Understanding the Rising Phase of the PM2.5 Concentration Evolution in Large China Cities

Long-term air quality observations are seldom analyzed from a dynamic view. This study analyzed fine particulate matter (PM 2.5 ) pollution processes using long-term PM 2.5 observations in three Chinese cities. Pollution processes were defined as linearly growing PM 2.5 concentrations following the criteria of coefficient of determination R 2  > 0.8 and duration time T  ≥ 18 hrs. The linear slopes quantitatively measured pollution levels by PM 2.5 concentrations rising rates (PMRR, μg/(m 3 ·hr)). The 741, 210 and 193 pollution processes were filtered out, respectively, in Beijing (BJ), Shanghai (SH), and Guangzhou (GZ). Then the relationships between PMRR and wind speed, wind direction, 24-hr backward points, gaseous pollutants (CO, NO 2 and SO 2 ) concentrations, and regional PM 2.5 levels were studied. Inverse relationships existed between PMRR and wind speed. The wind directions and 24-hr backward points converged in specific directions indicating long-range transport. Gaseous pollutants concentrations increased at variable rates in the three cities with growing PMRR values. PM 2.5 levels at the upwind regions of BJ and SH increased at high PMRRs. Regional transport dominated the PM 2.5 pollution processes of SH. In BJ, both local contributions and regional transport increased during high-PMRR pollution processes. In GZ, PM 2.5 pollution processes were mainly caused by local emissions.