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 71 % from 6.25 ± 5.73 µg m−3 in 2012 to 1.80 ± 1.54 µg m−3 in 2020
and 47 % decreases in the light extinction coefficient (bext, λ = 630 nm) 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. ΔeBC / ΔCO also showed an annual decrease from ∼ 7 to 4 ng m−3 ppbv−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 0.79 ± 0.11 to 0.88 ± 0.06, and mass extinction efficiency (MEE) increased from 3.2 to 3.8 m2 g−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 50 %, 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 (ΔFR) of BC decreased by 67 % from +3.36 W m−2 in 2012 to +1.09 W m−2 in 2020, and that of BrC decreased from +0.30 to +0.17 W m−2 during 2018–2020. Such changes might have important implications for affecting aerosol–boundary layer interactions and the improvement of future air quality.
Measurement report: Long-term changes in black carbon and aerosol optical properties from 2012 to 2020 in Beijing, China
