Middle Atmosphere Aerosols and Their Effects

Abstract

The middle atmosphere holds a rich variety of particulate matter, ranging from meteoritic debris to sulfate aerosols to polar stratospheric ice clouds. Volcanic eruptions strongly perturb the stratospheric sulfate (JUNGE) layer. High-altitude “noctilucent” ice clouds condense at the cold menopause. The properties of these particles—including composition, size and geographical distribution—are discussed, and their global effects—including chemical, radiative and climatic roles—are surveyed. Polar stratospheric clouds (PSCs) catalyze reactions of chlorine compounds that “activate” otherwise inert chlorine reservoirs, leading to severe ozone depletions in the southern polar stratosphere during austral spring. PSCs also modify the composition of the polar stratosphere through complex physicochemical processes, including dehydration and denitrification. The sulfate aerosol layer can reflect solar radiation and increase the planetary albedo, thereby cooling the surface and altering the climate. Major volcanic eruptions, which enhance the stratospheric sulfate aerosol burden by a factor of 100 or more, may cause significant global climatic anomalies. Sulfate aerosols also appear capable of activating stratospheric chlorine reservoirs on a global scale. Accordingly, if atmospheric concentrations of chlorine (associated with anthropogenic use of chlorocarbons) increase by a factor of two or more in future decades, significant worldwide ozone depletions could occur.