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

Abstract

Abstract. Black carbon (BC), which is formed from the incomplete combustion of fuel\nsources (mainly fossil fuel, biofuel and open biomass burning), is a\nchemically inert optical absorber in the atmosphere. It has significant impacts\non global climate, regional air quality and human health. During\ntransportation, its physical and chemical characteristics as well as\nits sources change dramatically. To investigate the properties of BC (i.e.,\nmass concentration, sources and optical properties) during intra-regional\ntransport between the southern edge of the North China Plain (SE-NCP) and\ncentral China (CC), simultaneous BC observations were conducted in a megacity\n(Wuhan – WH) in CC, in three borderline cities (Xiangyang – XY, Suixian –\nSX and Hong an – HA; from west to east) between the SE-NCP and CC, and in a\ncity (Luohe – LH) in the SE-NCP during typical winter haze episodes. Using\nan Aethalometer, the highest equivalent BC (eBC) mass concentrations and the\nhighest aerosol absorption coefficients ( σabs ) were found in\nLH in the SE-NCP, followed by the borderline cities (XY, SX and HA) and WH.\nThe levels, sources, optical properties (i.e., σabs and\nabsorption Angstrom exponent, AAE) and geographic origins of eBC were\ndifferent between clean and polluted periods. Compared with clean days,\nhigher eBC levels (26.4\u2009%–163\u2009% higher) and σabs \n(18.2\u2009%–236\u2009% higher) were found during pollution episodes due to\nthe increased combustion of fossil fuels (increased by\n51.1\u2009%–277\u2009%), which was supported by the decreased AAE values\n(decreased by 7.40\u2009%–12.7\u2009%). The conditional bivariate probability\nfunction (CBPF) and concentration-weighted trajectory (CWT) results showed\nthat the geographic origins of biomass burning (BC bb ) and fossil\nfuel (BC ff ) combustion-derived BC were different. Air parcels\nfrom the south dominated for border sites during clean days, with\ncontributions of 46.0\u2009%–58.2\u2009%, whereas trajectories from the\nnortheast showed higher contributions (37.5\u2009%–51.2\u2009%) during\npollution episodes. At the SE-NCP site (LH), transboundary influences from\nthe south (CC) exhibited a more frequent impact (with air parcels from this\ndirection comprising 47.8\u2009% of all parcels) on the ambient eBC levels\nduring pollution episodes. At WH, eBC was mainly from the northeast transport\nroute throughout the observation period. Two transportation cases showed that\nthe mass concentrations of eBC, BC ff and σabs \nall increased, from upwind to downwind, whereas AAE decreased. This study\nhighlights that intra-regional prevention and control for dominant sources at\neach specific site should be considered in order to improve the regional air\nquality.