Understanding Absorption by Black Versus Brown Carbon in Biomass Burning Plumes from the WE-CAN Campaign

Abstract. Aerosol absorption of visible light has an important impact on global radiative forcing. Wildfires are one of the major sources of light-absorbing aerosol, but there remains significant uncertainty about the magnitude, wavelength dependence and bleaching of absorption from biomass burning aerosol. We collected and analyzed data from 21 Western United States wildfire smoke plumes during the 2018 WE-CAN airborne measurement campaign to determine the contribution of black carbon (BC), brown carbon (BrC), and lensing to the aerosol mass absorption cross-section (MAC). MAC_BC, MAC of organics (MAC_BrC+lensing), and the MAC of water-soluble BrC (MAC_{ws_BrC660}) are calculated using Photoacoustic Absorption Spectrometer, Single Particle Soot Photometer and Particle-into-Liquid Sampler measurements. MAC_BC660 does not change significantly with physical age, organic aerosol (OA) concentration, oxygen to carbon ratio (O:C), gas-phase toluene:benzene ratio, modified combustion efficiency (MCE), altitude, or temperature, and has a relatively stable average value of 10.9 ± 2.1 m² g^-1. On average, 54 % of non-BC absorption (23 % total absorption) at 660 nm is from water-soluble BrC. MAC_{ws_BrC660} is 0.06 ± 0.04 m² g^-1 while MAC_BrC+lensing is 0.11 ± 0.06 m² g^-1 at 660 nm, increasing to 0.59 ± 0.19 m² g^-1 at 405 nm. MAC_BrC+lensing is constant with physical age and MCE, but increases slightly with increasing O:C or decreasing toluene:benzene, while total absorption (normalized to CO) slightly decreases with increasing O:C or decreasing toluene:benzene due to decreasing OA. No evidence of BrC bleaching is observed. Comparison to commonly used parameterizations, modeling studies, and the FIREX-AQ observations suggest model overestimation of absorption is likely due to incorrect BrC refractive indices. Quantification of significant brown carbon in the red wavelengths and the stability of MAC_BC, the observation of minimal bleaching, and the observation of changes in OA with O:C and toluene:benzene markers all serve as important constraints on aerosol absorption in regional and global climate models.