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Sulfur dioxide and nitrogen compounds are among the major air pollutants emitted by industrial and domestic sources. SO2 is further oxidized to sulfate, which exists in the atmosphere mainly as aerosols. The main anthropogenic components of emissions of nitrogen compounds to the atmosphere are NOx and ammonia (NH3). Increased atmospheric SO2 concentrations from anthropogenic sources are known to have negative effects on tree growth and crop yield (Kropff, 1989; Semenov et al., 1998) and are described below. Concentrations of nitrogen compounds are not considered because scenarios seldom are required for impact studies. However, wet and dry deposition of sulfur and nitrogen from the atmosphere onto the Earth's surface can lead to acidification, with detrimental effects on soils, surface waters, building materials, and ecosystems (Grennfelt et al., 1996). Nitrogen deposition may serve simultaneously as a plant fertilizer, positively influencing carbon gain in forests (Reich et al., 1990; Woodward, 1992; Petterson et al., 1993). Thus, deposition scenarios also are important.
Current global background concentrations of SO2 are monitored at stations belonging to the Background Atmospheric Pollution Monitoring Network (BAPMoN), established by the World Meteorological Organization (WMO) and UNEP, as well as in regional networks. Annual mean SO2 concentrations ([SO2]) over land areas are estimated to be approximately 0.1-10 mg m-3 (Rovinsky and Yegorov, 1986; Ryaboshapko et al., 1998). However, they can be much higher locally (Table 3-2). For example, annual average values of more than 80 mg m-3 were measured at some sites in Czechoslovakia in the 1970s (Materna, 1981). Model results have shown that [SO2] averaged over the vegetative season reached 35 mg m-3 in some regions of Europe during 1987-1993 (Semenov et al., 1998, 1999). In recent years reductions of SO2 and NO2 emissions have been recorded in many regions, accompanied by large-scale decreases in concentrations, especially evident in remote areas (Whelpdale and Kaiser, 1997). Typical rates of regional total (dry + wet) deposition of sulfur and nitrogen compounds, based on model simulations, are shown in Table 3-2.
Reference concentrations of SO2 adopted in impact assessments vary according to the objective of the study. For example, in some field experiments an enhanced [SO2] treatment is compared to a control case at ambient background concentrations. The latter concentrations can vary from year to year, depending on ambient weather and air quality conditions (Kropff, 1989). Alternatively, other experiments at locations close to pollution sources have used air purification systems to attain preindustrial levels of [SO2] in closed chambers, comparing plant responses to those under (locally high) ambient concentrations.
Several models have been developed to project atmospheric concentrations and deposition of sulfur and (in some cases) nitrogen compounds. At the regional scale these models include: for Europe, RAINS (Alcamo et al., 1990; Schöpp et al., 1999) and ASAM (ApSimon et al., 1994), both of which use output from mechanistic models developed by the Co-operative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe (EMEP); for Asia, RAINS-Asia (Foell et al., 1995); and for North America and Asia, ATMOS (Arndt et al., 1997). There also are global models: GRANTOUR (Penner et al., 1994), MOGUNTIA (Langner and Rodhe, 1991), ECHAM (Feichter et al., 1996), and STOCHEM (Collins et al., 1997).
There have been few studies of the joint impacts of acidifying compounds and climate change. Some of these studies are multifactorial model simulations of plant response (e.g., Semenov et al., 1998). There also have been some modeling studies based on the IS92a emissions scenario (Posch et al., 1996; Fischer and Rosenzweig, 1996), under which a substantial increase in annual sulfur deposition is projected to occur by 2050, with commensurate suppression of modeled GHG warming in some regions. However, this scenario is now thought to overestimate future emissions of sulfur (Grübler, 1998), as reflected in the new SRES scenarios (see Section 3.8.1). Not all of the models used in developing the SRES scenarios provide information on nitrogen emissions, but those that do can be used to produce consistent scenarios of [NOx], [SO2], sulfur and nitrogen deposition, and climate change for impact studies (Mayerhofer et al., 2000; Stevenson et al., 2000; see Table 3-2).
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