SO₄²⁻, H₂O₂ and O₃ appropriate for the eastern U.S. summer simulations use one-dimensional timedependent convective cloud and scavenging models; winter simulations use two-dimensional steady-state warm-frontal models. Sulfur scavenging mechanisms include nucleation scavenging of aerosol, aqueous reactions of H₂O₂, O₃ and HCHO with S(IV), and nonreactive S(IV) scavenging. Over the wide range of conditions that have been examined, the relation between sulfur inflow and sulfur wet deposition varies from nearly linear to strongly nonlinear. The degree of nonlinearity is most affected by aerosol SO₄²⁻ levels and relative levels of SO₂ vs H₂O₂. Higher aerosol SO₄²⁻ levels (as found in summer) produce a more linear relation. The greatest nonlinearity occurs when SO₂ exceeds H₂O₂. Winter simulations show more nonlinearity than summer simulations.     

The scavenging of atmospheric sulfate by arctic snow

Scavenging ratios for sulfate on the south-central Greenland Ice Sheet at Dye 3 have been computed for 1982–1984. The ratios are based on measured concentrations in snow and estimated concentrations in air. The snow data have been obtained from snowpit samples which were dated by comparing δ¹⁸O values with meteorological records. The airborne concentrations have been estimated from data collected at coastal Greenland sites. Scavenging ratios resulting from this process are found to be in the range ~ 100–200 in winter and ~ 200–400 in summer. The greater summer values are attributed to increased riming, resulting in scavenging of sulfate as condensation nuclei and possible oxidation of SO₂ in cloudwater droplets. Using the airborne and snowpit concentrations with assumed dry deposition velocities of 0.02–0.05 cms⁻, it is estimated that dry deposition is responsible for roughly 10–30% of the total sulfate deposition on a year-round basis at Dye 3. During portions of the Arctic winter, however, when the snow is unrimed and when there is less precipitation, dry deposition may be dominant.     

Study of aerosol transport through precipitation chemistry over Arabian Sea during winter and summer monsoons

Precipitation samples over the Arabian Sea collected during Arabian Sea Monsoon Experiment (ARMEX) in 2002–2003 were examined for major water soluble components and acidity of aerosols during the period of winter and summer monsoon seasons. The pH of rain water was alkaline during summer monsoon and acidic during winter monsoon. Summer monsoon precipitation showed dominance of sea-salt components (∼90%) and significant amounts of non-sea salt (nss) Ca²⁺ and SO₄²⁻. Winter monsoon precipitation samples showed higher concentration of NO₃⁻ and NH₄⁺ compared to that of summer monsoon, indicating more influence of anthropogenic sources. The rain water data is interpreted in terms of long-range transport and background pollution. In summer monsoon, air masses passing over the north African and Gulf continents which may be carrying nss components are advected towards the observational location. Also, prevailing strong southwesterly winds at surface level produced sea-salt aerosols which led to high sea-salt contribution in precipitation. While in winter monsoon, it was observed that, air masses coming from Asian region towards observational location carry more pollutants like NO₃⁻and nss SO₄²⁻ that acidify the precipitation.     

Spatial and temporal variations of aerosol sulfate and trace elements in a sourcedominated urban environment

Time-resolved measurements of SO₂, sulfate, particulate carbon and trace metal (Pb, As, K, Mn, Fe and V) concentrations were performed simultaneously at four locations in Ljubljana, Yugoslavia, during February and April of 1985. During the winter three different SO₄²⁻ formation regimes are identified: A—morning period coinciding with maximum emissions and high humidity resulting in maximum SO₄²⁻ concentrations, with the sulfate formation during this period attributed to fast heterogeneous, aqueous oxidation of local SO₂ involving combustion products; B—late evening period with low humidity and high emissions when most SO₄²⁻ is primary; C—the remainder of the day when SO₄²⁻ appears to be of a regional origin and formed by a combination of heterogeneous and homogeneous processes. During the non-heating season, the SO₄²⁻ appears to be of regional origin.     

