Lennart Granat

Brown Clouds over South Asia: Biomass or Fossil Fuel Combustion?

Carbonaceous aerosols cause strong atmospheric heating and large surface cooling that is as important to South Asian climate forcing as greenhouse gases, yet the aerosol sources are poorly understood. Emission inventory models suggest that biofuel burning accounts for 50 to 90% of emissions, whereas the elemental composition of ambient aerosols points to fossil fuel combustion. We used radiocarbon measurements of winter monsoon aerosols from western India and the Indian Ocean to determine that biomass combustion produced two-thirds of the bulk carbonaceous aerosols, as well as one-half and two-thirds of two black carbon subfractions, respectively. These constraints show that both biomass combustion (such as residential cooking and agricultural burning) and fossil fuel combustion should be targeted to mitigate climate effects and improve air quality.

Dry deposition of nitrogen dioxide and ozone to coniferous forests

The exchange of NO2, NO, and O3 between the atmosphere and coniferous forests has been studied by using a dynamic flow-through chamber technique. The measurements were performed during summer at two coniferous forest sites in Sweden, Jadraas (Scots pine) and Simlangsdalen (Scots pine and Norway spruce). In Simlangsdalen, the flux of NO2 was found to be quantitatively determined by the stomatal openings. Generally, the flux of NO2 was towards the vegetation surface, and only in a few cases a small emission of NO2 was detected. The average deposition velocities per projected needle area were 1.5 ± 1.3 mm s−1 for Norway spruce and 1.4 ± 1.1 mm s −1 for Scots pine. The results from Jadraas are slightly different; the deposition velocity of NO2, at concentrations lower than 1 ppbv, was always lower than the stomatal conductance and net emissions of NO2 were observed at concentrations below 0.5–0.7 ppbv. The average deposition velocity per projected needle area observed in Jadraas was 0.8 ± 0.7 mm s−1. In only a few cases, both for Jadraas and Simlangsdalen, the uptake of NO2 was limited by mesophyllic resistance. The difference between Simlangsdalen and Jadraas may be attributed to physiological variations or nutrient supply. The exchange rate of NO between the air and vegetation per projected needle area was generally found to be less than the detection limit of the method (0.1 ng N m−2 s−1 ). The deposition velocity per projected needle area for O3 varied typically from 0 to 15.7 mm s−1 in Jadraas and between 0 and 9.0 mm s−1 in Simlangsdalen, with average values of 3.1 and 2.5 mm s−1, respectively. The ratio between the deposition velocity for O3 and stomatal conductance was in general larger than one, indicating deposition of O3 to the external surfaces of the vegetation. The residual deposition of O3 showed a marked diurnal variation with maximum values around noon. It seems to follow a complex mechanism in which several factors such as light intensity, temperature, humidity and chemical properties of the surface of the cuticle might be important. No significant differences between Norway spruce and Scots pine species were observed in terms of the dry deposition of NO2 and O3. The dry deposition of NO2 to the forest floor is of the same magnitude as the deposition to the canopy. The average deposition velocity of NO2 to the forest floor in Jadraas was equal to 4 mm s−1. The maximum contribution from the dry deposition of NO2 to the total deposition of oxidized nitrogen compounds to the forest was estimated to be about 5% and 30% (as nitrogen) for Jadraas and Simlangsdalen, respectively. The emission flux of NO from the forest soil is at least 4 times smaller than the deposition rate of NO2 to the floor.

