Importance of tropospheric ClNO2 chemistry across the Northern Hemisphere

Laboratory and field experiments have revealed that uptake of dinitrogen pentoxide (N2O5) on aerosols containing chloride produces nitryl chloride (ClNO2) and nitric acid. We incorporate heterogeneous ClNO2 formation into the hemispheric Community Multiscale Air Quality model. This heterogeneous chemistry substantially enhances ClNO2 levels in several areas of the Northern Hemisphere and alters the composition of airborne reactive nitrogen, comprising more than 15% of monthly mean values in some areas. Model results suggest that this heterogeneous chemistry reduces monthly mean total nitrate by up to 25% and enhances monthly mean daily maximum 8 h ozone by up to 7.0 ppbv. The pathway also enhances hydroxyl radical by more than 20% in some locations which in turn increases sulfate and other secondary pollutants. The largest ClNO2 concentrations and impacts occur over China and Western Europe, two areas in which few relevant field measurements have been made.