Yuan, W., Huang, R. J.*, Shen, J., Wang, K., Yang, L., Wang, T., Gong, Y., Cao, W., Guo, J., Ni, H., Duan, J., and Hoffmann, T.: More water-soluble brown carbon after the residential “coal-to-gas” conversion measure in urban Beijing, npj Clim. and Atmos. Sci., 6, 20, 2023.

The implementation of air pollution reduction measures has significantly reduced the concentration of atmospheric fine particles (PM2.5) in Beijing, among which “coal-to-gas” conversion measure may play a crucial role. However, the effect of this conversion measure on brown carbon (BrC) are not well known.

A research group led by Prof. HUANG Rujin from the Institute of Earth Environment (IEE) of the Chinese Academy of Sciences studied the chemical composition of BrC in humic-like fraction (HULIS-BrC) and water-insoluble fraction (WI-BrC) in atmospheric fine particles in Beijing before and after “coal-to-gas” conversion measure.

After the conversion measure, the number of HULIS-BrC compounds increased by ~14%, while the number of WI-BrC compounds decreased by ~8%. The intensity of over 90% of HULIS-BrC compounds also increased after the conversion measure, and correspondingly the O/C ratios of CHO and CHON compounds in HULIS-BrC fraction generally increased with the increase of intensity ratios of after/before conversion measure, indicating that there were more water-soluble highly oxygenated BrC compounds after “coal-to-gas” conversion measure. On the contrary, the intensity of more than 80% of WI-BrC compounds decreased after the conversion measure, and the O/C ratios of CHO and CHON compounds in WI-BrC fraction generally decreased with the decrease of intensity ratios of after/before the conversion measure, indicating that after “coal-to-gas” conversion measure the water-insoluble low oxygenated BrC compounds decreased.

This study, published in npj Climate and Atmospheric Science, sheds light on the differences in the chemical composition of BrC between before and after “coal-to-gas” conversion measure and suggests that future studies on the residential coal combustion BrC and secondary BrC deserve further exploration.

Fig.1 Chromophores in each compound category of HULIS-BrC and WI-BrC fractions before (2015) and after (2019) the “coal-to-gas” conversion measure. a Number and b intensity of chromophores.

Fig.2 Characteristics of CHO chromophores in HULIS-BrC and WI-BrC fractions before (2015) and after (2019) the “coal-to-gas” conversion measure. a, b, c, d Van Krevelen diagram. e, f Corresponding intensity ratios of after/before (2019/2015) the measure. The insert plot represents the chromophores identified only before or after the measure.