AN EVALUATION OF SULFATE IN EUROPEAN PRECIPITATION 1955–1982

More than 25 years of data on sulfate in precipitation from the European Air Chemistry Network (EACN) are analysed for seasonal and long-term trends and their spatial variability. The number of stations has varied between about 50 and 100, all of them located in the central and northern parts of western Europe. Despite considerable shortcomings of the data (indicated among other things by poor ionic balance during several years, particularly in the late 1950s and the early 1960s) the following conclusions are drawn. n nIn Norway, Sweden, Denmark and Finland, the sulfate concentration increased by roughly 50 per cent between the late 1950s and the late 1960s. A further increase during the 1970s is indicated at the Danish stations but most stations in Sweden and Norway show a decline by on the average about 20 per cent since the early 1970s. Sulfate data from the U.K. and the European continent seem to exhibit less systematic variations. n nA comparison between the long-term changes in the Scandinavian data on sulfate in precipitation and the corresponding changes in anthropogenic SO2 emission in Europe indicates a fair agreement with regard both to the increase in emission during the 1950s and 1960s and, except for the Danish stations, also the subsequent decrease in emission in several of the West European countries. n nIn most parts of the network the annual cycle of concentration of sulfate exhibits a maximum in February through May and a minimum in July to October, whereas the wet deposition normally has its maximum in May to August and minimum in December to March. A comparison with the annual cycle of anthropogenic emission, which has a clear maximum in December to March, indicates that the fraction of sulfur exported out of the region is larger in winter than in summer.

Background aerosol composition in Southern Sweden—Fourteen micro and macro constituents measured in seven particle size intervals at one site during one year

Abstract The aerosol composition as a function of particle size, has been studied at a rural location in southern Sweden, during a period of one year. The sulphur and heavy metal concentrations have been measured in 113 cascade impactor samples by particle induced X-ray emission (PIXE). Elemental concentrations were orders of magnitude lower than in central Europe but only a factor of two to four higher than the winter concentrations in the Arctic. The size distributions were rather broad except for some of the anthropogenic components. Enrichment factors calculated in both the fine and coarse particle mode gave information on possible anthropogenic contributions in the fine mode for the expected soil derived elements K and Mn. The samples were classified according to calculated air mass trajectories. Air masses having traversed the British Isles or central Europe contained up to an order of magnitude higher concentrations of known anthropogenic components e.g. S, V, Ni, Cu, Zn and Pb than air masses originating in the north Atlantic. The foreign contribution of the elemental concentrations was estimated to 70–80% for S and Pb, 50% for Ni and 30% for V. The anthropogenic components (except sulphur) showed the largest seasonal variations being highest in the winter. Dry deposition estimates indicate that the wet deposition was dominant for the fine particle oriented elements (S, V, Ni, Cu, Zn and Pb), while for the large particle oriented elements Ti and Fe dry and wet deposition were of equal importance. An appendix concludes this paper and presents elemental concentration frequency distributions in the fine and coarse mode.

Sulfate in precipitation as observed by the European atmospheric chemistry network

Abstract The temporal variation in sulfur concentration/deposition involves long term fluctuations which show striking similarities within certain areas but are different between areas in different positions relative to major sulfur emission areas in Europe. As an example, the deposition has been constant or decreasing during the last ten years in most of the area covered by the network, indicating that an increasing amount of the sulfur emitted in Europe is transported and deposited elsewhere, possibly in an eastward direction. The seasonal variation in deposition resembles that for amount of precipitation and shows a maximum during the summer or autumn for most areas. The concentration shows a maximum in the spring and a minimum in the autumn at most stations. The concentration field has a maximum approximately over Belgium and Holland with decreasing concentrations towards SW to NE (over W) but with comparatively high levels extending up over Finland. (Areas to the east and south are not covered by the network.) The deposition field was, based on a brief discussion of the relation between concentration and amount of precipitation, obtained as the product of these latter fields. High deposition rates are found in the south of Norway and in the middle of Britain in addition to the areas with high concentration. Sulfur and hydrogen ion are the dominating ions in precipitation followed by ammonium and nitrate except in coastal areas and in places where the soil is bare. Sources of errors in the data base are discussed and the results from a large number of additional sampling sites around and between the regular network sampling sites are most helpful in this regard. These latter measurements also permit an estimate of the uncertainty in areal concentration averages which are due to local and mesoscale variability. Finally, past and future importance of continuous measurements (as in the EACN) is discussed briefly.

