The nitrate free radical NO3 has been observed by several investigators in the troposphere at night. It increases during the night much less than predicted in terms of known process, and there has been a search for an unknown NO3 scavenger. In this article we report evidence in the chemical literature for the thermal decomposition of No3 to form nitric oxide and molecular oxygen, where the reaction w is NO3→NO + O2, and the rate is such that this reaction is proposed as a scavenging process in the troposphere. By straightforward reinterpretation of three room temperature kinetic studies which used reactors of widely different volumes, we find the first-order rate constant at one atmosphere pressure to be w=(3¿2) + 10-3 s-1; in these systems the otherwise unexplained first-order loss of NO3 accounted for 5-74% of the observed rate. Using high-temperature shock tube experiments, Schott and Davidson <1958> evaluated rate constants for reaction e (NO2+NO3→NO+O2+NO2) and reaction g (2NO3→2NO2+O2), but these values are much higher than the extrapolated results of Graham and Johnson <1978>.

This difference is ascribed to the unrecognized contribution of reaction w in Schott and Davidson's system; with this assumption we evaluated by two methods the rate constants for w at high temperatures, which combine with the room temperature values, to give the Arrhenius expression w= 2.5 + 106exp (-6.1+103/T)s-1. this relation implies that the unimolecular reaction has a low frequency factor and a low activation energy, which are ascribed to a forbidden-transition to the low-lying excited electronic state (2E2 ) of NO3. The rate of this channel of decomposition is compared with the much slower process leading to NO2 and O as products. Although this article gives evidence that the reaction NO3→NO+O2 occurs, the quantitative values of rate coefficients are uncertain, and there are no data concerning the effect of foreign gas pressure. This article points out two methods (equations (14) and (17) whereby deliberate studies of this reaction can be used to establish it rate coefficients and to find its M gas dependence. The recommended rate expression for reaction e is 2.05 + 1013 exp (-1.24 + 104/T)/Keq and that for g is 8.5 + 10-13 exp (-2.45+103/T) cm3 molecule-1s-1, where Keq refers to the equilibrium N2O5=NO3+NO2.