Spatiotemporal characteristics of PM2.5 and PIV10 at urban and corresponding background sites in 23 cities in China

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 PM2.5 and PM10 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 29 km 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, PM2.5/PM10, 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, PM2.5 and PM10 concentrations averaged 56 +/- 26 and 91 +/- 44 mu g M-3, respectively, and China's daily and annual average air quality standards were frequently exceeded. PM2.5 and PM10 concentrations in most cities exceeded levels at the corresponding contrast sites, but by an average of only 14 +/- 14 and 26 +/- 27 mu g M-3, respectively. Seasonal changes in PM2.5 and PM10 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 PM2.5 is demonstrated by the small