Chemical properties, sources and size-resolved hygroscopicity of submicron black-carbon-containing aerosols in urban Shanghai

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 (R_BC) 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 (κ_rBCc) of rBCc particles was obtained based on their full chemical characterization, and was parameterized as κ_rBCc(x)=0.29–0.14 × $\exp (-\mathrm{0.006}\times x)$ (where x ranges from 150 to 1000 nm). Under critical supersaturations (SS_C) 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.