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Abstract. Light absorbing organic carbon, or brown carbon (BrC), can be a significantcontributor to the visible light absorption budget. However, the sources ofBrC and the contributions of BrC to light absorption are not wellunderstood. Biomass burning is thought to be a major source of BrC.Therefore, as part of the WE-CAN (Western Wildfire Experiment for CloudChemistry, Aerosol Absorption and Nitrogen) study, BrC absorption data werecollected on board the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) C-130 aircraft as it intercepted smoke fromwildfires in the western US in July–August 2018. BrC absorptionmeasurements were obtained in near real-time using two techniques. The firstcoupled a particle-into-liquid sampler (PILS) with a liquid waveguidecapillary cell and a total organic carbon analyzer for measurements ofwater-soluble BrC absorption and WSOC (water-soluble organic carbon). Thesecond employed a custom-built photoacoustic aerosol absorption spectrometer(PAS) to measure total absorption at 405 and 660 nm. The PAS BrC absorption at 405 nm (PAS total Abs 405 BrC) was calculated by assuming the absorption determined by the PAS at 660 nm was equivalent to the black carbon (BC) absorption and the BC aerosol absorption Ångström exponent was 1. Data from the PILS and PAS were combined to investigate the water-soluble vs. total BrC absorption at 405 nm in the various wildfire plumes sampled during WE-CAN. WSOC, PILS water-soluble Abs 405, and PAS total Abs 405 tracked each other in and out of the smoke plumes. BrC absorption was correlated with WSOC (R2 value for PAS =0.42 and PILS =0.60) and CO (carbon monoxide) (R2 value for PAS =0.76 and PILS =0.55) for all wildfires sampled. The PILS water-soluble Abs 405 was corrected for thenon-water-soluble fraction of the aerosol using the calculated UHSAS(ultra-high-sensitivity aerosol spectrometer) aerosol mass. The correctedPILS water-soluble Abs 405 showed good closure with the PAS total Abs 405BrC with a factor of ∼1.5 to 2 difference. This differencewas explained by particle vs. bulk solution absorption measured by the PASvs. PILS, respectively, and confirmed by Mie theory calculations. DuringWE-CAN, ∼ 45 % (ranging from 31 % to 65 %) of the BrCabsorption was observed to be due to water-soluble species. The ratio of BrC absorption to WSOC or ΔCO showed no clear dependence on firedynamics or the time since emission over 9 h. 

The evolution of organic aerosol (OA) and brown carbon (BrC) in wildfire plumes, including the relative contributions of primary versus secondary sources, has been uncertain in part because of limited knowledge of the precursor emissions and the chemical environment of smoke plumes. We made airborne measurements of a suite of reactive trace gases, particle composition, and optical properties in fresh western US wildfire smoke in July through August 2018. We use these observations to quantify primary versus secondary sources of biomass-burning OA (BBPOA versus BBSOA) and BrC in wildfire plumes. When a daytime wildfire plume dilutes by a factor of 5 to 10, we estimate that up to one-third of the primary OA has evaporated and subsequently reacted to form BBSOA with near unit yield. The reactions of measured BBSOA precursors contribute only 13 ± 3% of the total BBSOA source, with evaporated BBPOA comprising the rest. We find that oxidation of phenolic compounds contributes the majority of BBSOA from emitted vapors. The corresponding particulate nitrophenolic compounds are estimated to explain 29 ± 15% of average BrC light absorption at 405 nm (BrC Abs₄₀₅) measured in the first few hours of plume evolution, despite accounting for just 4 ± 2% of average OA mass. These measurements provide quantitative constraints on the role of dilution-driven evaporation of OA and subsequent radical-driven oxidation on the fate of biomass-burning OA and BrC in daytime wildfire plumes and point to the need to understand how processing of nighttime emissions differs.