A long-term analysis of atmospheric black carbon MERRA-2 concentration over China during 1980–2019

Black carbon (BC), an important component of atmospheric aerosols, has a great influence on regional and global radiation balance, climate and human health due to its small particle size, large specific surface area and radiation forcing. The long-term variation of atmospheric BC over China during 1980–2019 was investigated through MERRA-2 reanalysis data. MERRA-2 BC generally presented a good correlation (average R = 0.61) with 852 monthly samples from ground-based observations at 64 stations around China. In recent 40 years, the annual-averaged atmospheric BC concentration derived from MERRA-2 reanalysis data was 1.10 ± 0.22 μg/m3, with an average annual growth rate of 1.52%. The monthly BC concentrations showed a "U"-shaped trend. Based on the Mann-Kendall trend analysis, the BC concentration can be roughly divided into three stages: (1) the “low value” stage with slow growth rate (1.68%) (1980–1999, 0.91 ± 0.10 μg/m3), (2) the fluctuating “median value” stage with high growth rate (4.44%) (2000–2007, 1.28 ± 0.13 μg/m3), and (3) the “high value” stage with slow downtrend (−0.87%) (2008–2019, 1.32 ± 0.06 μg/m3). Peak times and multi-year average growth rates of BC concentration and emission were not synchronized. The spatial distribution, dividing by the Hu Line, a line dividing the population density and urbanization of China, formed three BC high-value areas in Sichuan Basin, Northern Henan area and Beijing-Tianjin-Hebei (BTH). The altitude where the concentration of BC increased fastest at an average annual growth rate of 3.47% during 1980–2019 was between 0 and 500 m. The growth rate of BC concentration was close to zero as the altitude increased. During the past 40 years, significant overall uptrends were detected in MERRA-2 BC concentration with Mann-Kendall trend analysis at pixel scale, especially during 1980–1999 and 2000–2007. This increasing trend was more obvious in the eastern Hu Line. Whereas, a downward trend was appeared in the plains and basins of southeast China during 2008–2019.