Abstract. Nitrous acid (HONO) is an important precursor to hydroxyl radical
(OH) that determines atmospheric oxidative capacity and thus impacts climate
and air quality. Wildfire is not only a major direct source of HONO, it also
results in highly polluted conditions that favor the heterogeneous formation of
HONO from nitrogen oxides (NOx= NO + NO2) and nitrate on both
ground and particle surfaces. However, these processes remain poorly
constrained. To quantitatively constrain the HONO budget under various
fire and/or smoke conditions, we combine a unique dataset of field concentrations
and isotopic ratios (15N / 14N and 18O / 16O) of NOx
and HONO with an isotopic box model. Here we report the first isotopic
evidence of secondary HONO production in near-ground wildfire plumes (over a
sample integration time of hours) and the subsequent quantification of the
relative importance of each pathway to total HONO production. Most
importantly, our results reveal that nitrate photolysis plays a minor role
(<5 %) in HONO formation in daytime aged smoke, while
NO2-to-HONO heterogeneous conversion contributes 85 %–95 % to total
HONO production, followed by OH + NO (5 %–15 %). At nighttime, heterogeneous
reduction of NO2 catalyzed by redox active species (e.g., iron oxide
and/or quinone) is essential (≥ 75 %) for HONO production in addition
to surface NO2 hydrolysis. Additionally, the 18O / 16O of HONO
is used for the first time to constrain the NO-to-NO2 oxidation
branching ratio between ozone and peroxy radicals. Our approach provides a
new and critical way to mechanistically constrain atmospheric chemistry and/or air
quality models on a diurnal timescale.
