observations are over land, and we need to understand whether they are affected by local urban haze. Anthropogenic absorbing aerosols, by themselves, can reduce land-averaged solar radiation by about 3 to 5 W m−2 (Ramanathan et al., 1995; Jacobson, 2002). Such large reductions in surface solar radiation have implications for the hydrological cycle, since roughly 70 percent of the absorbed solar radiation is balanced by the latent heat flux of evaporation.

This global dimming may also be related to changes in the ratio of direct and diffuse solar irradiance received by vegetation. As documented by Gu et al. (2003), the increase in diffuse irradiance for the two years following the eruption of Mount Pinatubo in 1991 resulted in a 23 percent increase of noontime photosynthesis of a deciduous forest in 1992 under cloudless conditions. The increased diffuse irradiance permits a greater penetration of photosynthetically active sunlight into the canopy. However, if there is a sufficient reduction of total solar irradiance received at the ground, vegetation growth could be stunted. Chameides et al. (1999), for example, reported on reductions in crop yield in China due to reduced total solar irradiance from pollution aerosols. Krakauer and Randerson (2003) concluded from tree ring data that with respect to volcanic eruptions, the beneficial effect of aerosol light scattering for high northern latitudes appears to be offset entirely by the deleterious effect of eruption-induced climate change. Using field observations, Niyogi et al. (2004) found that increased aerosol loading led to increases in carbon assimilation for forests and crops and decreases for grasslands. The effect on carbon assimilation was larger with aerosols than with clouds, since clouds reduced the total solar irradiance more than the aerosols did.

Deposition of BC aerosols over snow-covered areas can result in changes to the surface albedo (Chylek et al., 1983). Further reductions in albedo occur due to the enhanced melting that accompanies the heating of absorbing soot particles in snow. Chylek et al. (1983) estimate this enhancement to be up to a factor of ten in the rate of melting. Recent model results indicate radiative forcings of +0.3 W m−2 in the Northern Hemisphere associated with albedo effects of soot on snow and ice (Hansen and Nazarenko, 2004).

Model Discretization of Aerosols

Typically the behavior of aerosol particles in the atmosphere has been described in models by discretization of both size and chemical composition. The continuous particle size spectrum is described by a limited number of modes, moments, or sections (Seinfeld and Pandis, 1998). For many problems, such as the evolution of marine aerosol, the computational simplicity of a few modes is sufficient to characterize changes in the particle



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