Multiphase Atmospheric Chemistry: Implications for Climate
While it has been known for over a century that atmospheric aerosol particles are an important factor governing the interaction of solar radiation with the earth, both through direct influences on solar radiation and indirectly as cloud condensation nuclei, the large degree of variability in both aerosol composition and concentration precluded all but crude global appraisals of the actual climate forcing (W m2). The advent of global chemical reaction/transport/removal models, including parameterizations of heterogeneous processes, has made it possible to estimate the direct climatic forcing for sulfate aerosol and for condensed organic materials from biomass combustion. Thus the study of multiphase atmospheric chemistry has made possible a species-by-species and mechanism-by-mechanism approach to assessing these physical effects. Examination of the generalized heat balance equation for the earth suggests that there must be numerous other important aerosol species and several more mechanisms by which climate is affected. To date, the assessment of effects by anthropogenic sulfate and smoke from biomass combustion indicate that these aerosols cause a climatic forcing that, when averaged over the northern hemisphere, is comparable in magnitude but opposite in sign to the “greenhouse” forcing by CO2, CH4, chlorofluorocarbons, etc. Three steps are involved in these coupled chemical/radiative transfer models: simulation of the geographically, time and height dependent aerosol formation process, prediction of the microphysical properties as functions of the source processes or species characteristics, and coupling of these to geographically dependent radiative transfer calculations. We first present a description of the main aerosol production mechanisms and resultant physical properties. Subsequently we address the direct and indirect (CCN-cloud albedo) radiative forcings, leaving open the important questions of influence of aerosol particles on cloud amount and cloud-droplet longevity. Furthermore, we discuss multiphase chemical processes that affect the abundance of tropospheric O3, the latter being a potent greenhouse gas. We will emphasize the importance of the open scientific questions, concluding that it is not yet possible to quantify climate forcing by anthropogenic or natural aerosols fully. Finally, we describe the approach of the IGAC Multiphase Atmospheric Chemistry (MAC) Activity to improve the understanding of these issues.
KeywordsAerosol Particle Biomass Combustion Convective Cloud Cloud Droplet Sulfate Aerosol
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