Distinct Sources of Char and Soot Black Carbon Aerosols During Winter Haze at Pair Sites in North China: Source Apportionment From Dual-Carbon Isotope Constrained PMF

Char and soot black carbon (BC) are distinct in physicochemical properties and climate effects, but quantitative source apportionment of which in ambient aerosols remains limited. A comparison of char and soot in fine particles in megacity Beijing and its southwest upwind rural site, Gucheng, during winter haze in 2018 was conducted, utilizing dual-carbon isotope constrained positive matrix factorization for source apportionment. BC concentrations in Gucheng were twice those in Beijing, with char accounting for 90% of BC at both sites. Despite similar char/soot ratios, source contributions of char and soot differed significantly between the two sites. Fossil fuel combustion emerged as the major source of char in both Beijing (82.1%) and Gucheng (72.1%), with higher contributions to soot (96.3% and 86.2%, respectively). Heavy oil combustion (51.2%) was the predominant source of char in Beijing, the char/soot values from which had been previously underestimated. Traffic-related sources were identified as the principal contributors to soot in Beijing (51.2%). Residential coal combustion was the largest contributor to both char (35.3%) and soot (47.6%) in Gucheng, whereas biomass burning contributions (similar to 10% higher than Beijing) were also non-negligible. Our findings underscore the issue of elevated BC concentrations in the Beijing-Tianjin-Hebei region during winter haze and the complexity of char and soot sources in urban environments. Accurately quantifying the contributions of various fossil and nonfossil sources to char and soot, and developing comprehensive emission inventories, are essential for improving assessments of BC effects on climate and environment.