[Change in Characteristics of Pollution Gas and Water-soluble Ions at Different Development Stages of Haze]

To explore the variation in characteristics of atmospheric pollutants at different stages of haze, the monitor for aerosols and gases in ambient air (MARGA) was used to observe the concentrations of precursor pollutants (NH₃, HNO₃, and SO₂) and eight water-soluble ions in a regional haze in the Yangtze River Delta region from November 18 to December 07, 2018. Combined with environmental data (PM_2.5, NO₂, CO, and O₃) and meteorological data, the causes of regional haze formation, diurnal variation characteristics of air pollutants, and distribution characteristics of air pollutants in different stages of haze were analyzed. The results showed that the Yangtze River Delta region was mainly controlled by a ridge of high pressure during the haze process and the weather situation was stable, which was conducive to the accumulation of air pollutants. On hazy days, the concentrations of PM_2.5, NO₂, NO₃^-, SO₄^2-, NH₄⁺, Cl^-, and Na⁺ were (118.91±39.23), (61.62±26.34), (45.64±16.01), (18.80±8.02), (20.82±7.16), (3.02±2.25), and (0.23±0.22) μg·m^-3, respectively, and these were 2.73, 1.63, 2.64, 1.94, 2.50, 2.05, and 2.56 times the levels found on clean days, respectively. The concentration of CO was (1.34±0.39) mg·m^-3 on hazy days, which was 1.86 times that found on clean days. Diurnal variation characteristics of different air pollutants were different, as were the distribution characteristics of air pollutants at different haze stages. The concentrations of SO₂ was the highest in the haze occurrence stage. The concentrations of PM_2.5, NO₂, NH₃, CO, and SNA were highest in the haze development stage, and the concentrations of O₃, Cl^-, Na⁺, and K⁺ were highest in the haze dissipation stage. The relative contributions of SNA to PM_2.5 in different stages of haze could reach 94%-96%, and their growth rate was largest in the development stage. The order of growth rate was NO₃^- > NH₄⁺ > SO₄^2-. SNA mainly existed in the form of NH₄NO₃ on clean days and in the occurrence and development stages, and (NH₄)₂SO₄ in the dissipation stage. This haze process was mainly caused by the growth of NO₃^-, which was mainly generated by gas-phase homogeneous phase reaction, and NO₃^-contributes 51.06%, 51.85%, and 48.22%, respectively, to PM_2.5 in the occurrence, development, and dissipation stages of haze.