Abstract: Observational data of hourly concentrations and individual daily air quality indexes (IAQI) for six air pollutants during 2013 and 2014 over Shanghai were used to analyze the variation characteristics and meteorological influencing factors over Shanghai.The results show that the good to excellent rate of atmospheric environment quality was 77.0% in 2014,obviously higher than that in 2013.The air qualities in the two years varied seasonally,apparently worse in winter,especially in December with the highest AQI,while better in autumn.According to the variations of primary pollutants,PM_2.5 was the dominant contributor to the air qualities of Shanghai and its pollution mostly appears in winter,whereas O₃ was the major pollutant in summer.The concentrations of PM_2.5,PM₁₀,NO₂ and O₃ over Shanghai are all lower during the weekends than those during the weekdays,especially for PM_2.5 and NO₂.Haze days appeared much more frequently in the case that the PM_2.5 concentrations are beyond the moderate pollution level,but there is no one-to-one relationship.Since weather patterns influence PM_2.5 pollution significantly,the ground-level patterns can be divided into seven types,among which Type High Pressure Center and Type High Pressure Wedge are the dominate patterns.Due to the frequent cold air activities in winter,particles from upwind directions are transported to Shanghai under the northwest wind,leading to severe pollution.In addition,Type High Pressure Center and Type High Pressure Wedge weathers appear frequently under the control of Cold High,causing air pollution due to poor diffusion.During summer and autumn under control of the Subtropical High,the better horizontal and vertical diffusions are favorable to the removal of PM_2.5 pollution,but it is favorable to the O₃ pollution due to high temperature and sufficient sunlight conditions.

Key words: Air pollution, Haze, Primary pollutant, Weekend effect, Synoptic system type