Production of N₂O₅ and ClNO₂ through Nocturnal Processing of Biomass-Burning Aerosol

Abstract

Biomass burning is a source of both particulate chloride and nitrogen oxides, two important precursors for the formation of nitryl chloride (ClNO₂), a source of atmospheric oxidants that is poorly prescribed in atmospheric models. We investigated the ability of biomass burning to produce N₂O₅(g) and ClNO₂(g) through nocturnal chemistry using authentic biomass-burning emissions in a smog chamber. There was a positive relationship between the amount of ClNO₂ formed and the total amount of particulate chloride emitted and with the chloride fraction of nonrefractory particle mass. In every fuel tested, dinitrogen pentoxide (N₂O₅) formed quickly, following the addition of ozone to the smoke aerosol, and ClNO₂(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 N₂O₅(g). Despite this chloride acid displacement, the biomass-burning aerosol still converted on the order of 10% of reacted N₂O₅(g) into ClNO₂(g). These experiments directly confirm that biomass burning is a potentially significant source of atmospheric N₂O₅ and ClNO₂ to the atmosphere.