Influence of air mass trajectories on the chemical composition of precipitation in India

Chemical composition of precipitation was measured with wet-only samplers at a rural site at Bhubaneswar in eastern India during 1997–1998. All rain events were compared with trajectories and precipitation fields from the ECMWF. The pH and ionic concentrations were found to vary systematically with the origin of air and the amount of rainfall along the trajectory. A seasonal cycle for pH was found with a monthly median pH below 5.0 during October–December. The highest monthly median concentration of Ca2+ was found in May with 20 μmol l−1 and for SO42− in January with 52 μmol l−1. Samples with trajectories within 400 km from Bhubaneswar during the last 5 days were found to have a median pH slightly below 5.0 as an average. These samples also had the highest concentration for all measured ions, indicating large pollution sources within the region. Samples with continental origin showed a decrease of ∼70% in concentration if there had been rain during >50% of the last 5 days compared to rain during <50% of the last 5 days. High concentrations of Na+ and Cl− were also found in continental samples. Resuspension of previously deposited sea salt is believed to be the reason. The data were compared with data from three other sites in western India and higher concentrations of almost all ions (NH4+ being the exception) compared to Bhubaneswar were found at the west coast in monsoon samples.

Dry deposition of SO2 and NOx in winter

Abstract The deposition velocity of SO 2 , NO and NO 2 to a coniferous forest in winter is estimated to be less than 0.03 cm s −1 . Deposition velocity to a snow cover depends on the temperature.

An experimental study of the dry deposition of gaseous nitric acid to snow

A chamber placed in a constant temperature freezing room was used to study the surface resistance during deposition of HNO3 to a snow surface. The resistance decreased with increasing temperature from larger than 5 s mm−1 at − 18°C to about l s mm−1 at −3°C. Measurements of gaseous and particulate nitrate concentrations during winter at a rural site in south central Sweden gave concentrations in the range of 0.4–5 μg HNO3 m−1 and 0.3–3 μg NO−3 m−3 with a mean value of 1.3 μg HNO3 m−3 and 0.7 μg NO−3 m−3, respectively. The results indicate that for periods with temperatures below − 2°C estimated dry deposition of HNO3 to snow is at most 4 % of measured wet deposition of nitrate in the area.

Foliar Nitrogen as an Indicator of Nitrogen Deposition and Critical Loads Exceedance on a European Scale

The foliar N content of bryophytes and Calluna vulgaris (L.) has been shown to be an indicator of atmospheric N deposition in the UK at a regional scale (1000km) and more recently on a smaller scale in the vicinity of intensive livestock farms. This work extends the geographical scale of the relationship between foliar N concentration of Calluna vulgaris and other ericaceous shrubs and N deposition with 2 measurement transects, one extending from northern Finland to southern Norway (2000 km) and the other extending from central Sweden to Stockholm, south east Sweden (330 km). Included in the second transect is a region of complex terrain in the Transtrand uplands, where the variation in N deposition with altitude and canopy cover was quantified using 210Pb inventories in organic soil. The relationship between foliar N (FN) and N deposition was shown to increase linearly with N deposition (ND) over the range 0.8% N to 1.4% N according to FN = 0.040ND + 0.793 (r2 = 0.70). The data are entirely consistent with earlier studies which together provide a valuable indicator of critical loads exceedance, the threshold value being approximately 1.5% N, which is equivalent to a N deposition of 20 kg N ha−1 y−1.

Regional background acidity and chemical composition of precipitation in Thailand

Abstract In a joint Thai-Swedish project, within the framework of the IGAC/DEBITS project, precipitation has been collected for chemical analysis on a daily basis since mid-1991 at two sites in the countryside of Thailand. The quality assurance plan includes investigation of the effect of different sampling equipment (including wet-only collectors), sampling time and stability of samples collected in tropical conditions. Consistent results, unbiased by local conditions, have been obtained. The data show striking day-to-day fluctuations, reflecting changes in transport direction and an inhomogeneous distribution of sources. For days with little precipitation the concentration of the components ranges from low to high values while the range is smaller and concentration generally lower during days of high precipitation amount. Mean concentrations are similar at the two stations but the correlation between daily samples is low as can be expected for a distance between the stations of 500 km. A seasonal variation could not be significantly determined. The best correlation between components is found for H + and SO 4 2− , mutually between Na + Cl − , Mg 2+ and Ca 2+ and between NH 4 + and NO 3 − . Absence of correlation between Ca 2+ and SO 4 2− suggests that SO 4 2− is not soil derived. The samples are generally slightly acidic which, stochiometrically, can be explained as an imbalance between acidifying components in modest to low concentration and neutralizing components in somewhat lower concentration. The concentrations are lower than those previously reported from stations in eastern and southern Asia, except for some very remote locations.