POTENTIAL ATMOSPHERIC SOURCES AND SINKS OF NITROUS-OXIDE .2. POSSIBILITIES FROM EXCITED O-2, EMBRYONIC O-3, AND OPTICALLY PUMPED EXCITED O-3

Nitrous oxide (N2O) is an important constituent of the atmosphere beca use it is not only the dominant source of ozone (O-3) destroying odd n itrogen 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 i n 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. Th is need motivated the present study, which has brought to light a tota lly unsuspected aspect of atmospheric chemistry, that is, short-lived (10 ps less than or equal to lifetime less than or equal to 10 ns) exc ited species (e.g., O-2(B-3 Sigma) and electronically excited O-3) may be quite significant in the N2O photochemistry despite their relative insignificance in many other cases. Other specific findings of the pr esent study are the following: (1) O-2(B (3) Sigma), which is efficien tly produced in the stratosphere by resonant absorption in the Schuman n-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 ele ctronic energy in O-2(A(3) Sigma) is insufficient so that the potentia l reaction O-2(A) + N-2 --> N2O + O is only marginally possible unless assisted by high-vibrational excitation (nu greater than or equal to 6) in O-2(A). This source, if it exists, may be significant only at hi gher altitudes around 50 km. (3) Bimolecular and possibly termolecular reactions of O-2(b(1) Sigma) have the potential to be sinks of N2O. ( 4) The O-2(B-3 Sigma) source, while insignificant for the source defic iency problem, may produce N2O with an isotopic enrichment close to th e observations since its optical pumping of O-2(B-3 Sigma) is isotope sensitive. (5) The O-2(B-3 Sigma) source has another intriguing featur e, namely, it maximizes in the same altitude region where UARS/cryogen ic limb array etalon spectrometer (CLAES) and cryogenic whole air samp ler (CWAS) observations show a fold in the N2O mixing ratios which is more pronounced than the same in CH, and chlorofluorocarbons which hav e no atmospheric sources. (6) The O-2-mediated production of N2O from O(D-1) via the ''embryonic'' O-3 is potentially more efficient in the atmosphere relative to the highly inefficient direct reaction N-2 + O( D-1) + M --> N2O + M. The optical pumping of the ground state O-3 to i ts electronically excited states may also lead to potentially substant ial N2O production. The combined production of N2O from these two proc esses may approach 25% of the currently estimated microbiological prod uction. (7) Since O, is already isotopically enriched, it is quite pos sible that the N2O produced by the optically pumped excited O-3 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 he re suggest hitherto unrecognized, mainly biogenic, sinks of this speci es which significantly reduce the net emission of N2O from the soil an d the aquatic environments. It is hoped that the discussions in this p aper will create a greater appreciation of the problems with the class ical N2O chemistry and the potentials of the new chemistry and will th ereby 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 mec hanisms must avoid both the initial presence and the subsequent build up of O-3 in the reaction chamber.