A 2D microphysical model of the polar stratospheric CN layer

Each spring a layer of small particles forms between 20 and 30 km altitude in the polar regions. We present the first self-consistent explanation of t he observed "CN layer" from a 2D microphysical model of sulfate aerosol. Ou r theory relies on photolysis of H2SO4 and SO3, consistent with recent labo ratory measurements, to produce SO2 in the upper stratosphere and mesospher e. An additional source of SO2 may be required. Nucleation throughout the p olar winter extends the top of the aerosol layer to higher altitudes, despi te strong downward transport of ambient air. This may affect heterogeneous chemistry at the top of the aerosol layer in polar winter and spring.