Nitrous Acid Formation in Wildfire Plumes: Implications for Atmospheric Oxidation and Ozone Pollution

Abstract Nitrous acid (HONO) is a vital precursor for hydroxyl (OH) radicals in wildfire plumes, yet chemical transport models often underrepresent observed in-plume HONO levels. We implemented wildfire HONO emissions and literature-based secondary formation pathways in the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) and evaluated simulations against airborne measurements from the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign during 2019 summer wildfires in the northwestern United States. The revised model substantially improves in-plume HONO representation. Source attribution revealed that emissions contributed up to 40% of HONO in near-source smoke, while heterogeneous NO2 uptake on aerosol surfaces acted as the dominant pathway (47%–74%) due to the dense reactive surface area of smoke aerosols. HONO photolysis accounted for approximately 80% of the primary OH production in the near-field smoke, identifying HONO as the leading primary radical-initiation pathway during this stage of plume evolution. HONO-initiated radical production further accelerated radical cycling, yielding a net increase in O3 concentrations both near the surface and at the plume transport altitude. Our findings underscore the pivotal role of HONO in driving smoke-plume chemical evolution and highlight its importance for assessing regional air pollution and tropospheric O3 burdens.