Optical Property and Hygroscopicity of Solid Strongly Absorptive Brown Carbon in Wildfire Smoke

ZEZHEN CHENG, Amna Ijaz, ManishKumar Shrivastava, Daniel Veghte, Gregory W. Vandergrift, Kuo-Pin Tseng, Nurun Nahar Lata, Will Kew, Kaitlyn J. Suski, Johannes Weis, Gourihar Kulkarni, Larry Berg, Jerome Fast, Libor Kovarik, Lynn Mazzoleni, Alla Zelenyuk, Swarup China, Pacific Northwest National Laboratory

     Abstract Number: 289
     Working Group: Aerosols, Clouds and Climate

Abstract
Brown carbon (BrC) emitted from wildfires is a significant contributor to atmospheric aerosols. They affect climate directly by absorbing and scattering light and indirectly by interacting with clouds. However, the knowledge of the climate effects of wildfire BrC is still limited. One reason might be a limited understanding of strongly absorptive BrC (solid S-BrC, commonly known as tar ball) in wildfire aerosol. Literature reported solid S-BrC optical properties are highly variable, leading to significant uncertainties about their direct climate effects. Moreover, the interaction between solid S-BrC and clouds is typically neglected due to the common belief that they are hydrophobic. Here, we investigate the physicochemical and optical properties of solid S-BrC collected from the Pacific Northwest wildfire in September 2017, where more than 90% of particles are solid S-BrC with measured refractive index at 550 nm wavelength equal to 1.49+0.059i. Sensitivity studies of solid S-BrC absorption aerosol optical depth (AAOD) using the Weather Research and Forecasting Model coupled to chemistry (WRF-Chem) show that variation in RI can lead to ~ 200% difference in AAOD, which could lead to surface temperature difference. Furthermore, we found that ~50% of solid S-BrC particles are hydrophilic and can form core-shell morphology at 5 ºC and > 97% RH, possibly due to a highly oxygenated surface layer, making them potential cloud condensation nuclei. This core-shell morphology could enhance their absorption cross-section at 550 nm by a factor of 2 due to the lensing effect. In addition, SEM images show that at least 1% solid S-BrC has organic coatings, which leads to higher lensing enhancement than water coating. We conclude that more studies are necessary for models to improve the parametrization of the light-absorption properties of wildfire aerosol to incorporate the contribution of solid S-BrC and their interactions with water to predict Earth’s radiation budget accurately.