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Zusammenfassung:
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.
