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

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) combustionderived 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 Published by Copernicus Publications on behalf of the European Geosciences Union. 4500 H. Zheng et al.: Intra-regional transport of black carbon 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.

with 1-hour resolution each day (starting from 0:00 to 23:00) at 200 m AGL (Fig. S2). These trajectories were than clustered 185 according to their geographic origins (Fig. 1) Figure S3 shows the hourly 193 averaged meteorological parameters at the five sites. Meteorological conditions at the five sites followed similar variation 194 trends. However, significant differences (p < 0.01) of these parameters were found (Table S3)   and Henan provinces were about 0.6−1.0 g C m −2 yr −1 , which were higher than other regions . Simulation 216 results also suggested that the near-surface concentrations of BC (6−8 μg m −3 ) in Hubei and Henan were higher than those in 217 south China (4−6 μg m −3 ) during winter . Compared to the data in other countries (Table S1)   282 . In this study, the BC, PM2.5 and CO were well correlated with each other (Fig. S8)

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Overall, BC in this study was not likely from industrial emissions (Fig. 7a)  BCbb was due to more biomass burning in the southeast direction of HA and WH (Fig. S10). At LH, the CBPF plots of BCbb 308 and BCff were the same with the eBC as discussed above.

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In order to describe the BC transportation from upwind to downwind directions, we used Eq. (13) in section 2.3.3 to calculate 310 the surface transport (ST) of eBC (Fig. 9)

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Employing CWT method, the potential geographic origins of eBC for the five sites were explored (Fig. S11). Overall, CWT 318 results of eBC at the five sites suggested that high eBC levels were found both in the north and south directions of LH and 319 WH, while the high levels (i.e., > 4 μg m −3 ) of eBC were only found from northeast directions of HA, SX and XY (Fig. S11).

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Additionally, the potential geographic source regions of BCbb and BCff at HA, LH and WH were also discussed as shown in and CBPF results at WH suggested that there were intensive biomass burning activities in the south direction of WH during 327 the observation period, which was verified by the MODIS fire-points distribution (Fig. S10).

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We also discussed the source region differences of BC under different air quality (Fig. 11).

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To explore the BC variations (i.e., mass concentration, sources and AAE) during the transportation, we chose two cases. LH 342 and HA were selected as the study sites due to the same instrument deployment (AE33) and they are representative of SE-NCP 343 and CC. BC transportation from HA to LH and from LH to HA were both considered. Figure 13a show the hourly backward 12 trajectories reaching at HA on 2018-1-12 and the trajectory at 13:00 (GMT) (black line) was found passing through LH and 345 the travelling time was about 28 h. Therefore, the eBC mass concentration (including BCff and BCbb), σabs and AAE at the 346 upwind site LH on 8:00 2018-1-11 (GMT) and downwind site HA on 13:00, 2018-1-13 (GMT) were compared (Fig. 13b) (Figure 13c). The eBC, BCff and σabs increased from upwind (HA) to 351 downwind (LH), while BCbb and AAE decreased from 2.37 ± 0.23 μg m −3 and 1.43 ± 0.02 to 2.14 ± 0.14 μg m −3 and 1.32 ± 352 0.01, respectively (Fig. 13d)

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Therefore, the diameter of BC particle increased during the transportation due to the aging processes supported by the increased 371 absorb coefficients and decreased AAE as discussed above.     Table   S1 and S2 in the supplementary materials.   (Table S4).