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FIGURE 14.31 Calculated direct radiative forcing by sulfate, biomass, and fossil fuel black carbon (BC) + organic carbon (OC) particles in the (a) Northern Hemisphere, (b) Southern Hemisphere, and (c) global average (adapted from Penner et al., 1998).

by biomass particles is predicted to be large in the Southern Hemisphere and negative because of their contribution to light scattering. However, over some regions, e.g., the Sahara desert, such particles are predicted to lead to positive radiative forcing because they are over an already highly reflecting surface (see the earlier discussion). (Note that the size distribution assumed for the particles affects the absolute value pre-

dieted for radiative forcing; for example, input of a different size distribution than was used for Fig. 14.31 for the biomass particles gave a minimum direct radiative forcing in the Southern Hemisphere of —0.52 W m~2, significantly more negative than shown in Fig. 14.31b.) The contribution of fossil fuel black and organic carbon leads to a positive radiative forcing because of the absorption of solar radiation by black carbon (vide infra).

Organic constituents of particles are also likely to make a contribution to light scattering. For example, Li et al. (1998) studied aerosol particles over the east coast of Canada and found that unidentified species, likely organics, account for a large fraction ( ~ 2/3) of the mass in the 0.1- to l-^m range, in air masses with origins over the continental United States, these unidentified compounds were calculated to be responsible for 45-80% of the direct backscatter coefficient. There is also evidence of new particle formation likely involving organics from biological processes in coastal regions. For example, nucleation of new particles in the ultrafine size range (1.5- 5 nm) has been observed in coastal regions at Mace Head, Ireland, and the Outer Hebrides and was correlated to solar radiation and low tide (O'Dowd et al., 1998). This suggests that photochemical processes may lead to new organic particle formation and that the precursors may be biogenics from the shore regions.

The contribution of carbonaceous components to tropospheric aerosols off the east coast of the United States has been reported to vary from ~10% at low altitudes to >90% of the total aerosol mass at altitudes of ~3 km (Novakov et al., 1997). These carbonaceous components include both organics and elemental carbon, the latter estimated to be ~10% of the total carbonaceous mass. The larger fraction at higher altitudes may reflect more rapid removal of the inorganic components such as sulfate and nitrate through wet deposition at the surface. These carbonaceous materials contributed 66 ± 16% of the light scattering coefficient (Hegg et al., f 997), i.e., were major contributors to the direct effect of aerosol particles. Figure f4.32 shows some typical contributions to the light scattering measured during these studies (Hegg et al., 1997). While the total scattering, shown here in terms of the aerosol optical depth, varied from one set of measurements to another, it can be seen that liquid water in the particles, carbonaceous compounds, and sulfate were consistently the largest contributors, and in that order. Light absorption was a minor contributor to the aerosol optical depth in these studies. Since these measurements were made close to the earth's surface, the relative importance of organics when averaged over the troposphere may, however, be somewhat less.

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FIGURE 14.32 Typical contributions of aerosol liquid water, carbonaceous compounds, and sulfate in the lower troposphere of the east coast of the United States to the total aerosol optical depth. The contribution of light absorption is also shown. The different bars represent different sets of measurements during different flights (adapted from Hegg et al, 1997).

FIGURE 14.32 Typical contributions of aerosol liquid water, carbonaceous compounds, and sulfate in the lower troposphere of the east coast of the United States to the total aerosol optical depth. The contribution of light absorption is also shown. The different bars represent different sets of measurements during different flights (adapted from Hegg et al, 1997).

ft should be noted that while the focus of most of these studies has been on the contributions of species that are believed to have changed over time, e.g., those associated with human activities, there are also contributions due to natural processes. The latter are presumably not changing with time, or at least not at a significant rate compared to those due to anthropogenic activities (see discussion in Section E, however). For example, the carbonaceous aerosols measured by Novakov et al. (1997b) and Hegg et al. (1997) over the east coast of the United States may have been largely natural in origin (see Chapter 9.C.2) and hence not expected to show secular changes associated with anthropogenic activities. Similarly, Murphy et al. (1998) have measured the size-dependent composition of aerosol particles in the marine boundary layer in the Southern Ocean and shown that the contribution of sea salt particles to backscattering dwarfs that due to non-sea salt sulfate (nss). in addition, these particles comprised more than 50% of the cloud condensation nuclei, CCN (vide infra). (Note, however, that since the concentration of sea salt particles is highest in the marine boundary layer, this contribution to backscattering will not be characteristic of the global troposphere.) Sea salt particles are generated by natural processes, so that this contribution to backscattering has undoubtedly always been present and will not contribute to a secular trend. However, understanding its contribution, as well as those from other natural processes, in scattering of solar radiation is important to place the contribution of anthropogenically derived aerosol particles in perspective. In addition, as discussed later with respect to indirect effects of sea salt o

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