Comparison of chemical characteristics of PM2.5 during two winters in Xiangtan City in south central China

To assess the efficacy of the “Implementation Details of Air Pollution Prevention and Control Action Plan”, the chemical composition of PM_2.5 and other pollutants was determined during the winters of 2013–2014 and 2016–2017 at two urban sites in Xiangtan City, Hunan. The concentrations of PM_2.5, SO₂, and NO₂ decreased from 146.0 to 94.5?μg/m³, 75.9 to 33.5?μg/m³, and 80.6 to 55.8?μg/m³, respectively, from winter 2013–2014 to winter 2016–2017. The concentrations of almost all the major chemical components of PM_2.5 decreased as well, particularly secondary inorganic aerosols (SIAs). These results indicate that the implementation of the air quality control plan was very effective in improving air quality. Analysis of the data also suggests that SIA formation is likely responsible for high winter PM_2.5 pollution and that high relative humidity levels and low wind speed can promote the formation of SIA. A 72-h back trajectory analysis shows that both regional transport and the accumulation of local pollutants under stagnant meteorological conditions promote the occurrence of episodes of high wintertime pollution levels.