The climatology and trend of black carbon in China from 12-year ground observations

Black carbon (BC) is one of the most important climate pollutants due to its strong positive radiative forcing on the climate system. However, long-term observation of BC is inadequate, and its trend remains unknown at both regional and global scales. In this study, using quality-controlled ground-based black carbon observations at 34 stations, the climatology of BC mass concentration during 2006-2017 in China was evaluated for the first time. The national annual average BC concentration was 3534 ng/m(3), and the concentration exhibited large spatial variation, from a minimum of 272 ng/m(3) to a maximum of 10,228 ng/m(3). Significant seasonal variations were also observed. The peak BC concentration (4763 +/- 3380 ng/m(3)) occurred during winter, followed by autumn (3752 +/- 2623 ng/m(3)) and spring (3022 +/- 2121 ng/m(3)). The lowest BC concentration occurred in summer (2583 +/- 1780 ng/m(3)) due to decreased emissions, wet removal by precipitation, and diffusion induced by strong turbulent mixing. Additionally, the diurnal variability at about 80% of the stations in China showed a bimodal pattern. Daily maximum BC concentration during 0000-0600, 0600-1200, 1200-1800, and 1800-2400 local solar time accounted for 0, 41, 12, and 47% of the total, respectively. In particular, the diurnal and monthly variations in BC were explicitly analyzed at seven baseline stations. Surface BC has exhibited a significant declining trend across China, which is partly corroborated by the increase in single scattering albedo during the same period in Beijing. Also, the BC radiative forcing at the top of the atmosphere has declined at the rate of -0.9 +/- 0.1 W/m(2)/10 years over China, which differs from with the trend in the simulation using the CMIP6 emission. The decreasing BC has led to a larger reduction rate in atmospheric heating (-3.7 +/- 0.4 W/m(2)/10 years), implying a less stable atmosphere that facilitates the dispersion of air pollutants. The declining BC burden implies for both the air quality in China and regional climate change.