Variation, sources and historical trend of black carbon in Beijing, China based on ground observation and MERRA-2 reanalysis data.

Based on the ground-measurements and MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, Version 2) reanalysis data, the temporal-spatial variation of black carbon (BC) in Beijing and the affecting factors were investigated. According to the ground-measured BC concentration in November months of 2014, 2015 and 2016, the before-heating period in November 2014 showed the lowest BC concentration as a result of the efficient emission controls for the Asia-Pacific Economic Cooperation (APEC) meeting. Except for November 2014, the BC mass concentration during the heating periods was notably lower than the before-heating periods in November 2015 and 2016. Wind speed and relative humidity appeared to be two important meteorological parameters affecting BC pollution. The MERRA-2 BC concentration was validated through comparison with the continuous ground BC measurements in 2015 and 2016, affirming its reliability in demonstrating the large scale and long term variations of the ground BC concentration. The MERRA-2 BC spatial distribution, the potential source regions determined by concentration weighted trajectory (CWT) analysis, and the regional emission inventories were combined to reveal the potential source regions and source types of BC in Beijing. Transportation emission in Beijing and residential emissions in the neighboring regions such as Hebei appeared to be important sources of BC in Beijing. According to the historical trends of MERRA-2 BC concentration and typical fossil fuel consumption (1980-2017), local coal and coke are no longer the major factor affecting the BC concentration, instead, liquid fuels such as gasoline, kerosene, and diesel may highly contribute to the BC pollution in Beijing in recent years. Regional transport of BC may have also contributed to the loading of BC in Beijing. Open biomass burning may be a non-negligible factor for the short-term variation of BC in the atmosphere.