Brown carbon absorptivity in fresh wildfire smoke: associations with volatility and chemical compound groups

Abstract

Organic aerosol (OA) emissions from wildfires across the western United States have significant impacts on the climate and air quality. Brown carbon (BrC)—the light-absorbing component of OA—has been at the forefront of atmospheric aerosol research. In particular, how the different classes of organic compounds comprising OA relate to BrC light absorption remains an outstanding research question. In this study, we investigated the impact of OA chemical composition and volatility on the optical properties of BrC from fresh smoke emitted from three wildfires in the western United States. Our findings indicate that low volatility organics are well-correlated with both the methanol-soluble and water-insoluble components of BrC. We determined the volatility of the aerosols using two distinct methods and obtained consistent results using both approaches. Higher elemental carbon (EC) concentrations were associated with greater light absorption by BrC, suggesting the possible co-emission of strongly light-absorbing chromophores with EC. Using chemical speciation data, we performed a multivariate regression with the different compound groups and identified that a combination of polycyclic aromatic hydrocarbons, oxygenated aromatics, and nitrogen-containing organics is the best predictor of light-absorption by the methanol- and water-soluble components of BrC. Water-insoluble BrC accounted for 43 ± 11% of the methanol-soluble BrC light absorption at 405 nm, which is consistent with previous studies. This study validates previous laboratory observations of increasing light absorptivity of biomass burning BrC with decreasing volatility of OA molecules and solvent-solubility. We provide additional insight into specific compound groups that may act as BrC chromophores in wildfire emissions.