Importance of tropospheric ClNO₂ chemistry across the Northern Hemisphere

Laboratory and field experiments have revealed that uptake of dinitrogen pentoxide (N 2 O 5 ) on aerosols containing chloride produces nitryl chloride (ClNO 2 ) and nitric acid. We incorporate heterogeneous ClNO 2 formation into the hemispheric Community Multiscale Air Quality model. This heterogeneous chemistry substantially enhances ClNO 2 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 ClNO 2 concentrations and impacts occur over China and Western Europe, two areas in which few relevant field measurements have been made.