Theoretical Study Of The Gaseous Hydrolysis Of NO₂ In The Presence Of NH₃ As A Source Of Atmospheric HONO

Nitrogen-containing particulate matter and nitrous acid （HONO） are two typical pollutants in atmospheric chemistry and air pollution, and they are inseparable. Nitrous acid has long been recognized as an important trace gas in the troposphere where it rapidly undergoes photolysis by sunlight to release a significant amount of hydroxyl （OH） radicals. The sources of HONO and the formation mechanism, are not well understood despite its well recognized importance.The effects of water and ammonia molecules on the hydrolysis of nitrogen dioxide as well as product accumulation are investigated by theoretical calculations of three series of the molecular clusters 2NO₂-mH₂O （m=1-3）,2NO₂-mH₂O-NH₃ （m=1,2） and 2NO₂-mH₂O-2NH₃ （w=1,2）. These reactions are described as:2NO₂+mH₂O→products （m=1-3）2NO₂+mH₂O+NH₃→products （m=1,2）2NO₂+mH₂O+2NH₃→products （w=1,2）The gas-phase reaction 2NO₂+H₂O→HONO+HNO₃ is thermodynamically unfavourable. The additional water or ammonia in the clusters can not only stabilise the products by forming stable complexes, but also reduce the energy barrier for the reaction. There is a considerable energy barrier for the reaction at the reactant cluster 2NO₂-H₂O: 11.7 kcal mol"1. With ammonia and an additional water in the cluster,2NO₂-H₂O-NH₃, the thermodynamically stable products t-HONO+NH₄NO₃-H₂O can be formed without an energy barrier. With two ammonia molecules, as in the cluster 2NO₂-mH₂O-2NH₃ （m=1,2）, the dissociation reaction is barrierless and the product complex NH₄NO₂-NH₄NO₃ can be further stabilised. The present study, including natural bond orbital analysis on a series of species, shows that ammonia is more effective than water in promoting the hydrolysis reaction of NO₂. The product cluster NH₄NO₂-NH₄NO₃ resembles an alternating layered structure containing the ion units NH₄NO₂- and NH₄⁺NO₃^-. The decomposition processes of NH₄NO₂-NH₄NO₃ and its monohydrate are all spontaneous and endothermic.