Chemical and optical distinctions between elemental carbon subcategories Char-EC and Soot-EC: Insights from a haze event in North China.

Elemental carbon (EC), an important pollutant component in the atmosphere, is composed of two subcategories: Char-EC and Soot-EC. Their differences in chemical structure and light absorption capacity need further investigation. This study focused on an hourly time-resolved haze event in Wangdu, Hebei Province, eliminating potential interference from organic carbon, to compare the differences in aromatic condensation degree and mass absorption cross-section (MAC) between Soot-EC and Char-EC. The results indicate that, in terms of chemical structure, Soot-EC is a EC component with a higher aromatic condensation degree than Char-EC. During the haze event, the average number of carboxyl groups (NCOOH) and the ratio of B6CA/B6CA (B6/B5) in Soot-EC were higher than those in Char-EC (NCOOHSoot-EC 5.88 ± 0.17 > NCOOHChar-EC 5.42 ± 0.20; (B6/B5)Soot-EC 10.57 ± 4.27>(B6/B5)Char-EC 4.71 ± 2.59). In terms of light absorption capacity, the average MAC of Char-EC (9.52 ± 3.17 m2/g) was 1.64 times higher than that of Soot-EC (5.82 ± 2.88 m2/g) during this haze event. However, aromatic condensation degree could only explain a small portion of the difference in MAC between Soot-EC and Char-EC. Source apportionment results indicated that EC emissions during this haze event were mainly attributed to significant contributions from biomass burning (BB). The higher light absorption capacity of Char-EC may be due to the higher oxygen-containing group content in Char-EC emitted from BB.