Wildfires
are an important source of nitrous acid (HONO), a photolabile
radical precursor, yet in situ measurements and quantification of
primary HONO emissions from open wildfires have been scarce. We present
airborne observations of HONO within wildfire plumes sampled during
the Western Wildfire Experiment for Cloud chemistry, Aerosol absorption
and Nitrogen (WE-CAN) campaign. ΔHONO/ΔCO close to the
fire locations ranged from 0.7 to 17 pptv ppbv–1 using a maximum enhancement method, with the median similar to previous
observations of temperate forest fire plumes. Measured HONO to
NO
x
enhancement ratios were generally
factors of 2, or higher, at early plume ages than previous studies.
Enhancement ratios scale with modified combustion efficiency and certain
nitrogenous trace gases, which may be useful to estimate HONO release
when HONO observations are lacking or plumes have photochemical exposures
exceeding an hour as emitted HONO is rapidly photolyzed. We find that
HONO photolysis is the dominant contributor to hydrogen oxide radicals
(HO
x
= OH + HO2) in early stage
(<3 h) wildfire plume evolution. These results highlight the role
of HONO as a major component of reactive nitrogen emissions from wildfires
and the main driver of initial photochemical oxidation.
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