Atomic oxygen depletion in the vicinity of noctilucent clouds Original Research Article

There is growing evidence that noctilucent clouds play an important role in the chemistry of the upper mesosphere. Their volumetric surface area appears to be sufficiently large for heterogeneous reactions to compete with their gasphase counterparts, so long as the relevant uptake coefficients are large enough. Rocket-borne measurements have shown that atomic oxygen is substantially depleted in the vicinity of noctilucent clouds. Gumbel et al. (1998) proposed that this is caused by the efficient uptake of atomic O on the surface of the ice particles, requiring an uptake coefficient of at least 2 × 10−5 In the present study a low-temperature fast flow tube was used to measure the uptake coefficient of O on ice as a function of temperature and O2 concentration. Under the conditions pertinent to NLC in the upper mesosphere, the uptake coefficient is 1 × 10−5, and hence probably too small to explain the magnitude of the observed depletions of O. Instead it appears that a greater contribution to the removal of O (and O3) arises from gas-phase catalytic cycles driven b y odd hydrogen s pecies (H, OH and HO2). The sources of additional odd hydrogen are most likely the Lyman-α photolysis of enhanced gas-phase H2O around the clouds, and the photolysis of the ice particles themselves.