Characterization of black carbon-containing fine particles in Beijing during wintertime

Abstract. Refractory black carbon (BC) is a product of incomplete combustion of fossil fuel, biomass and biofuel, etc. By mixing with other species, BC can play significant roles in climate change, visibility impairment and human health. Such BC-containing particles in densely populated megacities like Beijing may have specific sources and properties that are important to haze formation and air quality. In this work, we exclusively characterized the BC-containing particles in urban Beijing by using a laser-only Aerodyne soot particle aerosol mass spectrometer (SP-AMS), as part of the Atmospheric Pollution & Human Health (APHH) 2016 winter campaign. The average mass ratio of coating to BC core (RBC) was found to be ∼5.0. Positive matrix factorization shows the presence of significant primary fossil fuel and biomass-burning organics (64 % of total organics). Yet secondary species, including sulfate, nitrate and oxygenated organic aerosol (OA) species, could have significant impacts on the properties of BC-containing particles, especially for ones with larger BC core sizes and thicker coatings. Analyses of sources and diurnal cycles of organic coating reveal significant afternoon photochemical production of secondary OA (SOA), as well as nighttime aqueous production of a portion of highly oxygenated OA. Besides SOA, photochemical production of nitrate, not sulfate, appeared to be important. Further investigations on BC-containing particles during different periods show that, on average, more polluted periods would have more contributions from secondary species and more thickly coated BC tended to associate with more secondary species, indicating the important role of chemical aging to the pollution of BC-containing particles in urban Beijing during wintertime. However, for individual pollution events, primary species (fossil fuel, coal and biomass-burning emissions) could also play a dominant role, as revealed by the compositions of BC-containing particles in two polluted episodes during the sampling period.