Middle atmospheric sources of nitrous oxide (N2O):: O2(B) and N2(A) chemistry

Experimental evidence is presented that shows that the short-lived O-2(B(3)Sigma) molecules [ tau = 10 - 100 ps] that are excited efficiently in the stratosphere and mesosphere by optical pumping of the Schumann-Runge bands, react with N-2 to form N2O with a yield of 5.0 x 10(-4) molecules per absorbed SRB photon at an air pressure of 250 torr. Although insignificant in the total budget of atmospheric N2O, this source is expected to be important in understanding of O-17,O- 18 enrichment in the middle atmospheric N2O. Since N2O is produced from O-2(B (3)Sigma) reacting with N-2, production of N2O from the reactions of N-2(A (3)Sigma) With O-2 is likely. Unfortunately experimental data pertinent to this production are in significant disagreement. We have, therefore, revisited nine such experiments in the light of the recent new developments in the N2O chemistry. We find that a high (approximate to 30%) branching ratio for N2O formation in the reactions of N2(A (3)Sigma) with O-2 is supported by Malcolme-Lawes's experiments and by the most reliable part of Black et al.'s experimental data which are in the limit of the lowest n(O-2) and n(N-2(A)) concentrations. The decreasing yields reported by Black et al. with increasing n(O-2) and n(N-2(A)) are less reliable due to their greater susceptibility to uncertainties in the N2(A) energy pooling reaction and the role of O-2 in the cascade of N-2(C) to N-2(A). The anomalous low N2O yield (almost none) from low pressure flowing afterglow experiments with Nz(A (3)Sigma) derived from a microwave discharge in Ar, Nz mixture does not necessarily mean that the N-2(A (3)Sigma) + O-2 reaction does not produce N2O. The low yield may mean that nascent N2O from the N-2(A (3)Sigma) + O-2 reaction is highly excited in the N-2..O stretch mode and in the low pressure environment is able to reach the wall in a vibrationally "hot" condition where it is destroyed by surface reaction with O-2. The nascent N2O may also be destroyed by N-2 (high v greater than or equal to 15, epsilon greater than or equal to 94 kcal/mole). This interpretation of the low pressure flowing afterglow experiment is consistent with the results of Malcolme-Lawis's experiment and with the most reliable part of Black ct al.'s data. Like the O-2(B 3(Sigma)) source, the N-2(A (3)Sigma) source is also insignificant for the N2O source-sink budget. But it is potentially important in the understanding of the middle atmospheric effects of solar proton and relativistic electron precipitation and also as a mechanism for the heavy N-atom enrichment in N2O. The new perspective on N2O formation from N-2(A), therefore, deserves attention. (C) 2000 Elsevier Science Ltd. All rights reserved.