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

Abstract

The heterogeneous hydrolysis of N₂O₅ was investigated on NaHSO₄, Na₂SO₄, and NaNO₃ aerosols. The experiments were performed in the large Aerosol Chamber at FZ Jülich 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 γ_N2O5 were in the range of 0.02–0.04. For NaNO₃ aerosol at similar relative humidities we observed γ_N2O5 of 0.0018–0.0032. With increasing relative humidity, i.e. with increasing dilution of the nitrate concentration in the aerosol droplets, γ_N2O5 increases to 0.023 at 90% relative humidity. Our observation of decreasing γ_N2O5 with increasing nitrate concentration can be explained within the framework of an ionic mechanism for the hydrolysis of N₂O₅, if the recombination reaction of NO₂⁺ with NO₃^- to N₂O₅ is considered. By a steady state analysis we derived analytical expressions of γ_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 N₂O₅(aq) and as a consequence the heterogeneous hydrolysis of N₂O₅ 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 N₂O₅ 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.