Evaluation of aerosol sources at European high altitude background sites with trajectory statistical methods

This study has investigated the influence of synoptic weather patterns and long-range transport episodes on the concentrations of several compounds related to different aerosol sources (EC, OC, SO₄^2?, Ca²⁺, Na⁺, K⁺, ²¹⁰Pb, levoglucosan and dicarboxylic acids) registered in PM10 or PM2.5 aerosol samples collected at three remote background sites in central Europe. Air mass back-trajectories arriving at these sites have been analysed by statistical methods. Firstly, air mass back-trajectories have been grouped into clusters. Each cluster corresponds to specific meteorological scenarios, which were extracted and discussed. Finally, redistributed concentration fields have been computed to identify the main potential source regions of the different key aerosol components. A marked seasonal pattern is observed in the occurrence of the different clusters, with fast westerly and northerly Atlantic flows during winter and weak circulation flows in summer. Spring and fall were characterised by advection of moderate flows from northeastern and eastern Europe. Significant inter-cluster differences were observed for concentrations of receptor aerosol components, with the highest concentrations of EC, OC, SO₄^2?, K⁺ and ²¹⁰Pb associated with local and mesoscale aerosol sources located over central Europe related to enhanced photochemical processes. Emissions produced by fossil fuel and biomass burning processes from the Baltic countries, Byelorussia, western regions of Russia and Kazakhstan in spring and fall also contribute to elevated levels of EC, OC, SO₄^2?, K⁺ and ²¹⁰Pb. In the summer period long-range transport episodes of mineral dust from North-African deserts were also frequently detected, which caused elevated concentrations of coarse Ca²⁺ at sites. The baseline aerosol concentrations in central Europe at the high altitude background sites were registered in winter, with the exception of coarse Na⁺. While the relatively high concentrations of Na⁺ can be explained by sea salt advected from the Atlantic, the low levels of other aerosol components are caused by efficient aerosol scavenging associated to advections of Atlantic air masses, as well as lower emissions of these species over the Atlantic compared to those over the European continent and very limited vertical air mass exchange over the continent.     

Organic and inorganic components of aerosols over the central Himalayas: winter and summer variations in stable carbon and nitrogen isotopic composition

The aerosol samples were collected from a high elevation mountain site, Nainital, in India (1958 m asl) during September 2006 to June 2007 and were analyzed for water-soluble inorganic species, total carbon, nitrogen, and their isotopic composition (δ¹³C and δ¹⁵N, respectively). The chemical and isotopic composition of aerosols revealed significant anthropogenic influence over this remote free-troposphere site. The amount of total carbon and nitrogen and their isotopic composition suggest a considerable contribution of biomass burning to the aerosols during winter. On the other hand, fossil fuel combustion sources are found to be dominant during summer. The carbon aerosol in winter is characterized by greater isotope ratios (av. ?24.0?‰), mostly originated from biomass burning of C₄ plants. On the contrary, the aerosols in summer showed smaller δ¹³C values (?26.0?‰), indicating that they are originated from vascular plants (mostly of C₃ plants). The secondary ions (i.e., SO₄ ^2?, NH₄ ⁺, and NO₃ ^?) were abundant due to the atmospheric reactions during long-range transport in both seasons. The water-soluble organic and inorganic compositions revealed that they are aged in winter but comparatively fresh in summer. This study validates that the pollutants generated from far distant sources could reach high altitudes over the Himalayan region under favorable meteorological conditions.     

Transport and chemical composition of the summer aerosol in the Norwegian Arctic

The chemical composition and transfer routes of the Arctic aerosol during summer have been studied at Ny-Alesund, Bjørnøya, Hopen and Jan Mayen in the period August/September 1983. Samples were also collected on mainland Norway to assess the origin of aerosols transported to the Norwegian Arctic. The concentrations of Si, Al, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Pb, Zn and Cu were measured in samples from a six-stage cascade impactor of Battelle design by particle-induced X-ray emissions (PIXE). The concentrations of Cd, Ni, Pb and Zn were also measured in samples from high-volume samplers by atomic absorption spectrophotometry (AAS).Three interesting periods were identified from the element concentrations. At the beginning of the measurement campaign, the air pollutants measured at Ny-Alesund and Hopen most likely originated in northern America and Greenland. A few days later, very high concentrations of Cd and Zn at Ny-Alesund seemed to be due to air mass transfer from the Soviet Union. During the last episode, observed at Ny-Alesund and Hopen in September, elevated concentrations of several anthropogenic pollutants appeared to be due to emissions in Europe.The results show that anthropogenic emissions from sources in western Europe, Eurasia and northern America may pollute the Arctic air not only in winter but in summer as well. Present levels of air pollutants in the Norwegian Arctic in summer are within the range of levels observed in other remote regions, but are one order of magnitude higher than in Antarctica.     

