Source-dependent wet scavenging of char and soot at a regional background site of the Yangtze River Delta region, China

Abstract. Atmospheric black carbon (BC) consists of two subfractions, char and soot, formed through different combustion processes and possessing distinct physicochemical properties. Here, we investigated the sources, precipitation-driven removal, and attenuation-derived apparent light absorption of char and soot in PM_2.5 at a regional background site in the Yangtze River Delta region, China, using a positive matrix factorization model constrained by ¹⁴C-Latin Hypercube Sampling estimates. Char dominated BC mass (77.5 ± 12.7 %), with higher concentrations in winter and autumn, whereas soot exhibited the opposite seasonal pattern. Wintertime char was mainly contributed by biomass burning (42.0 %) and coal combustion (34.2 %), consistent with enhanced regional transport from northern China. Traffic-related sources, including gasoline/diesel combustion and road dust, dominated char in spring (40.8 %) and summer (48.7 %), while heavy-oil combustion dominated in autumn (42.8 %), potentially reflecting coastal/marine shipping emission. Soot was largely fossil-derived, with coal combustion dominating in winter (74.9 %) and traffic-related sources prevailing in other seasons (58.3–65.4 %). Precipitation exhibited a distinct selective scavenging effect, removing char, particularly biomass-derived char, more efficiently and consequently reducing its absorption. By contrast, soot concentration remained statistically unchanged, whereas its apparent absorption and MAE increased with precipitation amount, which is potentially linked to humidity-related changes in particle mixing state. These results demonstrate that source- and subfraction-dependent wet scavenging alters BC composition and apparent absorption, with important implications for interpreting BC observations and improving model representations in remote background/receptor regions.