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Time series of the PM2.5 mass in Xianghe (a), Beijing (b), Tianjin (c), Tangshan (d) and Langfang (e) during the intensive measurement period. The blue, gray and yellow shaded areas represent three periods: before the control period (P1), during the control period (P2) and after the control period (P3). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
