A one-year study on black carbon in urban Beijing: Concentrations, sources and implications on visibility

Abstract

A one-year field measurement was conducted at an urban atmospheric environment monitoring station in Beijing. The concentrations, sources and implications on visibility of black carbon (BC) during different periods were investigated. The average BC concentration was 2.8 ± 2.5, 3.2 ± 1.9, 4.4 ± 3.6 and 5.3 ± 4.5 μg m−3 in spring, summer, autumn and winter, respectively. The source apportionment presented liquid fuel (traffic) was the main source of BC in spring (77.4%), summer (86.8%) and autumn (70.8%), while solid fuel (coal and biomass burning) dominated in winter (64.3%), implying regional air transport occurred because of the limited combustion activities in urban Beijing. Notably, the contribution of solid fuel decreased from Ⅲ (4 ≤ Vis <6 km) to Ⅴ (0 < Vis <2 km), indicating the effect of regional air transport weakened during the wintertime severe haze pollution. Combining the results of PSCF, BC during the winter haze period was not only influenced by the air masses from southern regions but also western areas, e.g., Datong city, where coal is still widely used. The ratio of σabs/Bext (light absorption coefficient/atmospheric extinction coefficient) during the haze periods was higher than that of non-haze periods in spring, autumn and winter, proving higher BC concentration favored the decline in visibility. However, in all seasons, the decreasing ratio of σabs/Bext with the haze pollution getting worse, indicated that BC contributed less and other components (e.g., SO42−, NO3− and organics) contributed more to the visibility reduction when severe haze event occurred.