Measurement report: Long-term changes in black carbon and aerosol optical properties from 2012 to 2020 in Beijing, China

Abstract

Atmospheric aerosols play an important role in the radiation balance of the earth-atmosphere system. However, our knowledge of the long-term changes in equivalent black carbon (eBC) and aerosol optical properties in China is very limited. Here we analyze the 9-year measurements of eBC and aerosol optical properties from 2012 to 2020 in Beijing, China. Our results showed large reductions in eBC by 71g% from 6.25g±g5.73gμggm-3 in 2012 to 1.80g±g1.54gμggm-3 in 2020 and 47g% decreases in the light extinction coefficient (bext, λgCombining double low lineg630gnm) of fine particles due to the Clean Air Action Plan that was implemented in 2013. The seasonal and diurnal variations of eBC illustrated the most significant reductions in the fall and at nighttime, respectively. eBCg/gCO also showed an annual decrease from g1/4g7 to 4gnggm-3gppbv-1 and presented strong seasonal variations with high values in spring and fall, indicating that primary emissions in Beijing have changed significantly. As a response to the Clean Air Action Plan, single-scattering albedo (SSA) showed a considerable increase from 0.79g±g0.11 to 0.88g±g0.06, and mass extinction efficiency (MEE) increased from 3.2 to 3.8gm2gg-1. These results highlight the increasing importance of scattering aerosols in radiative forcing and a future challenge in visibility improvement due to enhanced MEE. Brown carbon (BrC) showed similar changes and seasonal variations to eBC during 2018-2020. However, we found a large increase of secondary BrC in the total BrC in most seasons, particularly in summer with the contribution up to 50g%, demonstrating an enhanced role of secondary formation in BrC in recent years. The long-term changes in eBC and BrC have also affected the radiative forcing effect. The direct radiative forcing (FR) of BC decreased by 67g% from +3.36gWgm-2 in 2012 to +1.09gWgm-2 in 2020, and that of BrC decreased from +0.30 to +0.17gWgm-2 during 2018-2020. Such changes might have important implications for affecting aerosol-boundary layer interactions and the improvement of future air quality.