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The mass concentrations of air pollutants in different stages of COVID-19 pandemic in 2020 and in the same periods in 2019 (the cross symbols are mean values, the lines in the boxes are median values, the upper and lower boundaries of the boxes are 75th and 25th percentiles, and the whiskers above and below the boxes are 90th and 10th percentiles; S1-S5 represent Stages 1 to 5).
