[PM_2.5 Pollution Characterization and Cause Analysis of a Winter Heavy Pollution Event, Liaocheng City]

Heavy pollution events frequently occur during fall and winter seasons in northern areas. In order to understand the characteristics and chemical composition of PM_2.5 during heavy pollution in winter in Liaocheng City, ambient PM_2.5 samples were collected between January 7-11,2016. Mass concentration, water-soluble ions, carbonaceous species, and elements were analyzed, as well as the causes of pollution. Results showed that PM_2.5 mass concentration was 238.3 μg·m^-3 with the trend clearly that of an inverted V; this concentration represents exceedance of the National Ambient Air Quality Standard (GB 3095-2012) by more than 2.2 times. NO₃^-, SO₄^2-, and NH₄⁺(SNA)were the main water-soluble ions. As pollution increased or decreased, NH₄⁺, SO₄^2-, NO₃^-, and Cl^- exhibited the same trend, which contrasted with that of Ca²⁺. During the peak of pollution, NH₄⁺, NO₃^-, and SO₄^2- concentrations were 48.96, 68.45, and 80.55 μg·m^-3, with these representing levels 6.29, 7.31, and 7.84 times those of the initial stage, respectively. During the pollution event, OC and EC concentration variation ranges were 20.8-60.2 μg·m^-3, and 3.0-7.5 μg·m^-3, respectively. The concentration of OC was significantly higher than that of EC and the variation amplitude was significantly larger. During the event, the mass concentrations of 27 inorganic elements on each day were 10.2, 22.4, 16.0, 19.6, and 8.2 μg·m^-3, respectively. Enrichment factors (EF) of all elements were less than 10, indicating lack of enrichment and showing that sources were mainly natural. PM_2.5 mass concentration reconstruction results showed that organic matter (OM), SO₄^2-, and NO₃^- were major components, followed by NH₄⁺, crustal material, and other ions. EC and trace element content was relatively low. As PM_2.5 pollution worsened, secondary inorganic salt (NH₄⁺, SO₄^2-, NO₃^-) concentrations and proportions increased, OM concentration increased but its proportion decreased, while crustal material concentration and proportion both decreased, showing that secondary inorganic conversion was the main cause of this pollution event, mainly driven by coal and motor vehicle emissions.