Characteristics and evolution of brown carbon in western United States wildfires

Abstract. Brown carbon (BrC) associated with aerosol particles in western United States wildfires was measured between July and August 2019 aboard the NASA DC-8 research aircraft during Fire Influence on Regional to Global Environments Air Quality (FIREX-AQ) study. Two BrC measurement methods are investigated, highly spectrally resolved light absorption solvent (water methanol) extracts of collected filters situ bulk particle at three wavelengths (405, 532 664 nm) a photoacoustic spectrometer (PAS). A light-absorption closure analysis for 300 700 nm performed. The combined pure black material, including enhancements due internally mixed materials, plus soluble Mie-predicted factor conversion BrC, compared spectra from power law fit PAS wavelengths. For various parameters used, wavelength roughly 400 they agreed, lower individual component-predicted significantly exceeded higher consistently but more variable. Limitations extrapolation data below 405 missing species low solubility that strongly absorb may account differences. Based measurements closest fires, emission ratio PAS-measured relative monoxide (CO) average 0.13 Mm−1 ppbv−1; ratios also provided. As smoke moved away burning regions, evolution over time observed be complex; enhancement, depletion or constant levels age were all first 8 h after different plumes. Within following emissions, 4-nitrocatechol, well-characterized chromophore commonly found particles, largely depleted BrC. In descending plume where temperature increased by 15 K, 4-nitrocatechol dropped, possibly temperature-driven evaporation, remained unchanged. Evidence reactions ozone, related species, as pathway secondary formation under both high oxides nitrogen (NOx) conditions, while bleached regions ozone NOx, consistent complex behaviors laboratory studies. Although hours is variable, limited number aged (15 30 h) indicate net loss It yet determined how near-field affects characteristics longer timescales spatial scales, its environmental impacts likely greater.