Characteristics of light absorption, sources, and radiative effects of brown carbon aerosol in Chongqing during winter based on on-line measurement: Implications of secondary formation.

To investigate the light absorption characteristics and environmental effects of brown carbon (BrC) during pollution and clean periods, the light absorption coefficients of BrC were obtained by Aethalometer (AE33) from 20th January 2023 to 21st February 2023 in Chongqing. The results showed that the AAE values of PM2.5 were 1.58 and 1.55 during pollution and clean periods, implying the presence of BrC in urban PM2.5. During the campaign, BC and BrC contributed 58%~90% and 10%~42% to PM2.5 absorption at 370~660 nm, indicating σ_abs was mainly affected by BC, but the contribution of BrC can not be ignored. The light absorption coefficient of BrC at 370 nm [σ_BrC(370)] was (35.30±8.21) Mm-1 during pollution periods, 2.47 times higher than that during clean periods [(14.28±7.10) Mm-1], and its contribution to σ_abs (10%~42%) was higher than those during clean periods (8%~41%). It is worth noting that the AAEBrC values during pollution periods ranged from 3.2 to 4.7, generally lower than those during clean periods (3.6~4.9), which might be related to the difference of BrC components during the different periods. The light absorption coefficients of primary brown carbon (BrCpri) and secondary brown carbon (BrCsec) were obtained based on the minimum R squired method (MRS). The contributions of BrCpri to σ_BrC were 74%~80% and 57%~74% during pollution and clean periods, higher than that of BrCsec (20%~26% and 26%~43%). However, The AAE values of BrCpri during pollution and clean periods were 3.42 and 3.08, significantly lower than the corresponding AAE(BrCsec) (4.22 and 5.15), which was related to the strong wavelength dependence of BrCsec light absorption. In addition, the daily variation of σ_BrCsec(370) suggested that BrCsec production might be mainly affected by the liquid-phase reaction under high humidity conditions at night. The radiative absorption contribution of BrC relative to BC was more significant in short wavelengths and decreased obviously with increasing wavelengths. The radiative absorption contributions of BrCpri and BrCsec at 370~950 nm were 24.8% and 10.2% during pollution periods, higher than that during clean periods (18.4% and 9.8%), respectively. The results of the potential source contribution function (PSCF) showed that the local emissions were the main sources of BrC during pollution periods in Chongqing, while regional transport from Hunan province and Guizhou province had major impacts of BrC during clean periods. © 2024 Chinese Society for Environmental Sciences. All rights reserved.