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. 2020 Sep 7;754:142226. doi: 10.1016/j.scitotenv.2020.142226

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Fig. 6 — Map of East Asia showing the total column density of sulfur dioxide (SO₂) averaged in February 2019 and February 2020 over the study area. (a), (b), (c), and (d) refer to the BTH, Wuhan, SMA, and TMA regions, respectively. All units for the total column density of sulfur dioxide (SO₂) are in molecules/cm². The purple and green arrows show the wind direction from upwind regions of the TMA and the SMA, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)