(1999) suggesting that anthropogenic sulfur and carbonaceous aerosols may reduce the surface irradiance in these regions by as much as 30 percent, which may result in a decrease in crop yield. Modeling results based on projections of future sulfur emissions also predict cooling (Engardt and Rodhe, 1998; Wolf and Hidy, 1997).
China is also a significant source of black carbon (also called elemental carbon) aerosols from incomplete combustion processes. Black carbon absorbs solar radiation, which results in warming of the atmosphere. Using a global climate model to study the influence of black carbon emissions in China on both regional and global climate, Menon et al. (2002) found that increases in black carbon emissions correlated roughly with observed severe weather patterns (e.g., heavy rainfall and droughts) and influenced regional temperatures as far away as North America.
“Indirect” forcing of climate by aerosols emitted in China may also affect global climate. The indirect effects are attributable to changes in the optical properties of clouds associated with the presence of anthropogenic aerosol particles. Although much of China has a characteristic haziness, not much is known about the sources of aerosols and their impact on radiation balance, climate, and plant growth.
Although little information is available on aerosol radiative properties in China, several studies report extremely high mass concentrations of aerosols in urban areas. Measurements of total suspended-particulate (TSP) mass made over a one-year period in several major cities averaged ~300 µg/m3; major sources include windblown dust and fossil fuel combustion (Hashimoto et al., 1994). This value is roughly four times the maximum allowable in the United States, according to the national ambient air quality standard (NAAQS) for TSP, which is 75 µg/m3. Waldman et al. (1991) report values of aerosol particle mass with diameters of less than 2.5 µm (PM2.5) over a two-week intensive sampling period of 139 µg/m3 (with a range of 54 to 207 µg/m3) in Wuhan, an industrialized city in central China. Particles of this size may not only accumulate in the lungs, but may also scatter and absorb light in visible wavelengths; the mean value is greater by more than a factor of 2 than the proposed U.S. PM2.5 NAAQS 24-hour average value of 50 µg/m3 (Waggoner and Weiss, 1980). Based on a study by Waldman et al. (1991), the contributions of sulfate, nitrate, and carbonaceous aerosol to PM2.5 are ~30 percent, 20 percent, and 50 percent, respectively. A recent year-long monitoring study in Beijing also found relatively high values of PM2.5, with a reported annual mean concentration of ~120 µg/m3 (Cao et al., 2003). The results showed that organic carbon, sulfate, and nitrate collectively account for ~60 percent of the fine particle mass in Beijing. Measurements made at several stations in the Pearl River delta region during January and February of