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The effect of atmospheric aerosol on climate with special reference to surface temperature
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1970
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Description:A generalized model of the effect of thin atmospheric aerosol on the terrestrial heat budget is proposed, and applied to the problem of estimating the impact of aerosol on temperatures near the earth1s surface. The distinction between warming and cooling near the surface attributable to aerosol is found on the basis of this model to depend on whether the ratio of absorption (a) to backscatter (b) of incoming solar radiation by the aerosol is greater or less than the critical ratio (a/b)Q = C(l-A)(1-Ak)/[D(14A)-C(1-A)], where A is the surface albedo, C is the fraction of sensible to total (sensible plus latent) solar heating of the surface, D is the fraction of aerosol that is in convective contact with the surface, and k is a multiple of b that measures the relative aerosol backscattering efficiency with respect to solar radiation reflected upward from the surface.
A distinction is drawn between stratospheric aerosol (D = 0) which generally cools climate near the surface, and tropospheric aerosol (D—> 1) which may either cool or warm climate near the surface depending on various properties of the aerosol and of the surface itself. Over moist surfaces, such as vegetation areas and oceans, the critical ratio (a/b)Q is of order 0.1.Over drier surfaces, such as deserts and urban areas, (a/b)^ is of order unity. If the actual ratio a/b of most tropospheric aerosols is of order unity, as inferred by previous authors, then the dominant effect of such aerosols is to warm climate except over deserts and urban areas where the effect is somewhat marginal between warming and cooling.
Further climatic effects of aerosol are found likely to include a slight decrease of cloudiness and precipitation, and an increase of nplanetaryn albedo above the oceans although not necessarily above the continents. Suggestions by several previous authors that the apparent worldwide cooling of climate in recent decades is attributable to large-scale increases of particulate pollution of the atmosphere by human activities are not supported by this analysis.
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