Aircraft measurements of black carbon in the boundary layer over the North China Plain

Abstract. Black carbon aerosol (BC) is the principle absorber to modify the shortwave radiative balance between the lower atmosphere and the surface in polluted environment, which was in-situ characterized by aircraft measurements using a single particle soot photometer (DMT inc., SP2) throughout the boundary layer up to 3 km over the North China Plain around Beijing megacity. The flights were conducted in both hot season (late spring, Apr.–Jun. 2012, surface temperature > 20 °C, 10 flights) and cold season (winter, Dec. 2016, surface temperature < 5 °C, 6 flights). The BC mass in the daytime well-developed planetary boundary layer (BC_PBL) was found to be largely influenced by meteorology which modulated the local emission and regional transport. The BC_PBL in hot season showed no apparent vertical gradient or correlation with the PBL height (PBLH); whereas in winter the BC_PBL decreased at higher altitude and accumulated towards the surface due to reduced horizontal wind speed at low level, and was anti-correlated with the PBLH. The more homogenous vertical mixing of BC_PBL in hot season may result from the stronger convective mixing due to higher surface temperature, in addition the predominant southerly wind may have advected the largely polluted air mass, offsetting its dilution effect; whereas in winter the predominant cleaner air mass from the north had important dilution effect on local emissions. The overall averages showed lower BC mass loading in the surface layer during hot season but similar loadings (3.5–4 µg m⁻³) in the PBL for both seasons. The highly turbulent air conditions, as frequently observed in the early afternoon during hot season, largely diluted the BC mass down to the background level (~ 0.5 µg m⁻³); whereas in winter the BC_PBL was significantly elevated under dynamic air condition due to efficient transport of polluted air mass from the south.

The BC_PBL in winter showed systematically larger core size (mass median diameter, MMD = 217 ± 4 nm) than in late spring (209 ± 7 nm), which may be due to a higher contribution from the residential emission sector in cold season. The high BC mass loadings were mostly associated with a high fraction of thick coatings (F_coating), and the higher air moisture enhanced the coating content of BC_PBL. These suggest the primary sources with a range of co-emitted species significantly contributed to the coatings of BC, and the secondary formation which could be enhanced by elevated RH also played important role. The BC after significant removal showed much lower F_coating (~ 0.06) and smaller core size (MMD ~ 193 nm), implying that the coated and larger BC particle was preferentially removed. The scavenging efficiency of BC in the entrainment zone (EZ) showed positive correlation with the change of coatings between PBL and EZ for both seasons (F_coating,PBL > F_coating,EZ), however showed seasonally different behaviors in terms of core size (MMD_PBL > MMD_EZ in late spring, but the opposite in winter). These results provide the basis to evaluate the BC direct radiative forcing in the polluted planetary boundary layer over this region and also the indirect forcing of BC by interacting with low-level clouds.