Black carbon (BC) is one important component contributing to global warming and its climate-related impacts strongly depend on mixing state. Previous observations at ground level indicated BC aging was at a fast rate in daytime with efficient photochemical reactions, while BC aging significantly weakened at night. Here we present evidences that BC aging still occurs efficiently at night in the residual layer (RL). The ratio of thickly coated refractory BC (rBC) in total rBC (f(BC)) increased from 51.3% at 00:00 LST to 61.5% at 07:00 LST at the CITIC station, which located in the RL at night, with an increasing rate of 1.4% per hour. Such an increasing rate was even higher than that during noontime (11:00 to 15:00 LST, 0.7% per hour). Similar trend also reflected in the coating thickness (Dp/Dc) of rBC particles, which increased from 1.52 at 00:00 LST to 1.63 at 07:00 LST. The aging of rBC in the RL at night enhances light absorption of rBC particles correspondingly; calculated absorption enhancement (E-abs) increased from 1.64 at 00:00 LST to 1.79 in at 07:00 LST. Further analysis indicated that the Eabs depends not only on the D-p/D-c of rBC particles, but also on its size. An increase in the size of rBC particles in polluted episode can also enhance the Eabs. Combined observations of development of boundary layer and pollutants at the CITIC station suggested that rBC particles were upwards transported in daytime and trapped in the RL at night, where they were aged efficiently. These results will improve our understanding on rBC aging in the atmosphere, and hence help to evaluate its radiative forcing.

来源平台：ATMOSPHERIC ENVIRONMENT