Figure 1. Two PM_2.5 pollution processes in BJ and the corresponding daily mean PM_2.5 concentrations.

Brown points denote the data points within the PM_2.5 pollution processes filtered out in this study. Horizontal dashed lines denote the daily mean PM_2.5 levels, with the red lines referring to polluted days and the green lines referring to the attainment days measured by the static standard of 75 μg/m³.