Understanding the rising phase of the PM2.5 concentration evolution in Large China Cities

Abstract

Long-term air quality observations are seldom analyzed from a dynamic view. This study analyzed fine particulate matter (PM25) pollution processes using long-term PM25 observations in three Chinese cities. Pollution processes were defined as linearly growing PM25 concentrations following the criteria of coeficient of determination R2 > 0.8 and duration time T ≥ 18 hrs. The linear slopes quantitatively measured pollution levels by PM25 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 PM2.5 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. PM25 levels at the upwind regions of BJ and SH increased at high PMRRs. Regional transport dominated the PM25 pollution processes of SH. In BJ, both local contributions and regional transport increased during high-PMRR pollution processes. In GZ, PM2.5 pollution processes were mainly caused by local emissions.