Understanding the Rising Phase of the PM_2.5 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 R² > 0.8 and duration time T ≥ 18 hrs. The linear slopes quantitatively measured pollution levels by PM_2.5 concentrations rising rates (PMRR, μg/(m³·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, NO₂ and SO₂) 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.