A framework for delineating the regional boundaries of PM_2.5 pollution: A case study of China

Fine particulate matter (PM_2.5) pollution has been a major issue in many countries. Considerable studies have demonstrated that PM_2.5 pollution is a regional issue, but little research has been done to investigate the regional extent of PM_2.5 pollution or to define areas in which PM_2.5 pollutants interact. To allow for a better understanding of the regional nature and spatial patterns of PM_2.5 pollution, This study proposes a novel framework for delineating regional boundaries of PM_2.5 pollution. The framework consists of four steps, including cross-correlation analysis, time-series clustering, generation of Voronoi polygons, and polygon smoothing using polynomial approximation with exponential kernel method. Using the framework, the regional PM_2.5 boundaries for China are produced and the boundaries define areas where the monthly PM_2.5 time series of any two cities show, on average, more than 50% similarity with each other. These areas demonstrate straightforwardly that PM_2.5 pollution is not limited to a single city or a single province. We also found that the PM_2.5 areas in China tend to be larger in cold months, but more fragmented in warm months, suggesting that, in cold months, the interactions between PM_2.5 concentrations in adjacent cities are stronger than in warmer months. The proposed framework provides a tool to delineate PM_2.5 boundaries and identify areas where PM_2.5 pollutants interact. It can help define air pollution management zones and assess impacts related to PM_2.5 pollution. It can also be used in analyses of other air pollutants.