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Spatiotemporal characteristics of PM2.5 and PM10 at urban and corresponding background sites in 23 cities in China
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    Air pollution episodes in China are frequent and a more comprehensive understanding of pollution sources and impacts is needed to design appropriate strategies and set emission reduction targets. This study analyzes PM| and PM| concentrations measured in 23 cities at 178 urban sites and at 23 corresponding "urban contrast" sites in China with the goals of understanding spatial and temporal trends and quantifying the regional component of PM pollution. The contrast sites, located an average of 29km from cities in the upwind direction, are intended to represent "background" levels. Using daily measurements from April 2013 to March 2014, we assess compliance with air quality standards, PM|/PM| ratios and urban "increments," defined as the increase in PM levels in the city compared to the contrast site. Spatial and temporal patterns at daily, monthly and annual levels are shown using distributions, correlations, spatial autocorrelation, and factor analyses. At the contrast sites, PM| and PM| concentrations averaged 56±26 and 91±44μgm|, respectively, and China's daily and annual average air quality standards were frequently exceeded. PM| and PM| concentrations in most cities exceeded levels at the corresponding contrast sites, but by an average of only 14±14 and 26±27μgm|, respectively. Seasonal changes in PM| and PM| concentrations and urban increments were striking, e.g., levels increased 2 to 3-fold in winter at several sites. The significance of exurban and regional sources of PM| is demonstrated by the small urban increments, the strong correlations across broad regions, and the correlation between daily levels at city and contrast sites. These sources will require control to achieve air quality goals, in particular, the PM| and PM| targets announced by the Chinese government in 2013.

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