Spatiotemporal Distribution of Brown Carbon and Organic Aerosol Brownness Across the U.S. Chemical Speciation Network

L.-W. ANTONY CHEN, Judith Chow, Xiaoliang Wang, John Watson, Jingqiu Mao, University of Nevada, Las Vegas

     Abstract Number: 269
     Working Group: Aerosols Spanning Spatial Scales: Measurement Networks to Models and Satellites

Abstract
The Chemical Speciation Network (CSN) consists of 145 sites in U.S. urban areas to track fine PM_2.5 chemical compositions. Carbonaceous aerosols, including organic carbon (OC) and elemental carbon (EC), are commonly quantified by CSN using different thermal/optical analysis (TOA) protocols and instrumentation since 2001. From 2016 to September 2018, carbon analysis followed the IMPROVE_A protocol using the DRI Model 2015 Multiwavelength Thermal/Optical Carbon Analyzer (Aerosol Corp., Berkeley, CA, USA) that quantifies OC, EC, and eight thermal fractions while simultaneously monitoring filter reflectance and transmittance at seven wavelengths (405–980 nm). These analyses offer an opportunity to measure black carbon (BC), brown carbon (BrC), and non-light-absorbing white carbon (WtC) based on the aerosol light absorption properties. Multiwavelength measurements yielded valid carbon apportionments for 38,134 samples. The resulting network averages for BC, BrC and WtC are 0.45, 0.39, and 1.60 µg m^-3, respectively, leading to an overall BrC/OC ratio (organic brownness) of 0.20. EC measured by the conventional TOA is 0.52 µg m^-3, and the overall r² between EC and BC is 0.76. Organic brownness varies strongly by season, ranging from 0.09 in summer to 0.35 in winter, despite similar OC levels of 2.23 and 2.03 µg m^-3 in summer and winter, respectively. The seasonal contrast may reflect a higher fraction of weakly-absorbing secondary organic aerosol and/or more effective photobleaching during summer. Consequently, BrC light absorption at 405 nm achieved 13% and 36% of BC absorption in summer and winter, respectively. By site, BC averaged from 0.07 µg m^-3 at Whiteface, NY to 1.21 µg m^-3 at Charleston, WV, while organic brownness appeared to be inversely related to the BC/OC ratio. These findings can facilitate the incorporation of organic aerosol absorptivity into climate and visibility models.