Preliminary Study On The Formation Mechanism Of Secondary Inorganic Particles In Lanzhou Based On Thermodynamic And Kinetic Methods

With the strict implementation of environmental treatment measures in recent years,the annual average concentration of PM_2.5 in Lanzhou has dropped significantly,but the problem of PM_2.5 pollution in winter still exists,and sulfate(SO₄^2-)and nitrate（NO₃^-）are the main water-soluble ions in PM_2.5.Affected by emission source emissions and meteorological factors,the chemical reaction pathways and influencing factors that dominate the formation of SO₄^2-and NO₃^-in different regions are different.Studying the main chemical pathways and influencing factors involved in the formation of SO₄^2-and NO₃^-can provide scientific basis for source analysis.In this paper,based on the online water-soluble ion data from the Lanzhou Railway Design Institute site in the winter of 2017-2018,the empirical ratio method and the macrokinetics method are used to explore the reaction pathways of SO₄^2-and NO₃^-dominated formation;Based on the inorganic aerosol thermodynamic model ISORROPIA II,the response surface design method（RSM）was used to quantitatively explore the individual and interactive effects of various influencing factors on the formation of SO₄^2-and NO₃^-.The main conclusions are as follows:1.In order to judge the main way of NO₃^-formation in PM_2.5 in Lanzhou in winter,using J value(J=[NH₄⁺]/(2×[SO₄^2-]+[NO₃^-])map and distinguish acidic,neutral and alkaline samples,and modify the rich and poor ammonia threshold proposed by Pathak.The revised judgment threshold of rich and poor ammonia interval([NH₄⁺]/[SO₄^2-]=1.99)can more reasonably judge the generation pathway of NO₃^-in Lanzhou.The difference in the formation of NO₃^-in a typical PM_2.5 pollution process before,during and after the pollution process was studied.The results show that the main formation of NO₃^-in the three time periods is mainly formed by the homogeneous gas phase reaction,and the heterogeneous reaction is auxiliary.2.Using the positive definite matrix factorization method（PMF）to analyze the source and contribution of SO₄^2-in Lanzhou in winter from 2017 to 2018,the results show that secondary SO₄^2-can account for 52.50%of the total SO₄^2-mass.In the 10process of continuous growth of 10 SO₄^2-screened,based on the macroscopic chemical kinetics method,the results showed that the proportion of secondary SO₄^2-generated by the gas phase homogeneous reaction pathway of SO₂+in the total secondary SO₄^2-per unit time was 2.48%-16.31%during daytime and 2.03%-10.15%during nighttime,respectively.The liquid phase oxidation pathway contributed less to the formation of secondary SO₄^2-during daytime and at night.The proportion of secondary SO₄^2-generated by heterogeneous reaction pathway varied from 24.73%to 41.32%during daytime and 36.80%to 57.48%at night,respectively.Therefore,the heterogeneous reaction of SO₂ in Lanzhou city in winter is the main reaction pathway of secondary SO₄^2-.3.Input the measured data of Lanzhou in the winter of 2017-2018 into the model ISORROPIA II,and screen the parameter schemes simulated by the model with the simulation effects of SO₄^2-and NO₃^-in PM_2.5.The simulation effect is better when the forward+metastable scheme is selected.Under this parameter scheme,the influence of various factors on the formation of SO₄^2-and NO₃^-was investigated by single factor experiment.The results show that the order of the factors affecting the formation of NO₃^-are:HNO₃>NH₃>HCl>RH>Na⁺>Ca²⁺;the factors affecting the formation of SO₄^2-are:NH₃>Ca²⁺;the factors affecting the formation of aerosol phase are:RH>HNO₃>NH₃>HCl.In order to explore the interactive effects of multiple influencing factors in the formation of NO₃^-,a PM_2.5 excess process from December 26,2017 to January 1,2018 was selected as the study period.The thermodynamic ISORROPIA II model and RSM method were used to establish a mathematical model between the independent variables(SO₄^2-,NH₃,HCl and Ca²⁺)and the dependent variable（NO₃^-）.Studies have shown that SO₄^2-and NH₃,Ca²⁺and NH₃ have synergistic effect in the formation of NO₃^-.