The Simulation Of Precipitation Acidification Process Based On The Mechanism Of Aqueous-phase Chemical Reaction And Below-cloud Scavenging Of SO₂

When the pH value of atmospheric precipitation is lower than5.6, the acid rain is formed. With therapid development of industry, a large number of acidic waste gas arose in ambient atmosphere. Sulfurdioxide is a major contributor to the formation of acid rain. Acid rain not only damages the ecologicalenvironment, but also threats of human health. The study on the mechanism of liquid phase autoxidation ofSO₂in the atmospheric and the simulation of rain acidification process is of great significance to know theformation, transport and deposition mechanisms of acid rain, and has a guiding significance to develop themitigation for it.This paper studied the mechanism of aqueous phase and below-cloud scavenging of6kinds of gases,such as SO₂, H₂O₂, NH₃, HNO₃, CO₂and O₃, etc, and Matlab was employed to simulate the acidizingprocess of cloud droplet and raindrop. The process of acid precipitation in Xi’an was simulated as anexample, the comparative analysis was performed between the calculated data and the measured data.Finally, on the basis of the pollution situation and reasons of acid rain in Xi’an, the corresponding acid raincontrol measures are put forward. The conclusions were summarized as follows:1. The amount of gases dissolved in liquid phase depends on the Henry’s law constant, the equilibriumconstant for the hydrolysis of hydrates, the temperature, the liquid water content and the pH value of theliquid phase. In this work, HNO₃and NH₃are considered as highly soluble gas, H₂O₂and SO₂aremoderately soluble, CO₂and O₃are relatively insoluble.2. During the liquid phase oxidation process of SO₂, when the pH value is less than5, H₂O₂palys aleading role in the oxidation process of tetravalent sulfur; when pH value is equal or greater than5, O₃palys a major role in the process. Also, the oxidation of S（IV） by O₂catalyzed by Fe（Ⅲ） and Mn（Ⅱ） maybe important at high pH.3. The result of simulation of droplet acidification process shows that as temperature rises, the liquidphase reaction rate constant increases, while the Henry constant of the gas decreased, resulting in thedecrease of the S（VI） formation in cloud droplet. The higher the mixing ratios of NH₃on the surface of thedroplet, the higher concentration of SO₂-4and pH value in cloud droplets when the system come toequilibrium. The acidification of cloud droplets is not sensitive to the change of mixing ratios of CO₂onthe droplet surface. 4. The conclusion of simulation of below-cloud scavenging process indicate that the concentration ofSO₂-4and pH value in raindrop depends on the raindrop size, the concentrations of the reaction gases andso on. NH₃can neutralize the acidity of raindrop and the formation of SO₂-4in it. With different raindropsize, the concentration of SO₄^2-in it will be differ by an order of magnitude.5. Acid precipitation process was simulated based on data of SO₂concentration and meteorologicaldata of Xi’an in2011. The results show that SO₂-4concentration is36.01μmol/L and pH value is4.71whenrain falls on the ground. The measured pH value in Xi’an is5.97and the average concentration of SO₂-4is260μmol/L. Therefore, it can be seen that the simulation of acid deposition is a complex nonlinear process.