AbstractThe equilibrium constant, Keqof the reaction NO2+ NO3+ M 2 N2O5+ M has been determined for a small range of temperatures around room temperature in air at 740 torr by direct spectroscopical measurements of NO2, NO3, and N2O5. At 298 K, Keqwas determined as (3.73 ± 0.61) × 10−11cm3molecule−1. Averaging this and 11 other independent evaluations of Keqyields Keq= (3.31 ± 0.82) × 10−11cm3molecule−1, where the uncertainty is given as one standard deviation. The kinetics of the O3/NO2/N2O5/NO3/ air system was studied in a static chamber at room temperature and 740 torr total pressure. Evidence of a unimolecular decay reaction of NO3, NO3→ NO + O2, was found and its rate coefficient was estimated as (1.6 ± 0.7) × 10