Nitrate effect in the heterogeneous hydrolysis of dinitrogen pentoxide on aqueous aerosols

The heterogeneous hydrolysis of N(2)O(5) was investigated on NaHSO(4), Na(2)SO(4), and NaNO(3) aerosols. The experiments were performed in the large Aerosol Chamber at FZ Julich at room temperature and ambient pressure for several relative humidities. These salts are components of aerosols in the marine and coastal boundary layer. For the sodium sulfate aerosols at relative humidities of 50-70% the reaction probabilities gamma(N2O5) were in the range of 0.02-0.04. For NaNO(3) aerosol at similar relative humidities we observed gamma(N2O5) Of 0.0018-0.0032. With increasing relative humidity, i.e. with increasing dilution of the nitrate concentration in the aerosol droplets, gamma(N2O5) increases to 0.023 at 90% relative humidity. Our observation of decreasing gamma(N2O5) with increasing nitrate concentration can be explained within the framework of an ionic mechanism for the hydrolysis of N(2)O(5), if the recombination reaction of NO(2)(+) with NO(3)(-) to N(2)O(5) is considered. By a steady state analysis we derived analytical expressions of gamma(N2O5) as a function of the nitrate concentration for a reaction either throughout the aerosol volume or in a thin surface shell. Accordingly, increasing nitrate concentration should enhance the lifetime of physically dissolved N(2)O(5)(aq) and as a consequence the heterogeneous hydrolysis of N(2)O(5) should change from a near-surface to a volume reaction. The observation of such a specific nitrate effect can be regarded as further experimental evidence for the ionic reaction mechanism in the uptake of N(2)O(5) on aqueous aerosols. A nitrate effect may gain (local) importance in the atmosphere if increasing NO(x) emissions translate in an increasing nitrate fraction in the secondary aerosol of anthropogenic origin.