Water-soluble brown carbon in atmospheric aerosols along the transport pathway of Asian dust: Optical properties, chemical compositions, and potential sources

摘要

As an important type of light-absorbing aerosol, brown carbon (BrC) has the potential to affect the atmospheric photochemistry and Earth's energy budget. A comprehensive field campaign was carried out along the transport pathway of Asian dust during the spring of 2016, including a desert site (Erenhot), a rural site (Zhangbei), and an urban site (Jinan), in northern China. Optical properties, bulk chemical compositions, and potential sources of water-soluble brown carbon (WS-BrC) were investigated in atmospheric total suspended particulate (TSP) samples. Samples from Zhangbei had higher mass absorption efficiency at 365 nm (MAE_(365),1.32 ± 0.34 m~2 g~(-1)) than those from Jinan (1.00 ± 0.23 m~2 g~(-1)) and Erenhot (0.84 ± 0.30 m~2 g~(-1)). Compere to the non-dust samples, elevated water-soluble organic carbon (WSOC) concentrations and MAE_(365) values of dust samples from Erenhot are related to the input of high molecular weight organic compounds and biogenic matter from the Gobi Desert, while lower values from Zhangbei and Jinan are attributed to the dilution effect caused by strong northwesterly winds. Based on fluorescence excitation-emission matrix spectra and parallel factor analysis, two humic-like (C1 and C2) and two protein-like (C3 and C4) substances were identified. Together, Cl and C2 accounted for -64% of total fluorescence intensity at the highly polluted urban Jinan site; C3 represented -45% at the rural Zhangbei site where local biomass burning affects; and C4 contributed ~24% in the desert region (Erenhot) due to dust-sourced biogenic substances. The relative absorptive forcing of WS-BrC compared to black carbon at 300-400 nm was about 31.3%, 13.9%, and 9.2% during non-dust periods at Erenhot, Zhangbei, and Jinan, respectively, highlighting that WS-BrC may significantly affect the radiative balance of Earth's climate system and should be included in radiative forcing models.