Seasonal and diurnal variability of black carbon concentration in Beijing and its relationship with meteorological factors

Black carbon aerosol is not only a major air pollution species, but also causes significant climate impacts through the absorption of solar radiation and the interaction with the hydrological cycle. In this study, we report the variability of 2 years' black carbon (BC) concentration measured at an urban site in Beijing, as well as its relationship with meteorological factors. The observations are made using a Magee AE33 aethalometer, and span the period from August 2017 to June 2019. Results show that different from previous years, BC concentration is the lowest in winter and highest in the fall during these two years. The winter low concentration is mostly likely due to the strict pollution control strategy enforced over the Beijing-Tianjin-Hebei area in the recent two years. BC also exhibits a distinct diurnal cycle. Nighttime concentrations are higher than daytime, and the lowest value is generally found from noon to 2pm local time. BC concentration exhibits good correlation with scattering coefficients and PM2.5 concentrations. Wind direction and speed appears to be a major meteorological factor influencing BC concentration. We further develop an automatic boundary layer height (PBLH) retrieval algorithm based on normalized relative backscatter and liner depolarization ratio of the Micro Pulse Lidar data from Peking University's atmospheric physics comprehensive observation platform. We find that when wind speed is low, BC concentration is inversely correlated with PBLH. However, the relationship weakens when wind speed increases.