Potential atmospheric sources and sinks of nitrous oxide: 2. Possibilities from excited O2, “embryonic” O3, and optically pumped excited O3

Abstract

Nitrous oxide (N2O) is an important constituent of the atmosphere because it is not only the dominant source of ozone (O3) destroying odd nitrogen in the stratosphere but also a greenhouse gas. Unfortunately, the classical chemistry of N2O has at least two problems, namely, (1) a possible source gap in the source‐sink budget and (2) difficulties in explaining the observed heavy isotope enrichments. While the source gap can, in principle, be closed by sources of the classical type, the observed isotopic anomaly calls for atmospheric sources and sinks. This need motivated the present study, which has brought to light a totally unsuspected aspect of atmospheric chemistry, that is, short‐lived (10 ps≤lifetime≤10 ns) excited species (e.g., O2(B 3Σ) and electronically excited O3) may be quite significant in the N2O photochemistry despite their relative insignificance in many other cases. Other specific findings of the present study are the following: (1) O2(B 3Σ), which is efficiently produced in the stratosphere by resonant absorption in the Schumann‐Runge bands, is a possible source of N2O with a maximum strength of the order of 60 N2O cm−3 s−1 in the vicinity of 30 km. (2) The electronic energy in O2(A3Σ) is insufficient so that the potential reaction O2(A) + N2→ N2O + O is only marginally possible unless assisted by high‐vibrational excitation (v≥6) in O2(A). This source, if it exists, may be significant only at higher altitudes around 50 km. (3) Bimolecular and possibly termolecular reactions of O2(b1Σ) have the potential to be sinks of N2O. (4) The O2(B3Σ) source, while insignificant for the source deficiency problem, may produce N2O with an isotopic enrichment close to the observations since its optical pumping of O2(B3Σ) is isotope sensitive. (5) The O2(B3Σ) source has another intriguing feature, namely, it maximizes in the same altitude region where UARS/cryogenic limb array étalon spectrometer (CLAES) and cryogenic whole air sampler (CWAS) observations show a fold in the N2O mixing ratios which is more pronounced than the same in CH4 and chlorofluorocarbons which have no atmospheric sources. (6) The O2‐mediated production of N2O from O(1D) via the “embryonic” O3 is potentially more efficient in the atmosphere relative to the highly inefficient direct reaction N2+ O(1D)+M→N2O+M. The optical pumping of the ground state O3 to its electronically excited states may also lead to potentially substantial N2O production. The combined production of N2O from these two processes may approach 25% of the currently estimated microbiological production. (7) Since O3 is already isotopically enriched, it is quite possible that the N2O produced by the optically pumped excited O3 might also show isotopic enrichment. (8) If the current World Meteorological Organization position that the classical sources and sinks of N2O are in balance is accepted, then the new atmospheric sources discussed here suggest hitherto unrecognized, mainly biogenic, sinks of this species which significantly reduce the net emission of N2O from the soil and the aquatic environments. It is hoped that the discussions in this paper will create a greater appreciation of the problems with the classical N2O chemistry and the potentials of the new chemistry and will thereby stimulate further research. With this hope some suggestions for new laboratory and computational chemistry experiments are also made. In particular, new experiments to test the proposed N2O production mechanisms must avoid both the initial presence and the subsequent build up of O3 in the reaction chamber.