Mesospheric sulfate aerosol layer

We investigate the heretofore unstudied role of volcanic and nonvolcanic sulfur aerosols and gases in the mesosphere. Two‐dimensional microphysical calculations show that sulfur may be an important source of particles below the cold summer mesopause. Observed increases in SO 2 with altitude in the upper stratosphere had previously suggested ultraviolet destruction of H 2 SO 4 , discounting its survival in the mesosphere. Laboratory measurements have now ruled out ultraviolet photolysis of H 2 SO 4 , however, and a recent proposal of visible and near‐infrared photolysis of H 2 SO 4 explains the SO 2 observations. Our calculations show that enough H 2 SO 4 survives this weak photolysis mechanism to produce significant sulfate aerosol surface area in the mesosphere. Neutralization of H 2 SO 4 by metals on the surfaces of meteoritic dust is modeled and affects the mesospheric aerosol negligibly. We discuss the possible implications for this new class of particles for the formation of polar mesospheric summer echoes and polar mesospheric clouds in volcanically quiescent and active periods.