PROCESSING OF OXIDIZED NITROGEN-COMPOUNDS BY PASSAGE THROUGH WINTER-TIME OROGRAPHIC CLOUD

Four case studies are described, from a three-site field experiment in October/November 1991 using the Great Dun Fell flow-through reactor h ill cap cloud in rural Northern England. Measurements of total odd-nit rogen nitrogen oxides (NOy) made on either side of the hill, before an d after the air flowed through the cloud, showed that 10 to 50% of the NOy, called NOz, was neither NO nor NO2. This NOz failed to exhibit a diurnal variation and was often higher after passage through cloud th an before. No evidence of conversion of NOz to NO3- in cloud was found . A simple box model of gas-phase chemistry in air before it reached t he cloud, including scavenging of NO3 and N2O5 by aerosol of surface a rea 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 e vidence 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 t he 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 NOx to NOz were found. To explain these observations, scavenging of NOx and HNO2 by cloud droplets and/o r 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 li berated as HNO3 vapour. When no aqueous-phase production of acidity wa s occurring, the reverse, conversion of scavenged HNO3 to particulate NO3-, was observed.