Investigation of N2O Production from 266 and 532 nm Laser Flash Photolysis of O3/N2/O2 Mixtures

Tunable diode laser absorption spectroscopy has been employed to measure the amount of N 2 O produced from laser flash photolysis of O 3 /N 2 /O 2 mixtures at 266 and 532 nm. In the 532 nm photolysis experiments very little N 2 O is observed, thus allowing an upper limit yield of 7 × 10 -8 to be established for the process O 3 † + N 2 → N 2 O + O 2, where O 3 † is nascent O 3 that is newly formed via O( 3 P J ) + O 2 recombination (with vibrational excitation near the dissociation energy of O 3 ). The measured upper limit yield is a factor of ∼600 smaller than a previous literature value and is approximately a factor of 10 below the threshold for atmospheric importance. In the 266 nm photolysis experiments, significant N 2 O production is observed and the N 2 O quantum yield is found to increase linearly with pressure over the range 100−900 Torr in air bath gas. The source of N 2 O in the 266 nm photolysis experiments is believed to be the addition reaction O( 1 D 2 ) + N 2 + M N 2 O + M, although reaction of (very short-lived) electronically excited O 3 with N 2 cannot be ruled out by the available data. Assuming that all observed N 2 O comes from the O( 1 D 2 ) + N 2 + M reaction, the following expression describes the temperature dependence of k 6 (in its third-order low-pressure limit) that is consistent with the N 2 O yield data: k 6 = (2.8 ± 0.1) × 10 -36 ( T /300) -(0.88±0.36) cm 6 molecule -2 s -1, where the uncertainties are 2σ and represent precision only. The accuracy of the reported rate coefficients at the 95% confidence level is estimated to be 30−40% depending on the temperature. Model calculations suggest that gas phase processes initiated by ozone absorption of a UV photon represent about 1.4% of the currently estimated global source strength of atmospheric N 2 O. However, these processes could account for a significant fraction of the oxygen mass-independent enrichment observed in atmospheric N 2 O, and they appear to be the first suggested photochemical mechanism that is capable of explaining the altitude dependence of the observed mass-independent isotopic signature.