Study On The Formation Pathway And Influencing Factor Of Secondary Inorganic Aerosol During Haze Process In Northern Henan Province

Haze episodes have been frequently observed in China with fine particulate matter(PM_2.5)as the primary pollutant,which has adverse impacts on air quality and human health.Sulfate,nitrate,and ammonium（SNA）transformed by the gaseous precursors were the most important components of PM_2.5.The formation of nitrate is dominated by the gas-phase and hydrolysis reactions.Sulfate can be mostly formed through multiple aqueous-phase reactions.However,due to various factors such as gaseous precursor,meteorological conditions,atmospheric oxidant,and particle p H,the dominant paths for SNA formation are still unclear,and there is a lack of comparative study on urban and rural sites.In addition,particle size is an important parameter that affects the generation of SNA,but the current understanding of the formation pathways of SNA in different particle size ranges is still insufficient.The northern area of Henan Province is a high-PM_2.5 area of China with high concentrations of gaseous precursor during haze events.Therefore,it is of great significance to pollution control through identifying the formation paths and influencing factors of SNA in this area.The observation was conducted using high-time-resolution instruments at two urban sites（U-ZZ and U-AY sites）and three rural sites（R-AY,R-XX,and R-PY sites）.Combined with observation data,thermodynamic model,and reaction rate equations,this study compared the PM_2.5 components,particle acidity,SNA formation pathways,and influencing factors during haze events between urban and rural sites,as well as investigated the formation mechanism of droplet-mode SNA.The main research contents and conclusions are as follows:（1）The characteristics of the chemical component of PM_2.5 indicates that SNA（53–63%）was the highest component of PM2.5,NO₃^-was the most important（24–32%）,followed by SO₄^2-（13–17%）.The proportion of SNA increased in the process of haze pollution,reaching 67%during periods of heavy pollution.Compared with the urban sites,higher proportions of organic carbon in PM_2.5 were observed at the rural sites.Moreover,the proportions of SNA in PM_2.5 at urban sites were higher than those at rural sites under stable and high relative humidity（RH）conditions.（2）The p H values of PM_2.5 calculated by the ISORROPIA-II model using observation data at rural sites were slightly higher than those at urban sites,with a range of 4.5–5.2.Sensitivity tests show that particle p H increased with a cation increase and decreases in anions,temperature,and RH.Ammonia was the dominant factor that influenced the predicted p H.When the sum of total ammonia（NH₄⁺+NH₃）and other cation concentrations can completely neutralize anions,similar p H values（3.0）for the five sites indicated that the presence of excess ammonia was likely important for the lower acidity of PM_2.5 in this region.Moreover,the concentrations of excess ammonia may drive the higher p H values at rural sites,because of the higher excess ammonia concentrations at rural sites than those at urban sites.（3）Gas-phase and N₂O₅ hydrolysis reactions were the main formation pathways of nitrate during the daytime and nighttime,respectively.The OH radical was the major factor for gas-phase reactions.HONO photolysis played a more important role in OH radical formation when O₃ concentration decreased during the haze episode.N₂O₅hydrolysis reaction was mainly affected by O₃ and aerosol water content（AWC）.High RH,AWC,and N₂O₅ concentrations at the U-ZZ site enhanced the hydrolysis reaction than that of the R-XX site.The aqueous-phase reactions dominated the sulfate formation with the highest rate of transition metal ion catalytic and H₂O₂ oxidation reactions at the U-ZZ and R-XX sites,respectively.Elevated RH and particle acidity at the U-ZZ site resulted in a higher formation rate of aqueous-phase sulfate than that of the R-XX site.The gas-particle partition coefficient of NH₃ had a negative correlation with the particle p H and the presence of NH₃ could promote the increase of SNA concentration.（4）The characteristics of the size distribution of particles indicate that SNA was mainly concentrated in the droplet mode with the dominant size in the range of 0.65–2.1μm.High AWC concentration and large surface area in droplet-mode provided adequate carriers for the adsorption and condensation of HNO₃ and were beneficial for the hydrolysis reactions of N₂O₅.Elevated AWC,surface area,and particle acidity enhanced the H₂O₂ and transition metal ion catalytic oxidation reactions for droplet-mode sulfate formation.Moreover,droplet-mode particles exhibited moderate acidity,which enhanced the partitioning of NH₃ to NH₄⁺.In summary,based on the characteristics of the chemical composition and evolution trend of atmospheric particulate matter in northern Henan Province,this study elaborates the significant influence of SNA formation on the haze pollution;clarifies that gas-phase,hydrolysis,and aqueous-phase reactions are the main pathways for SNA formation;and reveals that AWC,particle p H,and the concentration of oxidant are responsible for the differences in SNA formation pathways between urban and rural sites.These results can support the improvement of air quality in Henan Province.