A simple model for diffusion of SO2 in Bergen

A model for computing daily mean SO₂ concentrations in Bergen, Norway, is developed and tested using SO₂ measurements from seven winter seasons during the 1970s. The meteorological predictors used are daily mean temperature, wind speed and temperature at two levels. Source strength is estimated from daily mean temperature. Correlations between observed and estimated SO₂ concentrations ranged between 0.8 and 0.9. Observed SO₂ levels declined by about 40 % from beginning to end of the observation period due to decreased SO₂ emission.     

Atmospheric transport and deposition of trace elements onto the Greenland Ice Sheet

Airborne particles of diameter > 0.4 μm reaching Dye 3, Greenland during April–May 1983 were highly variable in size and concentration from day to day. Five-day backward air mass trajectories suggest the importance of long-range transport from more northerly latitudes on days with high concentrations; particle sizes were larger on these days. Lower concentrations and smaller particle sizes were associated with transport from the south. It is inferred that Dye 3 may receive material emitted from Eurasian sources and transported over the Pole, similar to inferences for more northern Arctic sites.Elemental analysis of individual particles showed an abundance of crustal material, with many particles also containing sulfur. Bulk chemical analyses of airborne particles and fresh snow, collected during three snowstorms where ice nucleation dominated, provided data which were used to estimate mass-basis scavenging ratios. Average scavenging ratios were in the range ~1000–2000 for the crustal elements Al, Fe, K, Mg, Mn, and Na. Similar values were observed for Cd, Cu and NO₃⁻. The corresponding ratios for Pb and SO₄²⁻ averaged less than 200. These ratios were used with precipitation rate data to estimate wet deposition velocities in the order of ~2 cm s⁻¹ for the first nine species, and ~0.2 cm s⁻¹ for Pb and SO₄²⁻. Comparing fresh and older surface snow concentrations gave an average dry deposition velocity of roughly 0.2 cm s⁻¹ for the crustal elements, with the small fraction of large particles (~5–10 μm) dominating deposition; much smaller values were associated with the remaining species. When used with other data in the literature, the results of this study suggest that total deposition velocities of Pb and SO₄²⁻ may be as small as 0.05 cm s⁻¹ in relatively dry regions of the Arctic.     

Five years of air chemistry observations in the Canadian Arctic

Arctic air chemistry observations made in Canada between 1979 and 1984 are discussed. The weekly average concentration of 25 aerosol constituents has been measured routinely at three locations. Anthropogenic pollution typified by SO₄²⁻ and V has a persistent seasonal cycle. SO₄²⁻ concentrations are similar at all three locations, although they tend to be somewhat higher at Alert than at Mould Bay and Igloolik. The seasonal variation of an aerosol constituent depends on its source. There are four distinctive seasonal variations for:

• 1.(i) anthropogenic constituents Cr, Cu, Mn, Ni, Pb, Sr, V, Zn, H⁺, NH₄⁺, SO₄²⁻, NO₃⁻,
• 2.(ii) halogens (excepting Cl) Br, I, F,
• 3.(iii) sea salt elements Na, Mg, Cl and
• 4.(iv) soil constituents Al, Ba, Ca, Fe and Ti. In the Arctic winter, the mean concentrations of anthropogenic aerosol constituents, except SO₄²⁻, are 2–4 times lower than annual mean concentrations in southern Sweden near a major source region. SO₄²⁻ concentrations are only 30% lower mainly because of production from SO₂. Light scattering (b_scat) and SO₄²⁻ observations indicate that the SO₄²⁻ fraction of the fine particle mass fluctuates between 3 and 65% during the polluted winter months. Daily mean b_sact, at Mould Bay that exceeds 50 × 10⁻⁶m⁻¹ is associated with air originating from the northwest. The soluble major ion composition of aerosols during winter varies markedly with particle size. H⁺, NH₄⁺ and SO₄²⁻ dominate submicrometre particles while sea-salt ions Mg²⁺, Na⁺ and Cl⁻ predominate in supermicrometre particles. Winter SO₂ concentrations at Mould Bay and Igloolik ranged from 0.2 to 1.5 ppb
• 5.(v). The fraction of airborne S as SO₂ ranged from 20 to 90% and peaked in late December-early January. The concentration of total NO₃⁻ (0.025–0.090 ppb(v)) is much lower than that of SO₄²⁻ (0.3–1.2 ppb (v)).

