The chemistry of the stratosphere is strongly influenced by the presence of small particles composed of sulfuric acid, nitric acid and other materials. The ubiquitous background stratospheric aerosol layer is composed of sulfuric acid droplets, while the clouds observed in the polar winter stratosphere (the polar stratospheric clouds, or PSCs) are composed of nitric acid ices. Chemical reactions can occur efficiently on the particle surfaces, and in solution in the case of liquid droplets. Such reactions affect the concentrations of chlorine and nitrogen species in the lower stratosphere, and play a critical role in ozone depletion. Indeed, the “ozone hole” has been shown to be initiated by “heterogeneous” reactions occurring on PSC particles. The origins and properties of sulfate aerosols, PSCs and other observed stratospheric particles are surveyed. Anthropogenic influences on these aerosols are discussed. The heterogeneous chemistry of polar stratospheric clouds, and the chemical processing of air in contact with such clouds, are illustrated using detailed model simulations. The injection of sulfur and chlorine into the stratosphere by volcanic eruptions is also investigated. HCl scavenging in volcanic eruption plumes is quantified based on an analysis of the dynamics, physical chemistry and microphysics of eruption columns. It is shown that very little chlorine is likely to enter the stratosphere in volcanic plumes because of efficient HCl absorption in supercooled water that condenses on sulfuric acid aerosols. The possible role of sulfate aerosols — both of volcanic and background origin — as a medium for heterogeneous chemical reactions is assessed. It is argued that the sulfate aerosols can produce significant chemical pertubations in regions of the atmosphere where temperatures drop below about 200 K. The potential contribution of sulfate aerosols to ozone depletion at high latitudes is discussed. Outstanding scientific issues concerning stratospheric aerosols and their chemical effects are summmarized.
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- Heterogeneous Chemistry of Polar Stratospheric Clouds and Volcanic Aerosols
R. P. Turco
- Springer Berlin Heidelberg