Natural atmospheric sources and sinks of nitrous oxide: 1. An evaluation based on 10 laboratory experiments

Abstract

This paper discusses potentially potent new atmospheric sources and sinks of nitrous oxide (N2O), which is an important species by virtue of its being the dominant source of odd nitrogen (NOx) in the stratosphere and also a greenhouse gas. It is suggested that some of the excited nascent ozone from O, O2 recombination may form N2O via its reaction with N2. It is also conceivable that other metastable states of O3 accessible by optical pumping from the ground state might also be a potential source of N2O in the natural atmosphere. Additional atmospheric sources of N2O include reactions of excited NO3(Ā2E'), NO2(Ā2B1, 2B2) and OH(A) with N2, and of N2(A3∑) with O2. Highly vibrationally excited O2†(ν > 12) could be an important new sink of N2O in the stratosphere. This new N2O sink might also be a new NOx source in the stratosphere, depending on what fraction of the reactions of O2†(high ν) with N2O produces NO and NO2. At the lower altitudes, where O2†(ν > 12) density would become too small to matter, reactions of NO2 with N2O might be a potential N2O sink. The new sources need not conflict with the observed vertical profile of N2O, because: (1) the new O2†(high ν) sink could ensure the observed decrease of N2O mixing ratios in the upper atmosphere, and (2) the sources in the troposphere would be indistinguishable from the surface sources due to strong mixing and N2O long photochemical lifetime in this region. While some of these new sources and sinks are based on direct experimental data, others are based on new theoretical analysis of relevant experiments. Even so it is encouraging that the new sources could help us in removing the source deficit in the current N2O budget. So long as a prospect exists that NOxmight be produced in reactions of O2†(high ν) with N2O, it is constructive to speculate that the “new” N2O chemistry might have an effects on our current perceptions of NOx catalyzed stratospheric ozone depletion. If an important new source of NOx has been missed then an equally potent sink of NOx has also been missed, and it is possible that the inclusion of the latter into the current assessment models might affect our current perception of the atmospheric environmental effects of many aerospace activities.