     

Size mass distribution of water-soluble ionic species and gas conversion to sulfate and nitrate in particulate matter in southern Taiwan

A Micro-Orifice Uniform Deposition Impactor (MOUDI) and a Nano-MOUDI were employed to determine the size-segregated mass distributions of ambient particulate matter (PM) and water-soluble ionic species for particulate constituents. In addition, gas precursors, including HCl, HONO, HNO₃, SO₂, and NH₃ gases, were analyzed by an annular denuder system. PM size mass distribution, mass concentration, and ionic species concentration were measured during the day and at night during episode and non-episode periods in winter and summer. Average total suspended particle (TSP) concentrations during episode days in winter were as high as 153?±?33 μg/m³, and PM mass concentrations in summer were as low as one-third of that in winter. Generally, PM concentration at night was higher than that in the daytime in southern Taiwan during the sampling periods. In winter during the episode periods, the size-segregated mass distribution of PM mass concentration was mostly in the 0.32–3.2-μm range, and the PM concentration increased significantly in the range of 0.32–3.2 μm at night. Ammonium, nitrate, and sulfate were the dominant water-soluble ionic species in PM, contributing 34–48 % of TSP mass. High concentrations of ammonia (12.9–49 μg/m³) and SO₂ (2.6–27 μg/m³) were observed in the gas precursors. The conversion ratio was high in the PM size range of 0.18–3.2 μm both during the day and at night in winter, and the conversion ratio of episode days was 20 % higher than that of non-episode days. The conversion factor was high for both nitrogen and sulfur species at nighttime, especially on episode days.     

Wet and dry deposition monitoring in Southeastern Michigan

Wet and dry deposition as collected by a bucket were measured at two sites in southeastern Michigan for two years. The precipitation had an average pH of 4.27 and a SO²⁻₄ to NO⁻₃ ratio of 2.0. Particulate dry deposition velocities of 0.6 cm s⁻¹ for SO²⁻₄ and NO⁻₃ and > 2 cm s⁻¹ for Cl⁻, Ca²⁺, Mg²⁺,Na⁺ and K⁺ were calculated. The ambient particle composition, dry bucket collection and wet deposition were compared at two sites, one urban and the other rural. Higher ambient particle concentrations and dry deposition rates were measured at the urban site than the rural site, indicating the influence of local emissions. However, local emissions had no effect on the wet deposition concentrations. The influence of more distant source regions was examined by separating the precipitation events by wind direction. The events from the south and east had the highest SO²⁻₄ to NO⁻₃ ratios, which corresponded to the areas with the highest sulfur emissions. NO⁻₃ showed no directional dependence.Wet deposition was examined for the effect of storm type and seasonal trends. Contrary to a recent study on Long Island, we found higher concentrations of H⁺, SO²⁻₄ and NH⁺₄ in winter rain compared to snow. The wet deposition concentrations of H⁺, SO²⁻₄, and NH⁺₄ were highest in the summer, while only Na⁺ and Cl⁻ concentrations were highest in the winter, presumably due to winter road salting. The total deposition of acidic ions was highest in the summer and lowest in the winter, due both to lower concentrations and lower precipitation volumes in the winter. The dry deposition as collected by a bucket accounted for 1 % of total H⁺ deposition, 21 % of SO²⁻₄ deposition, 27% of NO⁻₃ deposition, 50% of Cl⁻ deposition and 61 % of Ca²⁺ deposition.     

Seasonal variation of water-soluble inorganic ions in PM10 in a city of northwestern Turkey

The objective of this study was to determine the concentrations of inorganic ions present in particulate matter smaller than 10 µm (PM₁₀), released into the environment by industrial, domestic and mobile sources in Duzce. To assess spatial variations, samples were collected from two sampling sites that had urban and suburban characteristics. Further, the process was carried out in two seasons to understand the seasonal variations. An ion chromatography device was used for analyzing the inorganic ion content in the collected samples. The highest levels of inorganic ion concentrations were measured at the urban sampling site during the winter campaign. Furthermore, the highest ion concentrations were measured for SO₄^2? ion at both sampling sites and during both seasons, while the lowest concentrations were measured for Br^?. Moreover, there were significant relationships between meteorological parameters and ion concentrations. A comparison of the cation and anion equivalence values using seasonal CE/AE (cation equivalence/anion equivalence) ratios showed that the aerosol matter had alkaline characteristics during both seasons. The mean value for the CE/AE ratios was 1.58 in winter and 2.06 in summer at the urban sampling site and 1.36 in winter and 1.52 in summer at the suburban sampling site. The interrelationships among the ions were determined by Pearson correlation analysis. Based on the correlation analyses, the ions emitted from common sources and those exposed to similar atmospheric conditions displayed strong correlations with each other.     

