Variations in optical properties of water- and methanol-soluble organic carbon in PM2.5 in Tianjin and Handan over the Wintertime of 2018–2020

Abstract

The impact of the Three-year Action Plan for winning the “blue sky defense battle” on the pollution levels and chemical characteristics of fine particulate matter has been intensively studied over the past few years. However, the effect of air pollution control on light absorption of brown carbon remains unclear. In this study, the light absorption and fluorescence properties of water- and methanol-soluble organic carbon (WSOC and MSOC) in PM2.5 in Tianjin and Handan in North China Plain were measured and evaluated for three consecutive winters during the Three-year Blue-Sky Action (2018–2020). Overall, the WSOC and MSOC mass concentrations and light absorption coefficients of WSOC and MSOC at 365 nm generally decreased from 2018 to 2020, indicating that the emission of solvent-soluble organic carbon has been effectively controlled with the implementation of pollution abatement measures, such as the enforcement of clean energy policies. The light absorption coefficient and mass absorption efficiency at 365 nm of MSOC were 1.41–3.82 and 1.13–1.72 times higher than that of WSOC, respectively. Fluorescent spectra coupled with parallel factor analysis indicated that less‑/highly‑oxygenated humic-like (LO-/HO-HULIS) and non-N aromatic species (non-Nas) were the major fluorophores in both WSOC and MSOC in the two cities, with LO-HULIS and HO-HULIS as two major contributors to WSOC light absorption, while both HULIS and non-Nas fluorophores had a strong correlation with MSOC light absorption. Correlation analysis with source tracers and fluorescent indices indicated that the influence of combustion emissions remains significant, and secondary formation made a non-negligible contribution to the light absorption and fluorescent components of WSOC and MSOC in both cities. This work will contribute to a better understanding of the evolution of concentration levels and optical properties of WSOC and MSOC during pollution abatement and will inform the subsequent development of measures to achieve a win-win situation for both air quality and climate change.