Heterogeneous uptake of NO2 by biomass burning particulate matter: a potential source of HONO in wildfire-influenced regions

Abstract:

Nitrous acid (HONO) is a significant precursor of the hydroxyl radical (·OH), which is the most important oxidant in the atmosphere. The sources of HONO in the atmosphere, including direct emission and secondary production, are still not well understood. Field studies have reported enhanced HONO concentrations both in and downwind of biomass burning plumes, which implies that the heterogeneous conversion of nitrogen dioxide (NO₂) on biomass burning aerosol may be a major HONO formation pathway. Although previous studies have reported NO₂-to-HONO conversion at the surface of laboratory-generated soot, secondary organic aerosol, and individual components of biomass burning organic aerosol, no studies have examined the interaction of NO₂ with realistically produced wildfire particulate matter (PM).

Here, we report the heterogeneous uptake of NO₂ by PM generated from the combustion of boreal peat sampled in Alberta, Canada. In particular, we report NO₂ uptake coefficients (γNO₂) and HONO yields under both dark conditions and in the presence of simulated solar radiation as a function of peat properties (moisture content and sampling depth) and resultant combustion conditions (represented by modified combustion efficiency data). Our results provide valuable input for atmospheric models and will ultimately improve our ability to estimate the contribution of biomass burning to HONO concentrations in wildfire-impacted regions.