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The month of August 2015 featured extensive wildfires in the Northwestern U.S. and no significant fires in Alaska and Canada. With the majority of carbonaceous aerosols (CA), including black carbon (BC) and brown carbon (BrC), over the U.S. dominated by emissions from Northwestern wildfires, this month presented a unique opportunity for testing wildfire BrC representation in the Weather Research and Forecasting model with chemistry (WRF-Chem). We performed parallel simulations that (1) did not account for BrC absorption, (2) accounted for BrC absorption, and (3) accounted for BrC absorption as well as its decay due to photobleaching. We used a comprehensive set of extensive and pseudo-intensive optical properties, namely the aerosol optical depth (AOD), aerosol absorption optical depth (AAOD), absorption Ångström exponent (AAE), and single scattering albedo (SSA) to constrain the model output against observations from the Aerosol Robotic Network (AERONET). We found that accounting for BrC absorption and photobleaching resulted in the best agreement with observations in terms of aerosol absorption (AAOD and AAE). However, the model severely underestimated AOD and SSA compared to observations. We attributed this discrepancy to missing scattering due to missing secondary organic aerosol (SOA) formation from wildfire emissions in the model. To test this hypothesis, we applied a zeroth-order representation of wildfire SOA, which significantly improved the AOD and SSA model-observation comparison. Our findings indicate that BrC absorption, the decay of its absorption due to photobleaching, as well as SOA formation should be accounted for in chemical transport models in order to accurately represent CA emissions from wildfires. 

The light-absorption properties of brown carbon (BrC) are often estimated using offline, solvent-extraction methods. However, recent studies have found evidence of insoluble BrC species that are unaccounted for in solvent extraction. In this work, we produced carbonaceous aerosol particles from the combustion of three biomass fuels (pine needles, hickory twigs, and oak foliage). We utilized a combination of online and offline measurements and optical calculations to estimate the mass fractions and contribution to light absorption by methanol-soluble BrC (MSBrC), methanol-insoluble BrC (MIBrC), and elemental carbon (EC). Averaged over all experiments, the majority of the carbonaceous aerosol species were attributed to MSBrC (90% ± 5%), while MIBrC and EC constituted 9% ± 5% and 1% ± 0.5%, respectively. The BrC produced in all experiments was moderately absorbing, with an imaginary component of the refractive index ( k ) at 532 nm ranging between 0.01 and 0.05. However, the k values at 532 nm of the MSBrC (0.004 ± 0.002) and MIBrC (0.211 ± 0.113) fractions were separated by two orders of magnitude, with MSBrC categorized as weakly absorbing BrC and MIBrC as strongly absorbing BrC. Consequently, even though MSBrC constituted the majority of the aerosol mass, MIBrC had a dominant contribution to light absorption at 532 nm (72% ± 11%). The findings presented in this paper provide support for previous reports of the existence of strongly absorbing, methanol-insoluble BrC species and indicate that relying on methanol extraction to characterize BrC in biomass-burning emissions would severely underestimate its absorption.