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. Black carbon (BC), which is formed from the incomplete combustion of fuelsources (mainly fossil fuel, biofuel and open biomass burning), is achemically inert optical absorber in the atmosphere. It has significant impactson global climate, regional air quality and human health. Duringtransportation, its physical and chemical characteristics as well asits sources change dramatically. To investigate the properties of BC (i.e.,mass concentration, sources and optical properties) during intra-regionaltransport between the southern edge of the North China Plain (SE-NCP) andcentral China (CC), simultaneous BC observations were conducted in a megacity(Wuhan – WH) in CC, in three borderline cities (Xiangyang – XY, Suixian –SX and Hong'an – HA; from west to east) between the SE-NCP and CC, and in acity (Luohe – LH) in the SE-NCP during typical winter haze episodes. Usingan Aethalometer, the highest equivalent BC (eBC) mass concentrations and thehighest aerosol absorption coefficients ( σabs ) were found inLH in the SE-NCP, followed by the borderline cities (XY, SX and HA) and WH.The levels, sources, optical properties (i.e., σabs andabsorption Angstrom exponent, AAE) and geographic origins of eBC weredifferent between clean and polluted periods. Compared with clean days,higher eBC levels (26.4 %–163 % higher) and σabs (18.2 %–236 % higher) were found during pollution episodes due tothe increased combustion of fossil fuels (increased by51.1 %–277 %), which was supported by the decreased AAE values(decreased by 7.40 %–12.7 %). The conditional bivariate probabilityfunction (CBPF) and concentration-weighted trajectory (CWT) results showedthat the geographic origins of biomass burning (BC bb ) and fossilfuel (BC ff ) combustion-derived BC were different. Air parcelsfrom the south dominated for border sites during clean days, withcontributions of 46.0 %–58.2 %, whereas trajectories from thenortheast showed higher contributions (37.5 %–51.2 %) duringpollution episodes. At the SE-NCP site (LH), transboundary influences fromthe south (CC) exhibited a more frequent impact (with air parcels from thisdirection comprising 47.8 % of all parcels) on the ambient eBC levelsduring pollution episodes. At WH, eBC was mainly from the northeast transportroute throughout the observation period. Two transportation cases showed thatthe mass concentrations of eBC, BC ff and σabs all increased, from upwind to downwind, whereas AAE decreased. This studyhighlights that intra-regional prevention and control for dominant sources ateach specific site should be considered in order to improve the regional airquality.

Intra-regional transport of black carbon between the south edge of the North China Plain and central China during winter haze episodes