|Working Group II: Impacts, Adaptation and Vulnerability|
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3.4.4. Scenarios of Tropospheric Ozone
Tropospheric ozone forms part of the natural shield that protects living organisms from harmful UV-B rays. In the lowest portion of the atmosphere, however, excess accumulations of ozone can be toxic for a wide range of plant species (Fuhrer, 1996; Semenov et al., 1998, 1999).Ozone is produced by a chain of chemical and photochemical reactions involving, in particular, NO, NO2, and VOCs (Finlayson-Pitts and Pitts, 1986; Derwent et al., 1991; Alexandrov et al., 1992; Simpson, 1992, 1995a; Peters et al., 1995). These chemical precursors of ozone can be human-derived (e.g., energy production, transport) or natural (e.g., biogenic emissions, forest fires). Surface ozone concentrations are highly variable in space and time (Table 3-2); the highest values typically are over industrial regions and large cities.
Global background concentrations of ground-level ozone (annual means) are about 20-25 ppb (Semenov et al., 1999). Background concentrations have increased in Europe during the 20th century from 10-15 to 30 ppb (Grennfelt, 1996). In the northern hemisphere as a whole, trends in concentrations since 1970 show large regional differences: increases in Europe and Japan, decreases in Canada, and only small changes in the United States (Lelieveld and Thompson, 1998). In an effort to reverse the upward trends still recorded in many regions, a comprehensive protocol to abate acidification, eutrophication, and ground-level ozone was signed in 1999, setting emissions ceilings for sulfur, NOx, NH3, and VOCs for most of the United Nations Economic Commission for Europe (UN/ECE) region.
Results from the first intercomparison of model-based estimates of global tropospheric ozone concentration assuming the new SRES emissions scenarios (see Section 3.8.1) are reported in TAR WGI Chapter 4. Estimates of mean ground-level O3 concentrations during July over the industrialized continents of the northern hemisphere under the SRES A2 and A1FI scenarios are presented in Table 3-2. These scenarios produce concentrations at the high end of the SRES range, with values in excess of 70 ppb for 2100 emissions (TAR WGI Chapter 4). Local smog events could enhance these background levels substantially, posing severe problems in achieving the accepted clean-air standard of <80 ppb in most populated areas.
Regional projections of ozone concentration also are made routinely, assuming various emissions reduction scenarios (e.g., SEPA, 1993; Simpson, 1995b; Simpson et al., 1995). These projections sometimes are expressed in impact termsfor example, using AOT40 (the integrated excess of O3 concentration above a threshold of 40 ppb during the vegetative period), based on studies of decline in tree growth and crop yield (Fuhrer, 1996; Semenov et al., 1999).
There are few examples of impact studies that have evaluated the joint effects
of ozone and climate change. Some experiments have reported on plant response
to ozone and CO2 concentration (Barnes et al., 1995; Ojanperä
et al., 1998), and several model-based studies have been conducted (Sirotenko
et al., 1995; Martin, 1997; Semenov et al., 1997, 1998, 1999).
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