Mixing Characteristics of Refractory Black Carbon Aerosol during Winter in Beijing

Abstract:

Black carbon aerosol is an important contributor to positive radiative forcing, thus has significant impact on climate change. In addition, it has also received great attention due to the known health impact. The health and climate effects of black carbon aerosol are strongly linked to its characteristics including size distribution, mixing state and composition. Such characteristics could vary across season and location. In this study, the mixing characteristics of black carbon aerosol was investigated with high time resolution in an urban site in Beijing during winter of 2018, after enforcing the Air Pollution Prevention and Control Plan in China (APPCP, from 2013-2017). The mixing state of refractory black carbon (rBC) was measured realtime using a single-particle soot photometer (SP2), and their mixing composition was measured using a single particle aerosol mass spectrometry (SPAMS). The results showed that the average rBC concentration was 0.93 μg/m³, accounting for 2% of PM_2.5 mass on average, which was reduced by about 4 times after the implementation of the APPCP in China. A total of 1,171,891 BC containing particles were collected and classified into five major types, including pure BC, aged BC, BC-OC (BC mixing with organic carbon), BC-levoglucosan, and BC-metals, which were mainly from sources including vehicular exhaust, biomass burning, and coal combustion. From the non-haze to haze episodes, the contribution of vehicle exhaust was reduced while coal combustion was significantly increased, accompanied by increased airmass transported from south of Beijing, and increased BC types (aged BC and BC-OC). In addition, the mass median diameter of rBC was greatly increased during haze episodes, and the ratio of the diameter of rBC particles to its core increased by about 27%, due to enhanced secondary formation compared to non-haze periods. This study presents a more complete and detailed examination of sources, mixing states, and composition of rBC in different pollution levels during winter in Beijing, providing valuable and rich information for other studies which focus on studying health and climate effects of black carbon.