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

[摘要] The formation, evolution and eventual sublimation of noctilucentclouds (NLC) may have a significant effect on the odd oxygen andhydrogen chemistry of the high latitude summer mesosphere. Threemechanisms are considered here: the direct uptake of atomic oxygenon the surface of the ice particles; the redistribution of watervapour, which changes the photochemical source of odd hydrogenspecies; and the direct photolysis of the ice particles themselvesto produce odd hydrogen species in the gas phase. A 1-Dphotochemical model is employed to investigate the potentialimportance of these mechanisms. This shows, using the recentlymeasured uptake coefficients of O on ice, that the heterogeneousremoval of O on the surface of the cloud particles is too slow by atleast a factor of 5x10³ to compete with gas-phase Ochemistry. The second and third mechanisms involve the solarLyman-α photolysis of H₂O in the gas and solid phase,respectively. During twilight, Lyman-α radiation is severelyattenuated and these mechanisms are insignificant. In contrast, whenthe upper mesosphere is fully illuminated there is a dramatic impacton the O profile, with depletion of O at the base of the cloud layerof close to an order of magnitude. A correspondingly large depletionin O₃ is also predicted, while H, OH, HO₂ andH₂O₂ are found to be enhanced by factors of 3-5. In fact,rocket-borne mass spectrometer measurements during summer haverevealed local H₂O₂ enhancements in the region of theclouds. Rocket-borne measurements of atomic O and O₃ profilesin the presence of mesospheric clouds in the daytime are highlydesirable to test the predictions of this model and ourunderstanding of the genesis of mesospheric clouds.

[学科分类] 大气科学