Processing of oxidised nitrogen compounds by passage through winter-time orographie cloud

Abstract

Four case studies are described, from a three-site field experiment in October/November 1991 using the Great Dun Fell flow-through reactor hill cap cloud in rural Northern England. Measurements of total odd-nitrogen nitrogen oxides (NO_y) made on either side of the hill, before and after the air flowed through the cloud, showed that 10 to 50% of the NO_y, called NO_z, was neither NO nor NO₂. This NO_z failed to exhibit a diurnal variation and was often higher after passage through cloud than before. No evidence of conversion of NO_z to NO₃^- in cloud was found. A simple box model of gas-phase chemistry in air before it reached the cloud, including scavenging of NO₃ and N₂O₅ by aerosol of surface area proportional to the NO₂ mixing ratio, shows that NO₃ and N₂O₅ may build up in the boundary layer by night only if stable stratification insulates the air from emissions of NO. This may explain the lack of evidence for N₂O₅ forming NO₃^- in cloud under well-mixed conditions in 1991, in contrast with observations under stably stratified conditions during previous experiments when evidence of N₂O₅ was found. Inside the cloud, some variations in the calculated total atmospheric loading of HNO₂ and the cloud liquid water content were related to each other. Also, indications of conversion of NO_x to NO_z were found. To explain these observations, scavenging of NO_x and HNO₂ by cloud droplets and/or aqueous-phase oxidation of NO₂^- by nitrate radicals are considered. When cloud acidity was being produced by aqueous-phase oxidation of NO_x, or SO₂, NO₃^- which had entered the cloud as aerosol particles was liberated as HNO₃ vapour. When no aqueous-phase production of acidity was occurring, the reverse, conversion of scavenged HNO₃ to particulate NO₃^-, was observed.