Characterization, sources, and light absorption of fine organic aerosols during summer and winter at an urban site

Abstract

In this study, the 24-h PM2.5 samples were collected from an urban site in Korea during summer and winter. The collected samples were analyzed for organic carbon (OC), elemental carbon (EC), water-soluble OC, humic-like substances (HULIS), and ionic species. Furthermore, in-situ measurements of PM2.5 aerosol absorption coefficients were made by using a dual-spot multi-wavelength Aethalometer. Primary traffic emissions and secondary formation were the major contributors to the HULIS concentration during summer. Furthermore, biomass burning (BB) contributed to HULIS concentration to a certain extent. During winter, the HULIS with higher OC/EC ratio (6.4) were strongly associated with BB and secondary formation, while the HULIS with lower OC/EC ratio (2.9) was mainly attributed to secondary formation and, to a certain extent, primary combustion sources. Light absorption by brown carbon (BrC) was significantly enhanced in the near-ultraviolet and visible spectral regions and exhibited strong spectral dependence. Ambient BrC absorption was estimated to account for 2.1–16.6% and 5.9–29.3% of the total aerosol absorption during summer and winter, respectively. The average values of absorption Ångström exponent (AAE) in the wavelength range of 370–660 nm for light-absorbing BrC aerosols were estimated to be 4.0 and 4.4 during summer and winter, respectively. This study demonstrates that the AAE values for BrC can be derived from multi-wavelength Aethalometer data without filter sampling and detailed laboratory analysis.