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Abstract

Refractory black carbon (rBC) aerosols play an important role in air quality and climate change, yet high time-resolved and detailed investigation on the physicochemical properties of rBC and its associated coating is still scarce. In this work, we used a laser-only Aerodyne soot particle aerosol mass spectrometer (SP-AMS) to exclusively measure the rBC-containing (rBCc) particles, and compared their properties with the total non-refractory submicron particles (NR-PM1) measured in parallel by a high-resolution AMS (HR-AMS) in Shanghai. The observation shows that rBC was overall thickly coated with an average mass ratio of coating to rBC core (RBC) of ~5.0. However, mass of rBC coating species only occupied 19.1 % of those in NR-PM1; sulfate tended to condense preferentially on non-rBC particles therefore its portion on rBC was only 7.4 %, while the majority of primary organic aerosols (POA) were associated with rBC (72.7 %). Positive matrix factorization reveals that cooking emitted organics did not coat on rBC, and a portion of organics coated on rBC was from biomass burning which was unidentifiable in NR-PM1 organics. Small rBCc particles were predominantly from traffic, while large-sized ones were often mixed with secondary components and typically had thick coating. During this campaign, sulfate and secondary organic aerosol (SOA) species were generated mainly through daytime photochemical oxidation (SOA formation likely involved with in-situ chemical conversion of traffic-related POA to SOA), while nocturnal heterogeneous formation was dominant for nitrate; we also estimated the average times of 5~19 hours for those secondary species to coat on rBC. Particles during a short period that was affected by ship emissions, were characterized with a high vanadium concentration (on average 5.8 ng m−3) and a vanadium/nickel mass ratio of 2.0. Furthermore, the size-resolved hygroscopicity parameter (кrBCc) of rBCc particles was obtained based on its fully chemical characterization, and was parameterized as кrBCc(x)= 0.29−0.14 × exp(-0.006 × x) (x is from 150 to 1000 nm). Under critical supersaturations (SSC) of 0.1 % and 0.2 %, the D50 values were 166 ± 16 and 110 ± 5 nm, respectively, and with 16 ± 3 % and 59 ± 4 % of rBCc in number could be activated into cloud condensation nuclei (CCN). Our findings are valuable to advance the understanding of BC chemistry as well as the effective control of atmospheric BC pollution.