Vertical, meridional, seasonal, and local time dependence of non-LTE effects in stratospheric NO and implications for infrared remote sensing

Abstract

Further calculations have been carried out on possible non-local thermodynamic equilibrium (non-LTE) effects previously suggested for stratospheric nitric oxide (NO) associated with the direct photochemical production of vibrationally excited NO by the processes NO 2 + hν → NO(ν) + O O + NO 2 → NO(ν) + O 2 . The new calculations, which make use of improved calculations of the NO vibrational state distribution from NO 2 photolysis, are carried out as a function of altitude and latitude for a variety of seasons and local times. Non-LTE effects on the order of 30% for ν = 1 are obtained, maximizing in the middle stratosphere over the equator. For ν = 2, enhancements in the population by a factor of 3000 over that expected in LTE are obtained. The calculated enhancements are functions of latitude for both ν = 1 and 2. For ν = 1 there is only very small seasonal and local time variation; larger variations were obtained for ν = 2. The results of these calculations suggest that incorporation of this non-LTE effect into the retrieval algorithm for NO from infrared thermal emission measuring instruments on the Upper Atmosphere Research Satellite will need to be done carefully if correct distributions and variations of NO with altitude, latitude, season, and local time are to be obtained. The sensitivity of the calculated non-LTE effects to various assumed parameters, especially of the rate of quenching of NO by O 2, is reviewed.