Seasonal variations in atmospheric SOx and NOy species in the adirondacks

This paper reports the results of over 2 years of measurements of several of the species comprising atmospheric SO_x (=SO₂+SO₄²⁻) and NO_y (=NO+NO₂ + PAN + HNO₃+NO₃⁻+ organicnitrates + HONO + 2N₂O₅ …) at Whiteface Mountain, New York. Continuous real-time measurements of SO₂ and total gaseous NO_y provided data for about 50% and 65% of the period, respectively, and 122 filter pack samples were obtained for HNO₃, SO₂ and aerosol SO₄²⁻, NO₃⁻, H⁺ and NH₄⁺. Concentrations of SO₂ and NO_y were greatest in winter, whereas concentrations of the reaction products SO₄²⁻ and HNO₃were greatest in summer. The seasonal variation in SO₄²⁻ was considerably more pronounced than that of HNO₃and the high concentrations of SO₄²⁻ aerosol present in summer were also relatively more acidic than SO₄²⁻ aerosol in other seasons. As a result, SO₄²⁻ aerosol was the predominant acidic species present in summer, HNO₃was predominant in other seasons. Aerosol NO₃⁻ concentrations were low in all seasons and appeared unrelated to simultaneous NO_y and HNO₃concentrations. These data are consistent with seasonal variations in photochemical oxidation rates and with existing data on seasonal variations in precipitation composition. The results of this study suggest that emission reductions targeted at the summer season might be a cost-effective way to reduce deposition of S species, but would not be similarly cost-effective in reducing deposition of N species. kwAcid deposition, seasonal variation, sulfate, nitrate, nitric acid, sulfur dioxide, oxides of nitrogen, hydrogen peroxide, ozone, air pollution, Adirondack Mountains     

Chemical composition and seasonal variation of acid deposition in Guangzhou, South China: comparison with precipitation in other major Chinese cities

With the aim of understanding the origin of acid rains in South China, we analyzed rainwaters collected from Guangzhou, China, between March 2005 and February 2006. The pH of rainwater collected during the monitoring period varied from 4.22 to 5.87; acid rain represented about 94% of total precipitation during this period. The rainwater was characterized by high concentrations of SO₄²⁻, NO₃⁻, Ca²⁺, and NH₄⁺. SO₄²⁻ and NO₃⁻, the main precursors of acid rain, were related to the combustion of coal and fertilizer use/traffic emissions, respectively. Ca²⁺ and NH₄⁺ act as neutralizers of acid, accounting for the decoupling between high SO₄²⁻ concentrations and relatively high pH in the Guangzhou precipitation. The acid rain in Guangzhou is most pronounced during spring and summer. A comparison with acid precipitation in other Chinese cities reveals a decreasing neutralization capacity from north to south, probably related to the role and origin of alkaline bases in precipitation.     

Photochemical oxidation and dispersion of gaseous sulfur compounds from natural and anthropogenic sources around a coastal location

The photochemical oxidation and dispersion of reduced sulfur compounds (RSCs: H₂S, CH₃SH, DMS, CS₂, and DMDS) emitted from anthropogenic (A) and natural (N) sources were evaluated based on a numerical modeling approach. The anthropogenic emission concentrations of RSCs were measured from several sampling sites at the Donghae landfill (D-LF) (i.e., source type A) in South Korea during a series of field campaigns (May through December 2004). The emissions of natural RSCs in a coastal study area near the D-LF (i.e., source type N) were estimated from sea surface DMS concentrations and transfer velocity during the same study period. These emission data were then used as input to the CALPUFF dispersion model, revised with 34 chemical reactions for RSCs. A significant fraction of sulfur dioxide (SO₂) was produced photochemically during the summer (about 34% of total SO₂ concentrations) followed by fall (21%), spring (15%), and winter (5%). Photochemical production of SO₂ was dominated by H₂S (about 55% of total contributions) and DMS (24%). The largest impact of RSCs from source type A on SO₂ concentrations occurred around the D-LF during summer. The total SO₂ concentrations produced from source type N around the D-LF during the summer (a mean SO₂ concentration of 7.4 ppbv) were significantly higher than those (≤0.3 ppbv) during the other seasons. This may be because of the high RSC and SO₂ emissions and their photochemistry along with the wind convergence.     

