Spatial patterns and temporal variations of six criteria air pollutants during 2015 to 2017 in the city clusters of Sichuan Basin, China.

Spatiotemporal variations of six criteria air pollutants and influencing factors in the city clusters of Sichuan Basin were studied based on real-time hourly concentrations of PM2.5 (the particles with diameters smaller than 2.5μm), PM10 (the particles with diameters smaller than 10μm), SO2, NO2, CO and O3 and routine meteorological data during the years from 2015 to 2017. The Sichuan Basin was further categorized into four regions: West, south, northeast Sichuan Basin (WSB, SSB and NESB) and plateau of west Sichuan Basin (PWSB) to better understand regional air pollution characteristics. Heavy air pollution was mainly induced by high PM2.5 or ozone concentrations in the cities clusters of Sichuan Basin. The compound air pollution characteristics existed in WSB with simultaneously high concentrations of PM2.5 and ozone, while PM2.5 concentrations in SSB were the highest among the four regions and especially in the city of Zigong with maximum PM2.5 concentration of 109.3μgm-3 in winter. The MDA8 (daily maximum 8-hour average surface O3 concentrations) more frequently exceeded CAAQS (Chinese Ambient Air Quality Standards) Grade I and II standards in Ziyang, Guang'an and Liangshan than the other cities maybe due to joint effects of industry emissions and regional transportation from surrounding cities. Annual (diurnal) variations of the pollutants with the exception of ozone showed "U" (flat "W") shape, while the ozone exhibited the opposite trends inside Sichuan Basin (WSB, SSB and NESB). Ozone pollution was more dependent on vehicle emissions inside Sichuan Basin, and industry had more important effects on ozone in the cities of PWSB with less vehicles. Severe ozone pollution can be formed easily under the weather conditions of high temperature, long sunshine duration and low RH (relative humidity) inside Sichuan Basin. High ozone concentrations in winter in PWSB may be partly transported from the other surrounding cities.