Optical absorption characteristics of brown carbon aerosols during the KORUS-AQ campaign at an urban site

Abstract

This study investigates the absorption characteristics of brown carbon (BrC) obtained from water and methanol extracts of fine particulate matter measured at an urban site in Gwangju, Korea during the KOREA U.S. – Air Quality campaign (May 2–June 11, 2016). The measurement period was classified into two intervals: biomass burning (BB) and non-BB periods. During the non-BB period, water-soluble organic carbon (WSOC) and humic-like substances (HULIS) primarily resulted from secondary organic aerosol (SOA) formation and primary vehicle emissions. Water-soluble organic aerosols during the BB period, meanwhile, were closely related to SOA formation and regionally transported BB emissions.The light absorption coefficient measured at 365nm (babs,365) by methanol extracts was 2.6 and 6.1 times higher than the coefficients from the water and HULIS extracts, respectively, indicating the importance of BrC absorption by water-insoluble organic carbon. This was demonstrated by a good correlation between the water-insoluble BrC absorption and the elemental carbon concentration. A comparison of babs,365 between the methanol- and water-extracted BrC indicated that water-insoluble BrC accounted for approximately 61% (33–86%) of the total BrC absorption. The contributions of SOA, primary BB emissions, and traffic emissions to the water extract babs,365 were estimated using a stepwise multiple linear regression (MLR) analysis and found to be 1.17±0.55, 0.65±0.62, and 0.25±0.09Mm−1, respectively, accounting for 59.6, 26.1, and 14.3% of the absorption coefficient by the water-soluble BrC. Further, it was determined that the contribution of the BB emissions to the water-soluble BrC absorption was approximately two times higher in the BB period than in the non-BB period.The average absorption Ångstrӧm exponent was 4.8±0.3, 5.3±0.7, and 6.8±0.8 for the methanol, water, and HULIS extracts, respectively. The average mass absorption efficiency (MAE365) of methanol extracted BrC was 1.3±0.4m2/g·C. Water- and HULIS-extracted BrC had a MAE365 of 1.0±0.3 and 0.8±0.3m2/g·C, respectively. These results suggest that methanol-extracted BrC could provide a better estimation of BrC absorption compared to WSOC and HULIS.