Study On Physio-chemical Characteristics And Influencing Factors Of Formation Of Secondary Inorganic Particles Of PM_2.5 In Sulfur And Acid-rain Control Zone Cities

PM_2.5 has a significant impact on the Earth's radiation, reducing atmospheric visibility, participating acid deposition and it is also the main reason for haze. Sustained exposure to polluted environment with high mass concentration of PM_2.5 will cause a negative impact on human health. Secondary inorganic particulate matter is the major component of PM_2.5. Domestic and foreign research has shown that the concentration of PM_2.5 increases significantly and secondary inorganic particles primarily contribute to PM_2.5 content in haze. Secondary inorganic particles also play an important role of the formation of regional atmospheric compound pollution. They can directly affect the acidity of precipitation, and have comparatively great impact on extinction coefficient and atmospheric visibility. Therefore, carrying out research on the physio-chemical characteristics and the discussion of influencing factors of formation of secondary inorganic particles of PM_2.5 can improve the local ambient air quality and provide the basic information of physical and chemical characteristics of secondary inorganic particles in sulfur and acid-rain control zone cities.In this article, PM_2.5 samples were collected in different seasons of Guiyang, Duyun, Zunyi in 2014 and Ion Chromatography?IC?, Field Emission Scanning Electron Microscopy?FESEM? and Image Analysis?IA? were utilized to analyze the characteristics of seasonal, diurnal variation of mass concentrations of secondary water-soluble inorganic ions of PM_2.5 and correlation with meteorological factors. Microscopic properties, the number of percentage, volume fraction and size distribution of secondary inorganic particles were investigated in three typical sulfur and acid-rain control zone cities and case study of Guiyang to discuss the influencing factors of formation of secondary inorganic particles of PM_2.5 by analyzing the gaseous precursors, SOR?sulfur oxidation ratios?, NOR?nitrogen oxidation ratios?, existing forms of secondary water-soluble inorganic ions, meteorological factors and microscopic characteristics. The results showed that,?1? Energy spectrum analysis show that secondary inorganic particles of PM_2.5 samples are mainly plate-shaped, needle-like, tufted and columnar and exist as sulfate and K-rich sulfate in the study area. The seasonal variations of number percentages of the secondary inorganic particles in PM_2.5 show substantially higher in spring, autumn and lowest in summer at each sampling point and the volume percentages show higher in autumn and winter and lowest in summer. The number frequency of secondary inorganic particles varies from 0.12% to 4.35% and the volume percentage varies from 0.02% to 66.95% in different size at sampling points. The number-size and volume-size distribution characteristics of secondary inorganic particulate matter in PM_2.5 samples indicate that the number and volume of the secondary inorganic particulate matter are most distributing in 0.6⁰.7, 0.8⁰.9, 1.0².5?m.?2? The mass concentrations of NH4+, NO₃^- and SO₄^2- in PM_2.5 rank as SO₄^2-> NO₃^-> NH4+ at each sampling point in the study area. The ratio of total mass concentrations of these three ions and PM_2.5 is more than 40%. The mass concentrations of NH4+, NO₃^- and SO₄^2- show relatively high during autumn and winter while lowest in summer. The seasonal values of R?acidity and alkalinity? are less than 1in Guiyang, Duyun, Dingzikou and Fenghuang Mountain?except in winter?, indicating that PM_2.5 is mainly acidic at the sample points. The average ratios of NO₃^-/SO₄^2- are less than 1in different seasons at each sampling point, indicating that sulfur and nitrogen in the atmosphere in study area mainly come from stationary sources. The values of SOR, NOR reach the maximum in fall and the annual average values of SOR and NOR are more than 0.1.?3? Affecting by meteorological conditions, sources and other factors, the existing forms of NH4+, NO₃^-, SO₄^2- in PM_2.5 vary in different seasons at each sampling point in the study area. The results of correlation analysis indicate that NH4+, NO₃^- and SO₄^2- probably exist in?NH4?2SO4, NH4NO3 during spring and summer in Guiyang and spring, winter in Zunyi, NH4HSO4 and NH4NO3 during autumn in Guiyang, Zunyi and Duyun,?NH4?2SO4, NH4HSO4, NH4HSO4 during winter in Guiyang and spring, summer, winter in Duyun, and NH4HSO4 in summer in Zunyi.?4? The case study of Guiyang city showed the discussion of influencing factors of formation of secondary inorganic particles in PM_2.5. The following conclusions are:?1? The results show that higher relative humidity is conducive to the generation of ammonium and lower temperature, higher relative humidity are in favor of NO2 transforming NO₃^- by heterogeneous reaction, which also promote the conversion of HNO3 to NO₃^-. The mass concentrations of SO₄^2- are affected by SO2 content and solar radiation intensity significantly. NO₃^- has a good relationship with NO2 as same as SO₄^2-and SO2, indicating that there exist strong conversion of NO2 to NO₃^- and SO2 to SO₄^2- in Guiyang. The values of SOR and NOR are more than 0.1, showing that SO2 and NO2 are the mainly driving factors for the formation of sulfate and nitrate in Guiyang City.?2? Under conditions of stronger source emissions, lower temperatures and higher relative humidity, NH4+, NO₃^-, SO₄^2- typically exist as NH4NO3, NH4HSO4,?NH4?2SO4, while exist as NH4HSO4 and?NH4?2SO4 when temperature is higher and relative humidity is lower.?3? The results of correlation analysis indicate that NH4+, NO₃^- and SO₄^2- in PM_2.5 present certain degree of negative correlation with temperature, wind speed and the correlation coefficients are-0.58,-0.49,-0.40 and-0.47,-0.44,-0.51, respectively. While the relative humidity shows positive correlation with NH4+, NO₃^- and SO₄^2- and the correlation coefficients are 0.81, 0.89 and 0.76, respectively, suggesting that low temperature, low wind speed and high relative humidity are conducive to the accumulations of secondary water-soluble inorganic ions.?4? The number-size and volume-size distribution indicate that the secondary inorganic particles of PM_2.5 mainly appear in the accumulation mode?0.6⁰.8?m? and coarse modes?1.0².5?m? in spring and winter, revealing that secondary inorganic particles mainly generate through gas phase reactions and heterogeneous reactions. However, there are two mechanisms of the generation of secondary inorganic particulate matter in summer and autumn, one is coalescence of small particles and growth of heterogeneous phase reactions, the other is generating through heterogeneous phase reactions on the surface of aerosol and droplet particles.