Production of NO and ClNO through Nocturnal Processing of Biomass-Burning Aerosol.

Biomass burning is a source of both particulate chloride and nitrogen oxides, two important precursors for the formation of nitryl chloride (ClNO), a source of atmospheric oxidants that is poorly prescribed in atmospheric models. We investigated the ability of biomass burning to produce NO(g) and ClNO(g) through nocturnal chemistry using authentic biomass-burning emissions in a smog chamber. There was a positive relationship between the amount of ClNO formed and the total amount of particulate chloride emitted and with the chloride fraction of nonrefractory particle mass. In every fuel tested, dinitrogen pentoxide (NO) formed quickly, following the addition of ozone to the smoke aerosol, and ClNO(g) production promptly followed. At atmospherically relevant relative humidities, the particulate chloride in the biomass-burning aerosol was rapidly but incompletely displaced, likely by the nitric acid produced largely by the heterogeneous uptake of NO(g). Despite this chloride acid displacement, the biomass-burning aerosol still converted on the order of 10% of reacted NO(g) into ClNO(g). These experiments directly confirm that biomass burning is a potentially significant source of atmospheric NO and ClNO to the atmosphere.