Characteristics and source apportionment of black carbon aerosol in the North China Plain

In this study, the physical properties of refractory BC (rBC) including mass concentration, core size distribution and mixing state were in-situ characterized with a single particle soot photometer (SP2) at a polluted rural site in the North China Plain in November 2018. The average rBC mass concentration is 4.12 ± 2.47 μg m−3, accounting for 6.26% of PM1 mass. Its diurnal variation shows that the rBC concentration is affected by both the local residential activities and the evolution of planetary boundary layer (PBL). And a broader rBC core mass size distribution (CMSD) can be observed in the afternoon. The CMSD show a primary mode with peak diameter (Dm) of ~222 nm, and a small secondary mode with Dm of ~687 nm. The average absolute coating thickness (ACT) of rBC particles is 50 nm and the bulk relative coating thickness (BRCT) is about 1.7, indicating a relatively high aging level of rBC. The correlation analysis between SP2-separated rBC and PMF-separated OA factors indicate that rBC with different core size (Dc) and ACT may be originated from different sources. The rBC contributed by coal combustion is distributed throughout a large Dc range (~70–500 nm). The rBC particles emitted by traffic sources have small core sizes and thin coating thickness (Dc < 180 nm and ACT <50 nm); while the rBC particles emitted from biomass burning show a higher degree of aging (ACT >250 nm). In addition, the secondary mode of rBC particles are likely to be originated from coal combustion.