北京和西安棕碳气溶胶的光学性质及化学组成

Optical properties and chemical composition of atmospheric brown carbon aerosols over Beijing and Xi’an

关键词:  棕碳  光学性质  化学组成  污染水平  昼夜变化

Key words:  brown carbon  optical properties  chemical composition  pollution levels  diurnal variation

摘要:

对2016—2017年冬季北京和西安PM_2.5中的甲醇溶性棕碳的光吸收特性和10种多环芳烃发色团的浓度进行测定，探究不同污染水平及昼夜变化对棕碳光学性质和化学组成的影响。对光学性质的研究表明：棕碳在波长365 nm处的光吸收系数（Abs₃₆₅值）随污染水平的升高而明显增加，西安夜间Abs₃₆₅值明显高于白天，两城市Abs₃₆₅值的增加与有机碳浓度的增加密切相关。不同污染水平下，棕碳在365 nm处的质量吸收效率（MAE₃₆₅值）表现出与Abs₃₆₅值不同的变化趋势，表明不同污染水平下两城市棕碳的化学组成与来源存在差异。此外，西安夜间MAE₃₆₅值高于白天，可能与夜间增加的一次排放及二次形成有关。对化学组成的研究表明：随着污染水平的升高，10种多环芳烃在波长365 nm处对甲醇溶性棕碳光吸收的贡献明显增加，西安夜间高于白天。值得注意的是，两城市10种多环芳烃发色团在365 nm处的吸光占比分别是其碳浓度在有机碳浓度中占比的3.9倍（北京平均）和5.8倍（西安平均），说明即使少量的发色团也能产生较高的棕碳吸光贡献。

Abstract:

Background, aim, and scope Brown carbon (BrC) aerosol has an important effect on the atmospheric radiative forcing, but large uncertainties remain, mainly because the relationship between BrC chemical composition and optical properties is not well understood, and variable pollution levels and complex atmospheric aging process increase the difficulty of understanding this relationship. The purpose of this study is to investigate the optical properties and chemical composition of BrC in Beijing and Xi’an, as well as the contributions of components (such as polycyclic aromatic hydrocarbons, PAHs) to light absorption of BrC, and to compare the differences of different pollution levels and diurnal variation, to further understand the correlation between BrC components and optical properties. Materials and methods PM_2.5 samples were collected during the winter of 2016—2017 in Beijing and Xi’an. A total of 21 samples were analyzed for the light absorption at 365 nm (Ab₃₆₅) and the mass absorption efficiency at 365 nm (MAE₃₆₅) of methanol-soluble brown carbon (MS-BrC), together with the concentrations of organic carbon (OC), elemental carbon (EC) and 10 PAHs. These samples included 7 from Beijing (5 from heavy haze days and 2 from clean days) and 14 from Xi’an (8 from heavy haze days and 6 from low pollution days, with equal representation of 7 daytime and 7 nighttime samples). Results The Abs₃₆₅ in heavy haze days ((102.4±9.4) Mm⁻¹) in Beijing was 10.1 times higher than that in clean days ((10.2±3.2) Mm⁻¹), and in Xi’an, the Abs₃₆₅ on heavy haze days ((83.9±18.2) Mm⁻¹) was 2.8 times higher than that on low pollution days ((29.5±12.6) Mm⁻¹). The Abs₃₆₅ in Xi’an also showed evident diurnal differences of (53.9±30.9) Mm⁻¹ and (67.4±30.3) Mm⁻¹ in the day and night, respectively. The MAE₃₆₅ ((2.04±0.16) m²·g⁻¹) on heavy haze days in Beijing was higher than that on clean days ((1.45±0.17) m²·g⁻¹). However, the MAE₃₆₅ in low pollution days ((1.56±0.24) m²·g⁻¹) in Xi’an was higher than that in heavy haze days ((1.38±0.07) m²·g−1). In addition, the MAE₃₆₅ in Xi’an during the day ((1.38±0.09) m²·g⁻¹) and night ((1.53±0.23) m²·g⁻¹) were considerably different. For the chemical composition of BrC, the contributions of 10 PAHs at the wavelength of 365 nm were 1.7% and 4.4% in Beijing in clean and heavy haze days, respectively, and which were 2.2% and 3.5% in low pollution and heavy haze days in Xi’an, respectively. The light absorption contributions of 10 PAHs at 365 nm wavelength in Xi’an in the day and night were 2.3% and 3.6%, respectively, with a remarkable difference. Discussion The Abs₃₆₅ in the two cities increased substantially with the increasing of pollution levels, and the variation trends of Abs₃₆₅ and OC concentration in the two cities showed strong consistency (Pearson r=0.99), indicating that the increasing in OC concentration was the main reason for the elevated Abs₃₆₅ values during heavy haze days in both cities and during nighttime in Xi’an. The MAE₃₆₅ for MS-BrC in the two cities showed different trends compared with Abs₃₆₅ under different pollution levels, indicating the chemical components and sources of BrC in the two cities were discrepant under different pollution levels. In Xi’an, the MAE₃₆₅ at night was higher than that in the day, which may be related to the increased primary emission and secondary generation at night. It is worth noting that the light absorption ratio of 10 PAHs at 365 nm in the two cities are 3.9 times (Beijing mean) and 5.8 times (Xi’an mean) of their carbon concentration ratio in OC, respectively, indicating that even a small amount of chromophore can produce a high contribution to BrC absorption. Conclusions Our results show that pollution levels and diurnal variation have varying degrees of influence on the optical properties and composition of urban atmospheric BrC. Recommendations and perspectives In this study, only the quantitative correlation between PAHs chromophores and the optical properties of MS-BrC was analysed and more studies are needed in the future to deepen the understanding of the relationship between optical properties and chemical composition of BrC.