AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Impacts of Brown Carbon on Surface Shortwave Radiation in the California Sacramento Valley in Summer 2018
CHELSEA CORR, Maosi Chen, Zhibin Sun, Yan'an Liu, George Janson, Becky Olson, Scott Simpson, Amy P. Sullivan, Emily Fischer, Wei Gao, Colorado State University
Abstract Number: 508 Working Group: Biomass Combustion: Emissions, Chemistry, Air Quality, Climate, and Human Health
Abstract Biomass burning is a significant source of brown carbon (BrC) aerosol which exhibits stronger absorption at ultraviolet (UV) wavelengths (λ < 400nm) compared to the visible. The influence of BrC spectral absorption on surface UV and visible irradiance was examined using data collected at the USDA UV-B Monitoring and Research Program’s UC Davis site in Summer 2018. Situated in California’s Sacramento Valley, the UC Davis site was frequently influenced by wildfire smoke from the large Carr and Mendocino Complex Fires between July and September. Diffuse, direct and total irradiance from two multifilter rotating shadowband radiometers (MFRSRs) at 7 discrete UV wavelengths (300, 305, 311, 317, 325, 332, and 368nm) and 2 visible wavelengths (415 and 500nm) were used to calculate the diffuse fraction of total irradiance (DF) and the diffuse-to-direct ratio (DDR) for cloud-free days before and during the active fire period. Aerosol optical depths (AODs) determined by the Langley method for the UV-MFRSR 368nm channel were used to characterize smoke loading. Measurement-derived DF and DDR values were consistently higher than those modeled for aerosol-free conditions using the Tropospheric Ultraviolet Visible Model (TUV V5.3.2). Further, DF and DDR enhancements in the presence of aerosol increased with AOD, demonstrating the positive relationship between aerosol scattering and diffuse irradiance. Offsets between aerosol-free and measured DDR and DF did show a spectral dependence with largest values at longer wavelengths. This suggests an increase in aerosol absorption relative to scattering as wavelength decreases, a pattern consistent with the presence of BrC. Comparisons between irradiance ratios and measurements of BrC made aboard the NCAR C-130 during the WE-CAN (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen) airborne field campaign were performed to further assess the contribution of BrC to the spectral signatures of the DDR and DF enhancements.