Mixing characteristics of refractory black carbon aerosols at an urban site in Beijing

Black carbon aerosols play an important role in climate change because they directly absorb solar radiation. In this study, the mixing state of refractory black carbon (rBC) at an urban site in Beijing in the early summer of 2018 was studied with a single-particle soot photometer (SP2) as well as a tandem observation system with a centrifugal particle mass analyzer (CPMA) and a differential mobility analyzer (DMA). The results demonstrated that the mass-equivalent size distribution of rBC exhibited an approximately lognormal distribution with a mass median diameter (MMD) of 171 nm. When the site experienced prevailing southerly winds, the MMD of rBC increased notably, by 19 %. During the observational period, the ratio of the diameter of rBC-containing particles (D-p) to the rBC core (D-c) was 1.20 on average for D-c = 180 nm, indicating that the majority of rBC particles were thinly coated. The D-p/D-c value exhibited a clear diurnal pattern, with a maximum at 14:00 LST and a D-p growth rate of 2.3 nm h(-1); higher O-x conditions increased the coating growth rate. The microphysical properties of rBC were also studied. Bare rBC particles were mostly found in fractal structures with a mass fractal dimensions (D-fm) of 2.35, with limited variation during both clean and polluted periods. The morphology of rBC changed with coating thickness increasing. When the mass ratio of nonrefractory matter to rBC (M-R) was <1.5, rBC-containing particles were primarily found in external fractal structures, and they changed to a core-shell structure when M-R>6, at which point the measured scattering cross section of rBC-containing particles was consistent with that based on the Mie-scattering simulation. We found that only 28% of the rBC-containing particles were in coreshell structures with a particle mass of 10 fg in the clean period but that proportion increased considerably, to 45 %, in the polluted period. Due to the morphology change, the absorption enhancement (E-abs) was 12% lower than that predicted for core-shell structures.