Role of heterogeneous conversion of N2O5 on sulphate aerosols in global ozone losses

Abstract

THERE is a serious discrepancy between estimates of the downward trend in the column abundance of ozone at middle and high latitudes derived from ground-based and satellite data, and those obtained by models that calculate the effect of increases in atmospheric chlorine concentrations, but include only gas-phase chemistry1,2. Recent measurements3,4 of the reaction rate of N2O5 on sulphate aerosols yield very fast rates for a wide range of water content, indicating that this reaction could take place in the stratospheric sulphate aerosol layer, which is present around the globe and year-round between ~14–25 km altitude. Here we include this reaction in a two-dimensional model and find that the calculated decadal ozone trends at both high and middle latitudes agree much more closely with the trends deduced from observations. Inclusion of the reaction also significantly increases the predicted concentrations of key species such as OH, ClO and HNO3, and decreases those of NO and NO2. Measurements of these species in and near the sulphate layer are needed to confirm the importance of this reaction in the observed decrease of atmospheric ozone.