Characteristic Of Water-soluble Ions In PM_2.5 In The Northern Suburb Of Nanjing Based On On-line Monitoring

With the rapid development of social economy, human emissions of atmospheric particles continue to increase, leading to the deterioration of atmospheric environmental quality, which has aroused widespread concern of the public and scientists. In this study, the pollution characteristics of PM_2.5 and its water-soluble ions ?WSI?, the sources, the formation and evolution of secondary inorganic ions and its influencing factors were analyzed by using PM_2.5 and its water-soluble ion data from March 2016 to March 2017 in the northern suburb of Nanjing. The main conclusions are as follows:?1? The average concentration of total water-soluble ions ?TWSI? in PM_2.5 in winter, spring and summer in the northern suburb of Nanjing was 51.79?g/m3,47.96?g/m3 and 28.91?g/m3, respectively, and the average annual concentration was 41.89?g/m3, accounting for 72.28% of PM_2.5. The diurnal variation of TWSI and the main water-soluble ions sun as NO₃^-,SO₄^2-,NH₄⁺ showed a single-peak and single-valley type, the peak appreared at 8:00-10:00. The concentrations of K⁺, Na⁺and Cl^- showed a double-peak and single-valley type, and the peak values were in the range of 8:00-10:00, the valley were at 6:00-8:00 and 21:00-22:00, respectively.?2? The diurnal variation of the pollution gas are mainly affected by the factors such as the height of the boundary layer, atmospheric oxidation, self volatilization and emission sources. The concentration of HCl showed the diurnal variation of single-peak and single-valley type, with the peak value appeared at 10:00. SO₂ showed a double-peak and single-valley type diurnal variation, and the peak values appeared at 9:00-12:00 and 23:00-24:00, respectively. The concentration of HNO₂ showed significant diurnal variation characteristics of lower at night and higher at daytime in 3 seasons, while HNO₃ concentration showed the different diurnal variation characteristics. The seasonal concentration of NH₃ ranked summer>spring>winter.?3? With the increase of PM_2.5, the ratio of TWSI concentration in PM_2.5 increased, indicating that the contribution of water-soluble ions to PM_2.5 increased with the increase of atmospheric pollution level. More severe the pollution, the proportion of NO₃^- in TWSI was increased and replaced SO₄^2- become the most important water-soluble ions, indicating that the contribution of NO_x emissions from motor vehicles and factories on high pollution can not be ignored. The calculated results of the equivalent charge ratio Rc/A and the degree of neutralization NR show that the atmospheric aerosol in the northern suburb of Nanjing has neutral acidity characteristics.?4? The results of cluster analysis show that the air quality in Nanjing is mainly affected by the surrounding air mass in Nanjing, and the concentration of air mass from the south is relatively high. In the summer, the air mass from Shanghai is the most polluted, and NO₃^- is the highest ion among all air masses. The concentration of Cl^- from the East Ocean is higher than that of the other sources. In the winter, Nanjing is mainly affected by the northwest airflow, the ocean's influence is weak, Cl^- concentration is higher than other seasons, indicating that the winter is mainly affected by coal or biomass combustion.?5? Meteorological conditions have a significant effect on water-soluble ions,and when the wind speed is more than 3m/s, it can effectively remove atmospheric particles. Precipitation has a significant effect on the removal of water-soluble ions.The larger the precipitation is, the longer the rainfall duration is, and the better the removal effect. The concentration of NO₃^-, SO₄^2-, NH₄⁺ had a significantly positive correlation with humidity, indicating that liquid phase oxidation reaction is the main way of secondary ion formation; and were negative to the temperature.?6? The average values of SOR ?sulfur conversion rate? in spring, summer and winter were 0.48, 0.48 and 0.50, respectively, and increased with the increase of relative humidity, especially when RH> 60%. The proportion of SO₄^2- in PM_2.5 increased with the increasing of RH, while the summer showed a decreasing trend,which may be related to the increase of NO₃^-. There was a significant linear correlation between NH₄⁺ and SO₄^2-,[NO₃^-]/[SO₄^2-] and [NH₄⁺]/[SO₄^2-],indicating that the sulfate in the atmosphere of the northern suburbs of Nanjing was mainly exist as low acidity solid or Liquid state ?NH₄?₂SO₄, NH₄HSO₄.?7? Cl^- and NO₃^- in PM_2.5 are mainly present in the form of NH₄Cl and NH₄NO₃. When the ambient temperature is higher than 25?, the gas-particle distribution index (FHNO₃ of NO₃^- increases rapidly with the increase of temperature, which indicates that the higher temperature is beneficial to the conversion of solid NH₄NO₃ to gaseous HNO₃. The influence of relative humidity is opposite to temperature. The average FHNO₃ under clean day is 0.07, which is 1.94 times that of hazyday, which indicates that the formation of NH₄NO₃ is more favorable in hazyday and further aggravates the degree of air pollution.