Characterization of Black Carbon-Containing Fine Particles in Beijing during Summertime: Contrast between SP-AMS and HR-AMS

Refractory black carbon (rBC) containing aerosol (rBCc) plays an important role in atmospheric warming effect on both regional and global scale. Its capacity of light absorption varies greatly depending on the mixing state and chemical composition associated with rBC. However, knowledge of the mechanisms on how different secondary organic aerosol (SOA) mixing with rBCc is still rare. Here we report, chemical-resolved comparison of between rBC related non-refractory particulate matter (NR-PM_rBC, i.e., coatings) and total NR-PM₁ in summer urban Bejing, by using a laser-only soot particle aerosol mass spectrometer (SP-AMS) and a high-resolution aerosol mass spectrometer (HR-AMS). We found that, overall, NR-PM_rBC account for ~40.0% of the mass of all NR-PM₁, and the biggest difference was between organic aerosol (OA). Primary OA (POA) was observed mainly been co-emitted on rBC at night, while secondary OA (SOA) on rBC was majorly attributed to the condensation and/or coagulated of photochemical aging of volatile organic compounds (VOCs) in the afternoon. The rBCc has an average mass ratio of NR-PM_rBC to rBC (coating thickness, R_BC) of ∼5.0, and the photochemical process was the dominated driven factor in rBCc aging. Interestingly, the rBCc with the R_BC value larger than 7.5 (i.e., thickly coated) was less oxygenated than the moderated coated rBC (5.5 < R_BC < 7.5), mainly due to the influence of the coated biomass burning and cooking-related organic aerosol (BBOA and COA). In addition, for the first time, the coated COA was identified by using positive matrix factorization (PMF), with about two hours delay of the diurnal profile peak of COA/rBC compares to that of COA in total NR-PM₁, which proposed a synergistic growth with the striking enhancement of photochemical-related SOA (PCOOA). Nitrate fraction was observed less when R_BC > 7.5 due to its relatively high volatility property. rBC was found decreasing with the increase of coating thickness (R_BC), rBCc, and coated organics. We further evaluated the “synergistic effect” through calculating the similar compatible parameters based on those possible molecules from tracer ion fragments of different SOA mass spectrum. Our findings highlight that the contributions of BBOA and COA on rBC can be enhanced effectively through similar compatible with the PCOOA, thus increase the coating thickness and the light absorption ability of rBCc through “lensing effect”.