Analysis of the Influencing Factors and Sources of Brown Carbon Light Absorption in a Typical Megacity of the Yangtze River Delta, China

Abstract

Brown carbon (BrC) is a new term for organic aerosol (OA) with strong absorption ability from the visible to ultraviolet (UV) wavelengths, which plays a vital role in atmospheric visibility and climate change. Herein, we report field measurements from 1 March 2020 to 28 February 2021, sampled at urban Suzhou, Yangtze River Delta (YRD), China, to investigate the optical properties and sources of BrC. By analyzing the seasonal characteristics of the absorption of BrC at 370 nm (babs370), babs370 was found to be the highest (9.0 ± 7.2 Mm−1) in winter and the lowest (5.1 ± 3.3 Mm−1) in summer, respectively. The absorption Ångström exponent (AAE) value of BrC in winter was 1.22 ± 0.05, followed by 1.21 ± 0.05, 1.20 ± 0.05, and 1.19 ± 0.05 for fall, spring, and summer, respectively. The mass absorption cross-section (MAC) of secondary organic carbon (SOC) was 3.3 ± 0.2 m2g−1 in spring, 2.9 ± 0.1 m2g−1 in summer, 4.3 ± 0.1 m2g−1 in fall, and 2.8 ± 0.2 m2g−1 in winter, significantly lower than that of primary organic carbon (POC) at 370 nm, suggesting the aging process could weaken the light absorption of BrC. Five different BrC factors were identified by the positive matrix factorization (PMF) analysis, including biomass-burning-related, vehicle-related, sulfate-related, nitrate-related, and dust-related factors, which on average account for 7.4%, 73.4%, 11.9%, 1.9%, and 5.4% of babs370, respectively. Potential Source Contribution Factor (PSCF) analysis showed that those high babs370 periods were mainly contributed by air mass from the south. Moreover, for the influence degree of the potential source areas, the sequence was winter > spring > fall > summer. Our results improve the understanding of BrC in an important industrial city in YRD, which could reduce the uncertainty of the prediction of its climate effect in this region.