Size-Related Physical Properties of Black Carbon in the Lower Atmosphere over Beijing and Europe

The size-resolved properties of atmospheric black carbon (BC) importantly determine its absorption capacity and cloud condensation nuclei (CCN) ability. This study reports comprehensive vertical profiles of BC size-related properties over the Beijing area (BJ) and Continental Europe (CE). BC mass loadings over CE were in the range of clean background over BJ. For both planetary boundary layer (PBL) and lower free troposphere, the BC mass median core diameter over BJ during the cold season was 0.21 ± 0.02 μm, larger than the warm season over BJ and CE (0.18 ± 0.01 μm), which may reflect seasonal differences in emissions. The BC coatings were positively correlated with the pollution level, with background BC having a smaller coated count median diameter (0.19 ± 0.01 μm). The modeled absorption enhancement (E_abs) due to coatings was 1.23 ± 0.14 for the background but in the PBL following a linear expression (E_abs = 0.13 × Mass_BC,surface + 1.26). The CCN ability of BC was significantly enhanced in the polluted PBL, due to both enlarged size and increased hygroscopicity. In polluted BJ at predicted supersaturations, ∼0.08% half of the BC number could be activated, whereas the cleaner environment needs ∼0.14%. The results here suggest that the highly coated and absorbing BC can be efficiently incorporated into clouds and can exert important indirect radiative impacts over the polluted East Asia region.