Aerosol measurements in central Alaska, 1982–1984

Atmospheric aerosols in subarctic central Alaska were studied for two winter seasons. Both optical absorptivity and excess (non-marine) sulfate undergo seasonal variation similar to that reported in Arctic locations (maximum in late spring and minimum in summer), but the magnitudes are a factor of two smaller than in the Arctic. The meridional variation in aerosol blackness and sulfur content (cleaner air to the south) is contrary to the trend in the Scandinavian Arctic and is interpreted to indicate the existence of pollution sources generally north and west, outside of Alaska's borders.Aerosol size varies with air temperature. Submicrometer aerosol mass and geometric mean diameter (GMD) increase as temperature decreases. Aerosols in all air masses studied were rich in volatile particles. The volatility suggests the presence of aqueous solutions of H₂SO₄. On the basis of (a) the relativity high aerosol volatility, and (b) the opposite trends between mean diameters and air temperature, it is conjectured that condensation of H₂SO₄ vapor may be an important mechanism for aerosol evolution in the winter (dark) polar troposphere.     

Significant geographic gradients in particulate sulfate over Japan determined from multiple-site measurements and a chemical transport model: Impacts of transboundary pollution from the Asian continent

We found a significant geographic gradient (longitudinal and latitudinal) in the sulfate (SO₄^2?) concentrations measured at multiple sites over the East Asian Pacific Rim region. Furthermore, the observed gradient was well reproduced by a regional chemical transport model. The observed and modeled SO₄^2? concentrations were higher at the sites closer to the Asian continent. The concentrations of SO₄^2? from China as calculated by the model also showed the fundamental features of the longitudinal/latitudinal gradient. The proportional contribution of Chinese SO₄^2? to the total in Japan throughout the year was above 50–70% in the control case, using data for Chinese sulfur dioxide (SO₂) emission from the Regional Emission Inventory in Asia (40–60% in the low Chinese emissions case, using Chinese SO₂ emissions data from the State Environmental Protection Administration of China), with a winter maximum of approximately 65–80%, although the actual concentrations of SO₄^2? from China were highest in summer. The multiple-site measurements and the model analysis strongly suggest that the SO₄^2? concentrations in Japan were influenced by the outflow from the Asian continent, and this influence was greatest in the areas closer to the Asian continent. In contrast, we found no longitudinal/latitudinal gradient in SO₂ concentrations; instead SO₂ concentrations were significantly correlated with local SO₂ emissions. Our results show that large amounts of particulate sulfate are transported over long distances from the East Asian Pacific Rim region, and consequently the SO₄^2? concentrations in Japan are controlled by the transboundary outflow from the Asian continent.     

Anthropogenic sulphate aerosol from India: estimates of burden and direct radiative forcing

A one-box chemical-meteorological model had been formulated to make preliminary estimates of sulphate aerosol formation and direct radiative forcing over India. Anthropogenic SO₂ emissions from India, from industrial fuel use and biomass burning, were estimated at 2.0 Tg S yr^-1 for 1990 in the range of previous estimates of 1.54 and 2.55 Tg S yr ^-1 for 1987. Meteorological parameters for 1990 from 18 Indian Meteorological Department stations were used to estimate spatial average sulphate burdens through formation from SO₂ reactions in gas and aqueous phase and removal by dry and wet deposition. The hydrogen peroxide reaction was found dominating for undepleted oxidant-rich conditions. Monthly mean sulphate burdens ranged from 2–10 mg m^-2 with a seasonal variation of winter–spring highs and summer lows in agreement with previous GCM studies. The sulphate burdens are dominated by sulphate removal rates by wet deposition, which are high in the monsoon period from June–November. Monthly mean direct radiative forcing from sulphate aerosols is high (−3.5 and −2.3 W m^-2) in December and January, is moderate (−1.3 to −1.5 W m^-2) during February to April and November and low (−0.4 to −0.6 W m^-2) during May to October also in general agreement with previous GCM estimates. This model, in reasonable agreement with detailed GCM results, gives us a simple tool to make preliminary estimates of sulphate burdens and direct radiative forcing.