Brown carbon in biomass burning aerosol enhances HONO production

Nitrous acid (HONO) is an important source of hydroxyl radicals (•OH). •OH drives ozone and aerosol formation, affecting air quality, health, and climate. Elevated HONO concentrations have been observed in biomass burning (BB) plumes. However, the mechanism remains unclear, hindering parameterization and quantification of the impacts. Here, we conducted flow tube experiments with BB aerosol samples and proxies to investigate renoxification, a process that converts nitrate into HONO and NO x (NO and NO 2 ). Our results reveal that renoxification in BB aerosol is enhanced by brown carbon (BrC) through electron transfer to nitrate as an alternative to surface-enhanced nitrate photolysis. We develop parameterization to assess the potential global impacts of BrC-enhanced renoxification, which suggests that BrC enhances surface-level HONO production by up to one order of magnitude in BB-affected regions, with surface HONO production exceeding 1 part per trillion per hour in regions with high aerosol nitrate. These impacts are expected to worsen as BB intensifies in a warming climate.