Characteristics of light absorption and environmental effects of Brown carbon aerosol in Chongqing during summer and winter based on online measurement: Implications of secondary formation

Abstract

Light-absorbing organic carbon (i.e., brown carbon, BrC) significantly contributes to light absorption and radiative forcing in the atmospheric particles. However, the secondary formation of BrC and optical properties of secondary BrC are poorly understood. In this study, we analyzed and evaluated the light absorption and environmental effects of BrC and secondary BrC from July 1st to 31st, 2022 (summer) and January 20th to February 20th, 2023 (winter) in Chongqing. BrC and secondary BrC light absorption were estimated via a seven-wavelength aethalometer and the statistical approach. The average values of secondary BrC light absorption (Abs_BrC,sec,λ) accounted for 46.2–56.5% of Abs_BrC. Abs_BrC,370 and Abs_BrC,sec,370 were significantly higher during winter (26.2 ± 13.2 and 9.1 ± 5.2 Mm⁻¹ respectively) than that during summer (7.2 ± 4.1 and 5.2 ± 3.5 Mm⁻¹ respectively) (p < 0.001), suggesting secondary formation played an essential role in BrC. A diurnal cycle of Abs_BrC,sec,370 was explained by the photobleaching of light-absorbing chromophores under the oxidizing conditions in the daytime, and the formation of chromophores via aqueous reactions with NH₄⁺ and NO_x after sunset during winter. PSCF analysis showed that transport of anthropogenic emissions from the northeastern and southeastern areas of Chongqing was the important source of the secondary BrC in PP during winter. During winter, the average values of SFE_BrC and SFE_BrC,sec were 31.9 and 27.4 W g⁻¹ lower than that during summer (64.7 and 44.5 W g⁻¹), respectively. In contrast, J[NO₂] values of SFE_BrC and SFE_BrC,sec decreased by 23.3% and 8.7% during winter higher than that during summer (19.9% and 5.6%), indicating that BrC and secondary BrC cause substantial radiative effects and atmospheric photochemistry. Overall, this study is helpful in understanding the characterization and secondary formation of BrC and accurately evaluating the environmental effects of BrC in Chongqing.