In this paper, we report the results obtained from one year of real-time measurement (i.e., from December 2019 to November 2020) of atmospheric black carbon (BC) under a rural environment in Qingdao of Northeastern China. The annual average concentration of BC was 1.92 ± 1.89 μg m−3. The highest average concentration of BC was observed in winter (3.65 ± 2.66 μg m−3), followed by fall (1.73 ± 1.33 μg m−3), spring (1.53 ± 1.33 μg m−3), and summer (0.83 ± 0.56 μg m−3). A clear weekend effect was observed in winter, which was characterized by higher BC concentration (4.60 ± 2.86 μg m−3) during the weekend rather than that (3.22 ± 2.45 μg m−3) during weekdays. The influence of meteorological parameters, including surface horizontal wind speed, boundary layer height (BLH), and precipitation, on BC, was investigated. In particular, such BLH influence presented evidently seasonal dependence, while there was no significant seasonality for horizontal wind speed. These may reflect different roles of atmospheric vertical dilution on affecting BC in different seasons. The △BC/△CO ratio decreased with the increase of precipitation, indicative of the influence of below-cloud wet removal of BC, especially during summertime where rainfall events more frequently occurred than any of other seasons. The bivariate-polar-plot analysis showed that the high BC concentrations were mainly associated with low wind speed in all seasons, highlighting an important BC source originated from local emissions. By using concentration-weighted trajectory analysis, it was found that regional transports, especially from northeastern in winter, could not be negligible for contributing to BC pollution in rural Qingdao. In the coronavirus disease 2019 (COVID−19) case analysis, we observed an obvious increase in the BC/NO2 ratio during the COVID-19 lockdown, supporting the significant non-traffic source sector (such as residential coal combustion) for BC in rural Qingdao.

Abstract. Refractory black carbon (rBC) aerosols play an important role in air quality and climate change, yet highly time-resolved and detailed investigations on the physicochemical properties of rBC and its associated coating are still scarce. In this work, we used a laser-only Aerodyne soot particle aerosol mass spectrometer (SP-AMS) to exclusively measure rBC-containing (rBCc) particles, and we compared their properties with those of the total nonrefractory submicron particles (NR-PM1) measured in parallel by a high-resolution AMS (HR-AMS) in Shanghai. Observations showed that, overall, rBC was thickly coated, with an average mass ratio of coating to rBC core (RBC) of ∼5.0 (±1.7). However, the ratio of the mass of the rBC-coating species to the mass of those species in NR-PM1 was only 19.1 (±4.9) %; sulfate tended to condense preferentially on non-rBC particles, so the ratio of the sulfate on rBC to the NR-PM1 sulfate was only 7.4 (±2.2) %, while the majority (72.7±21.0 %) of the primary organic aerosols (POA) were associated with rBC. Positive matrix factorization revealed that organics emitted from cooking did not coat rBC, and a portion of the organics that coated rBC was from biomass burning; such organics were unidentifiable in NR-PM1. Small rBCc particles were predominantly from traffic, while large-sized ones were often mixed with secondary components and typically had a thick coating. Sulfate and secondary organic aerosol (SOA) species were generated mainly through daytime photochemical oxidation (SOA formation, likely associated with in situ chemical conversion of traffic-related POA to SOA), while nocturnal heterogeneous formation was dominant for nitrate; we also estimated an average time of 5–19 h for those secondary species to coat rBC. During a short period that was affected by ship emissions, particles were characterized as having a high vanadium concentration (on average 6.3±3.1 ng m−3) and a mean vanadium/nickel mass ratio of 2.0 (±0.6). Furthermore, the size-resolved hygroscopicity parameter (κrBCc) of rBCc particles was obtained based on their full chemical characterization, and was parameterized as κrBCc(x)=0.29–0.14 × exp⁡(-0.006×x) (where x ranges 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 16 (±3) % and 59 (±4) %, respectively, of the rBCc particles by number could be activated into cloud condensation nuclei (CCN). Our findings are valuable for advancing the understanding of BC chemistry as well as the effective control of atmospheric BC pollution.

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