Aqueous phase reaction of HNO4:: The impact on tropospheric chemistry

We use a global atmospheric chemistry transport model to study the possible influence of aqueous phase reactions of peroxynitric acid (HNO4) on the concentrations and budgets of NOx, SOx, O-3 and H2O2. Laboratory studies have shown that the aqueous reaction of HNO(4)aq with HSO- 3aq, and the uni-molecular decomposition of the NO4- anion to form NO2- (nitrite) occur on a time scale of about a second. Despite a substantial contribution of the reaction of HSO- 3aq with HNO(4)aq to the overall in-cloud conversion of SO2 to SO42-, a simultaneous decrease of other oxidants (most notably H2O2) more than compensated the increase in SO42- production. The strongest influence of heterogeneous HNO4 chemistry was found in the boundary layer, where calculated monthly average ozone concentrations were reduced between 2% to 10% and changes of H2O2 between -20% to +10% compared to a simulation which ignores this reaction. Furthermore, SO2 was increased by 10% to 20% and SO42- depleted by up to 10%. Since the resolution of our global model does not enable a detailed comparison with measurements in polluted regions, it is not possible to verify whether considering heterogeneous HNO4 reactions results in a substantial improvement of atmospheric chemistry transport models. However, the conversion of HNO4 in the aqueous phase seems to be efficient enough to warrant further laboratory investigations and more detailed model studies on this topic.