Evidence of the mitigated urban particulate matter island (UPI) effect in China during 2000-2015.

Urban fine particulate matter (PM2.5) pollution has been the subject of great concern, due to its remarkable adverse effects on public health. However, quantitative investigations of the spatial concentration trends from urban to background areas are still lacking. The urban particulate matter island (UPI) effect, referring to the phenomenon that higher particle concentrations in urban areas are gradually attenuated to background areas, is found and investigated in this study. UPI intensity (UPII) and its footprint (UPIFP) are defined to quantify the magnitude and extent of UPI, respectively. Based on observations from 338 Chinese prefectures for 2000-2015, we confirm the existence of the UPI effect, and further reveal its spatiotemporal patterns. We find that: 1) 84% (283/338) of the cities in China in various city levels and climatic zones showed the UPI phenomenon during 2000-2015, and this phenomenon is closely related to the land-use/cover patterns between the urban area and surrounding areas; 2) different spatial patterns of UPI effect are apparent, with high UPII values and small UPIFP values in Beijing-Tianjin-Hebei, moderate UPII values and large UPIFP values in northern China, the Pearl River Delta and the Yangtze River Delta, and high UPII and UPIFP values in the Western Taiwan Straits region; 3) UPI mitigation can be observed nationwide, with significant decreasing trends for both UPII and UPIFP, benefiting from the increase in urban green spaces and the built-up proportion differences between urban and suburban areas during urbanization. Additionally, it is indicated that more urban residents and faster urban expansion correspond to a steeper decline of UPII in China during 2000-2015. The existence and characteristics of the UPI effect in China will allow new insight and understanding of urban pollution patterns, and will provide scientific evidence for urban planning and pollution control.

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