Photolysis of sulfuric acid vapor by visible light as a source of the polar stratospheric CN layer

[1] We present the first microphysical calculations confirming that photolysis of sulfuric acid vapor by visible light is responsible for the formation of the springtime CN layer'' observed in the polar stratosphere. Our calculations show that the recently proposed photolysis mechanism is also sufficient to explain observations of vertically increasing SO2 mixing ratios in the upper stratosphere. Such photolysis, however, does not sufficiently explain the limited observations above 40 km of vertically decreasing H2SO4 and SO3 vapor, suggesting an additional loss mechanism there. We rule out previous speculation regarding reaction of H2SO4 with O(D-1) or OH as inconsistent with observations of SO2. Rather, the loss is consistent with a permanent sink for sulfur, such as H2SO4 neutralization by metals on meteoritic dust. In light of such removal, H2SO4 photolysis to SO2 gains added importance in preserving gaseous sulfur in the upper stratosphere. We also present new cross sections derived from ab initio calculations for H2SO4 absorption at visible and Lyman-alpha wavelengths and evaluate their atmospheric implications. Our atmospheric model reveals that photolysis of H2SO4 by Lyman-alpha radiation is responsible for up to one third of the SO2 in the mesosphere and 10% of the particles in the polar stratospheric CN layer.