The NO3 radical decomposition and NO3 scavenging in the troposphere

Studies designed to measure the apparent first order decay of the NO3 radical are reported here: NO3 (+M) → NO + O2 (+M) (R1). Two techniques were employed: (1) direct absorption measurements of [NO2] and [NO3] were made in the visible in a mixture of N2O5 in nitrogen; and (2) NO3 was followed indirectly through measured infrared absorptions of NO2 and N2O5 and calculated from the N2O5 equilibrium relation. Rate data were obtained in experiments over a range of temperatures (313–347 K) and pressures of added N2 (5–2000 torr). Rate coefficient data obtained in experiments at PN2 < 40 torr show no detectable increase with increase in PN2 within the large uncertainty, and the observed temperature dependence is qualitatively similar to that derived by Johnston et al. (1986) from experiments at 760 torr. However, the data obtained using the indirect method at PN2 > 40 torr show an apparent increase in k1 with PN2, and a negative temperature coefficient which is attributed to a wall‐catalyzed decomposition of NO3, presumably to form NO and O2 by the stoichiometry of reaction (R1). Although our observations on the kinetics of reaction (R1) are only qualitative, they support those of Johnston et al. The occurrence of a first order, homogeneous decomposition of NO3 in the atmosphere can explain most of Noxon's (1983) observations of NO3 scavenging in the troposphere. However, his reported loss of NOx during the nighttime hours requires, in addition, a contribution from some other, unidentified reaction pathway. Alternatives for this removal mechanism are discussed in view of our current knowledge.