Abstract
Abstract. Black carbon (BC), which is formed from the incomplete combustion of fuel sources (mainly fossil fuel, biofuel and open biomass burning), is a chemically inert optical absorber in the atmosphere. It has significant impacts on global climate, regional air quality and human health. During transportation, its physical and chemical characteristics as well as its sources change dramatically. To investigate the properties of BC (i.e., mass concentration, sources and optical properties) during intra-regional transport between the southern edge of the North China Plain (SE-NCP) and central 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 a city (Luohe – LH) in the SE-NCP during typical winter haze episodes. Using an Aethalometer, the highest equivalent BC (eBC) mass concentrations and the highest aerosol absorption coefficients (σabs) were found in LH in the SE-NCP, followed by the borderline cities (XY, SX and HA) and WH. The levels, sources, optical properties (i.e., σabs and absorption Ångström exponent, AAE) and geographic origins of eBC were different 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 to the increased combustion of fossil fuels (increased by 51.1 %–277 %), which was supported by the decreased AAE values (decreased by 7.40 %–12.7 %). The conditional bivariate probability function (CBPF) and concentration-weighted trajectory (CWT) results showed that the geographic origins of biomass burning (BCbb) and fossil fuel (BCff) combustion-derived BC were different. Air parcels from the south dominated for border sites during clean days, with contributions of 46.0 %–58.2 %, whereas trajectories from the northeast showed higher contributions (37.5 %–51.2 %) during pollution episodes. At the SE-NCP site (LH), transboundary influences from the south (CC) exhibited a more frequent impact (with air parcels from this direction comprising 47.8 % of all parcels) on the ambient eBC levels during pollution episodes. At WH, eBC was mainly from the northeast transport route throughout the observation period. Two transportation cases showed that the mass concentrations of eBC, BCff and σabs all increased, from upwind to downwind, whereas AAE decreased. This study highlights that intra-regional prevention and control for dominant sources at each specific site should be considered in order to improve the regional air quality.
Highlights
Black carbon (BC), a distinct type of carbonaceous material, has attracted wide attention over past decades mainly due to its climate effect (Hansen et al, 2000; Jacobson, 2000; Bond et al, 2013)
In order to investigate the regional transport of air pollutants and answer the question regarding whether the pollutants in central China (CC) can be transported to the southern edge of the North China Plain (SE-North China Plain (NCP)) in winter, five sites including Wuhan (WH), three borderline cities (Xiangyang – XY, Suixian – SX, and Hong’an – HA; from west to east in Fig. 1) between the NCP and CC, and a city (Luohe, LH) in the SE-NCP were selected
In order to understand the levels, optical properties, sources, regional transportation and aging of BC in central China and the south edge of North China Plain during winter haze episodes, simultaneous observations were conducted at rural (HA and SX), suburban (LH and XY) and megacity (WH) sites during January 2018
Summary
Black carbon (BC), a distinct type of carbonaceous material, has attracted wide attention over past decades mainly due to its climate effect (Hansen et al, 2000; Jacobson, 2000; Bond et al, 2013). BC can strongly absorb but reflects less light, with the direct radiative forcing of BC estimated to be +0.88 W m−2 (Bond et al, 2013) It is composed of small carbon spherules and has a large specific surface area, which allows it to absorb aerosol and provide a substrate for atmospheric chemical reactions In urban areas, BC can enhance haze pollution by modifying the planetary boundary layer height, which is unfavorable with respect to the vertical dispersion of air pollutants (Ding et al, 2016). This “dome effect” is more substantial in rural areas under the same BC conditions The properties of BC at rural and suburban sites needed to be emphasized, as they have generally been ignored in former field campaigns
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