Experimental Simulation Study On The Influence Of Relative Humidity On The Formation Of Sulfate Particles Under The Action Of Ammonia

Sulfate(SO₄^2-)is one of the important secondary inorganic pollutants in atmospheric particulates,which will have a profound impact on atmospheric visibility,human health,air quality,climate change,etc.In order to clarify the formation process of sulfate in the atmospheric environment,this paper mainly adopts diffuse reflection infrared Fourier transform spectroscopy technology combined with gas flow system to monitor trace gases（SO₂,NH₃,NO₂）andα-Fe₂O₃ online in situ and in real time.The heterogeneous reaction process between particles,focusing on the effect of RH on the reaction mechanism and reaction kinetics,mainly draws the following conclusions:（1）By studying the reaction system of SO₂,SO₂/NH₃,SO₂/NO₂ andα-Fe₂O₃particles respectively,the reaction mechanism was explored from the change of infrared spectrum characteristic peaks with reaction time.Compared with the SO₂ system,the SO₂/NH₃ and SO₂/NO₂ systems have increased sulfate production by 2.08 and 1.49 times,respectively,indicating that both NO₂ or NH₃ can promote the production of sulfate,but NH₃ has a stronger promoting effect.Due to the neutralization of acid and alkali,the role of ammonia is beneficial to the oxidation of SO₂ on the surface of particles to form sulfates.In the step-by-step experiment,the amount of sulfate that has reached a stable level after the reaction of SO₂ was first introduced,and the absorbance of the subsequent introduction of NH₃ increased by 0.12 times,which further verified the promotion of NH₃to the oxidation of SO₂ to form sulfate.（2）The heterogeneous chemical reaction process of SO₂ onα-Fe₂O₃ particles under the action of RH on ammonia gas was studied.Calculate the reaction rate and uptake coefficient（γ）from the relationship between the product sulfate peak area and time.The change of RH can affect the reaction kinetic parameters of SO₂/NH₃ andα-Fe₂O₃:when RH increases from 0%to 78%,the reaction rate decreases from 9.38×10^-17 ions^-1 g^-1 to4.33×10^-17 ions^-1 g^-1,the intake coefficient decreased from 2.04×10^-6 to 9.43×10^-7.Under high humidity conditions,the surface sulfate products gradually increase,and the hygroscopicity is strong,resulting in more obvious competition between H₂O and SO₂,NH₃ on the surface of the particles,and the formation of SO₄^2-is inhibited.Studies have found that the intervention of water vapor will lead to changes in the reaction mechanism.Under the intervention of water vapor,part of the sulfate is converted into bisulfate.In addition,through quantitative analysis of the influence of trace gas concentration on the formation of secondary particulate matter,it is found that an increase in ammonia concentration can increase the rate and amount of sulfate formation.Ammonia enhances the surface alkalinity of particulate matter and promotes sulfate formation.（3）The heterogeneous reaction process of mixed gas SO₂/NO₂/NH₃ on the surface ofα-Fe₂O₃ particles was studied.According to the change trend of sulfate product peak position and peak intensity with time,the effect of relative humidity on the reaction mechanism and kinetic parameters was analyzed.influences.The reaction produces N₂O₄under dry and low humidity conditions,but when the humidity exceeds 40%,the N₂O₄product disappears.Analysis shows that under dry conditions,ammonia gas enhances alkalinity on the surface ofα-Fe₂O₃ particles and forms a complex(H₄N⁺-SO₃^2-),which can quickly convert SO₂ into sulfate.With the increase of adsorbed water on the surface,NH₃ will hydrolyze with NO₂ dimer to generate NH₄⁺,HONO and NO₃^-.When the RH increased from 0%to 62%,the uptake coefficient increased from 1.06×10^-6 to 3.01×10^-6,and the reaction rate increased from 0.49×10¹⁸ ions^-1 g^-1 to 1.38×10¹⁸ ions^-1 g^-1.With the intervention of water vapor,NH₃ will undergo a hydrolysis reaction with NO₂ dimer to form HONO.The converted NO⁺NO₃^-is also conducive to the oxidation of surface S（IV）.In addition,more active oxygen or oxygen can be formed on the particle surface.The hydroxyl group is conducive to the formation of sulfate.But when the RH increased from62%to 72%,the intake coefficient decreased from 3.01×10^-6 to 2.36×10^-6,and the reaction rate decreased from 1.38×10¹⁸ ions^-1 g^-1 to 1.08×10¹⁸ ions^-1 g^-1.And the surface water layer formed under high humidity inhibited the reaction of trace gases on the surface active sites ofα-Fe₂O₃ particles.It shows that RH can promote and inhibit the absorption of SO₂ onα-Fe₂O₃ particles.