Elucidating the Impact of Wildfire Molecular Tracers on Nitrous Acid (HONO) Production from Aqueous Nitrate Photochemistry

Nitrous acid (HONO), a gaseous tropospheric pollutant, significantly influences the oxidative potential of the atmosphere as a major source of hydroxyl radicals (•OH). Recently, HONO has been identified as a primary and secondary product of wildfire smoke, yet mechanisms for its formation are poorly understood. In this study, wildfire-emitted organic compounds, such as levoglucosan (LG), syringic acid (SA), and vanillic acid (VA), were studied to investigate their impact on HONO production from aqueous nitrate photochemistry. The presence of SA and VA increased gas-phase HONO yields by 3- and 5-fold, respectively, while LG increased HONO production the most, 6-fold. Furthermore, a nonlinear relationship between HONO and LG concentration was found. Aqueous nitrate samples exposed to lower concentrations of LG were shown to produce an LG-derived carbonyl compound, whereas the samples exposed to a higher LG concentration resulted in an LG dimer. The mechanisms as to how these differences in products affect HONO enhancement are discussed. Gas-phase HONO yields were quantified using incoherent broadband cavity enhanced absorption spectroscopy (IBCEAS), and identification of any condensed phase products from aqueous nitrate samples was done by high resolution mass spectrometry. Studying nitrate photochemistry in the presence of wildfire-emitted organic compounds can potentially minimize the knowledge gap on the observed high concentrations of gaseous HONO formation during biomass burning events.

The current understanding of gas-phase HONO formation from wildfire smoke is limited and does not take smoke plume products into account. This study shows how wildfire-emitted organic compounds greatly enhance gas-phase HONO production.