Vertical characteristics of black carbon physical properties over Beijing region in warm and cold seasons

Abstract

The physical properties of black carbon (BC) including the mass loading, size distribution and mixing state were in-situ characterized by aircraft measurements using a single particle soot photometer (SP2) in the lower troposphere over Beijing area. The flights were conducted in both late spring and winter during the daytime with well-developed planetary boundary layer (PBL). The BC mass in the PBL (BCPBL) in late spring showed no apparent vertical gradient nor correlation with the PBL height (PBLH) due to strong convective mixing; in winter the BCPBL was more concentrated near ground and anti-correlated with the PBLH due to dilution effect of the dominant cleaner northerly air masses at higher altitude. The BC mass loading at height h, C(h) within the PBL can be extended from the surface level (C0) in late spring; for levels above the PBL, C(h) can be parameterized by applying an exponential decline function C(h) = C0*exp(h/hs), with the scale height (hs) of 0.31 ± 0.16 km and 0.66 ± 0.24 km for late spring and winter respectively. This parameterization excluded the profiles for: turbulent conditions when the BC mass was efficiently vented upwards and diluted, expressed as C(h) = C0 up to the top of the PBL; or in periods of strong southerly advection, when the entire column was significantly influenced by regional transport from the polluted south regions. The BC core mass median diameters (MMD) were commonly populated at 205–220 nm in both seasons, with additional mode of MMD ∼195 nm also frequently observed in late spring. The bulk relative coating thickness of BC (coated diameter divided by uncoated core diameter Dp/Dc) in the PBL mostly populated at 2.0–2.2 but at ∼1.2–1.6 in the lower free troposphere (FT). The mass absorption cross section of BC at 550 nm (MAC550), constrained by the SP2 measurements, was largely influenced by the coating thickness, was relatively consistent in the PBL at ∼8.6 m2 g−1, but reduced to 7–7.5 m2 g−1 in the FT or turbulent condition due to decreased coatings. The BC was found to exhibit smaller particle size in the FT but larger in the PBL, which may imply larger BC have been scavenged by low-level clouds. The BC particles trapped in the PBL or regionally transported from polluted region represent the most absorbing element in the particulate matter population and should be particularly considered in evaluating the radiative forcing impact of aerosols over this region.