Modelling the impact of noctilucent cloud formation on atomic oxygen and other minor constituents of the summer mesosphere

The formation, evolution and eventual sublimation of noctilucent clouds ( NLC) may have a significant effect on the odd oxygen and hydrogen chemistry of the high latitude summer mesosphere. Three mechanisms are considered here: the direct uptake of atomic oxygen on the surface of the ice particles; the redistribution of water vapour, which changes the photochemical source of odd hydrogen species; and the direct photolysis of the ice particles themselves to produce odd hydrogen species in the gas phase. A 1-D photochemical model is employed to investigate the potential importance of these mechanisms. This shows, using the recently measured uptake coefficients of O on ice, that the heterogeneous removal of O on the surface of the cloud particles is too slow by at least a factor of 5 x 10(3) to compete with gas-phase O chemistry. The second and third mechanisms involve the solar Lyman-alpha photolysis of H2O in the gas and solid phase, respectively. During twilight, Lyman-alpha radiation is severely attenuated and these mechanisms are insignificant. In contrast, when the upper mesosphere is fully illuminated there is a dramatic impact on the O profile, with depletion of O at the base of the cloud layer of close to an order of magnitude. A correspondingly large depletion in O-3 is also predicted, while H, OH, HO2 and H2O2 are found to be enhanced by factors of 3 - 5. In fact, rocket-borne mass spectrometer measurements during summer have revealed local H2O2 enhancements in the region of the clouds. Rocket-borne measurements of atomic O and O-3 profiles in the presence of mesospheric clouds in the daytime are highly desirable to test the predictions of this model and our understanding of the genesis of mesospheric clouds.