Sulfate atmospheric deposition

Health effects attributed to sulfur oxides are likely due to exposure to sulfur dioxide, sulfate aerosols, and sulfur dioxide adsorbed onto particulate matter. Alone, sulfur dioxide will dissolve in the watery fluids of the upper respiratory system and be absorbed into the bloodstream. Sulfur dioxide reacts with other substances in the atmosphere to form sulfate aerosols. Since most sulfate aerosols are part of PMj 5, they may have an important role in the health impacts associated with fine particulates. However, sulfate aerosols can be transported long distances through the atmosphere before deposition actually occurs. Average sulfate aerosol concentrations are about 40% of average fine particulate levels in regions where fuels with high sulfur content are commonly used. Sulfur dioxide adsorbed on particles can be carried deep into the pulmonary system. Therefore, reducing concentrations of particulate matter may also reduce the health impacts of sulfur dioxide. Acid aerosols affect respiratory and sensory functions. 

Acid rain monitoring data in North America have been gathered by Environment Canada and stored in the National Atmospheric Chemistry (NatChem) Database, details of which can be found at www.airquality.tor.ec.gc.ca/natchem. Analysis of the deposition chemistry data has confirmed that wet sulfate deposition did indeed decline in concert with the decline in SO2 emissions in both eastern Canada and the... 

Sulfur oxyanions are similar to nitrate and nitrite in that they are mobile in soil and can be converted to many different forms (see Chapter 4, Figure 4.8). However, they are different from the other oxyanions in that they are the source of the essential nutrient, sulfur, and are deposited on soil from the atmosphere. Sulfate species can be determined by X-ray fluoresence (XRF), making their determination easier than the other oxyanions [21,22], Sulfur is discussed in more detail in Section 6.2.23. 

There are a number of held measurements that have addressed this relationship between the mandated reductions in S02 emissions in the United States and the subsequent changes in sulfate deposition downwind. For example, one analysis of the trends in the atmospheric concentrations of sulfate in the northeastern United States suggests that from 1977 to 1989, the sulfate concentration decreased by about 22-28% during which the emissions of S02 were estimated to have decreased by 25% (Shreffler and Barnes, 1996). 

Fay. J. A., Golomb, D. Kumar, S. 1985. Source apportionment of wet sulfate deposition in Eastern North America. Atmospheric Environment, 19, 1773-1782. 

Atmospheric S Deposition. Increased S content of recent sediments is frequently attributed to increased atmospheric deposition of S042" (e.g., 7, 28-30, 49-51, 199). Increased supply of sulfate is thought to have in-... 

The discovery of the anomalous oxygen isotopic compositions of atmospheric sulfate provides a new means for identifying sulfate of atmospheric origin. Rainwater and aerosols from southern California were found to have A O values in the range of 0%o to -K.5%o (Lee et al., 2001). The average A O of snow sulfate in the Rocky Mountains (Colorado, USA) was - -1.3%o. Sulfate in ice cores, massive sulfate deposits, and Dry Valley soils from various locations also have MIF (Bao et al., 2000 Lee et al., 2001). There appears to be seasonality in the A O of sulfate in precipitation, with higher values in the winter and lower values in the summer, probably due to seasonal changes in climatic elfects that favor aqueous phase S(IV) oxidation in winter relative to summer (Lee and Thiemens, 2001). 

Using sulfur-35. Sulfur-35 ( S) is a naturally produced radioactive tracer (half-life = 87 d) that can be used to trace the movement of atmospherically derived sulfate in the environment. It is formed in the atmosphere from cosmic ray spallation of " °Ar (Peters, 1959), and deposits on the Earth s surface in precipitation or as dryfall. It can be used both to trace the timescales for movement of atmospheric sulfate through the hydrosphere and, in ideal cases, to trace the movement of young (<1 yr) water. It is an especially useful tracer in regions away from the ocean where sulfate concentrations are relatively low. 

Sulfur-35 was also used to determine the source of the sulfate in stream water in a highly polluted watershed in the Czech Republic (Novak et al., 2003). The sulfate input to this watershed has decreased over the past decade because of new controls on sulfur emissions, but the watershed continues to export sulfate far in excess of the atmospheric loading at the present time. Measurement of in runoff indicates that none of the recently deposited sulfate is exiting in the system. Apparently the atmospheric sulfate interacts with the soil layers and consequently takes more than a year to be removed from the watershed (Novak et al., 2003). 

Globally, about half of all atmospheric sulfate is derived from combustion of fossil fuels and half from natural sources (Berner and Bemer 1987). It has been estimated that anthropogenic sources are responsible for 90% of the total atmospheric sulfur deposition in eastern North America, which occurs as dry deposition of SO2 gas and sulfate particles or dissolved in rain. The highest amounts of sulfate and nitrate in U.S. rain, which are found in the northeast [Fig. 8.7(a) and Fig. 8.7(b)], are... 

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we noted in Chapter 17, the aqueous-phase pathway is estimated to be responsible for more than half of the ambient atmospheric sulfate concentrations, with the remainder produced by the gas-phase oxidation of S02 by OH (Walcek et al. 1990 Karamachandani and Venkatram 1992 Dennis et al. 1993 McHenry and Dennis 1994). These results are in agreement with box model calculations suggesting that gas-phase daytime S02 oxidation rates are l-5% per hour, while a representative in-cloud oxidation rate is 10% per minute for 1 ppb of H202. 

Lake ecosystems are also complex, but they sometimes respond more quickly than do forest ecosystems to reductions in sulfur dioxide emissions. In many lakes, however, the decline in sulfate concentrations is less than expected given the drop in atmospheric sulfur dioxide deposition, and the pH shows surprisingly little change. As Peter Dillon et al. describe in Chapter 4, water quality in lakes in Ontario improved very little despite a major reduction in atmospheric acid deposition because sulfates are carried into the lakes from the surrounding watershed in stream water, especially following drought years. Sulfur dioxide emissions will need to be lowered even more before accumulated sulfates are removed from this system. 

Formation sequences for compounds in zinc corrosion products formed under sheltered conditions dominated by chloride deposition (left sequence) and sulfate deposition (right sequence). (From Odnevall, I. and Leygraf, C., Reaction Sequences in Atmospheric Corrosion of Zinc, ASTM STP 1239, W. W. Kirk and H. H. Lawson, eds., p. 215, 1995, Courtesy of the American Society for Testing and Materials, Philadelphia, PA.)... 

Acid rain is precipitation polluted by acidification with atmospheric pollutants. These pollutants include emissions of oxides of nitrogen (NOx), oxides of sulfur (SOx), and hydrogen chloride radicals. Various strengths of nitric acid, sulfuric acid, and hydrochloric acid result. Key indicators of acid rain include emission levels of NOx and SOx, wet sulfate deposits, and trends in acidity in lakes and other freshwater bodies. An increase in emissions that increases the level of any of these indicators will bring environmental, r atory, and potentially public and special-interest group pressures to bear on a plant. Sample measurements of these indicators on a national or provincial scale arc illustrated in Figs. A-4 to A-7. 

Acid Deposition. Acid deposition, the deposition of acids from the atmosphere to the surface of the earth, can be dry or wet. Dry deposition involves acid gases or their precursors or acid particles coming in contact with the earth s surface and thence being retained. The principal species associated with dry acid deposition are S02(g), acid sulfate particles, ie, H2SO4 and NH HSO, and HN02(g). Measurements of dry deposition are quite sparse, however, and usually only speciated as total and total NO3. In general, dry acid deposition is estimated to be a small fraction of the total... 

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