Organic species can be a significant component of the atmospheric aerosol. Sources include biogenic emissions ( Mazurek et al. [1991]), combustion ( Penner et al. [1992b]), and partial oxidation of hydrocarbon precursors ( Hildemann et al. [1993] and Turpin and Huntzicker [1991]). Some of these organic species may be hygroscopic and may play a role in indirect climate effects ( Novakov and Penner [1993]). Black carbon has been detected even in regions considered remote from anthropogenic sources ( Clarke [1989]). In contrast to sulfate and organic aerosol, it is a strong absorber of solar radiation, and thus can lead to warming rather than cooling.
Estimates of climate forcing by aerosols from biomass burning are
presented by Penner et al. [1992b]. The estimates (a global
average direct forcing of -0.8 W m
) are based upon
assumptions similar to those applied in the case of sulfate aerosol,
except that the aerosol both scatters and absorbs. The uncertainties
in this computation are substantially larger than those for the
sulfate forcing estimate.