Parameterization And Numerical Simulation Of Heterogeneous Uptake Coefficient Of SO₂ On Multi-component Aerosols

Heterogeneous reactions on the surface of atmospheric aerosols are an important pathway for polluting gases to transform into secondary aerosols,changing atmospheric chemical compositions,hygroscopicity,and optical properties of aerosols,playing an important role in the formation of haze pollution in China.Sulfate,as the dominant chemical component of PM_2.5,can be significantly produced through the heterogeneous reactions of SO₂.However,due to the insufficient understanding of mechanisms and kinetics,the parametrization of heterogeneous reactions in most atmospheric chemistry models still lacks off,which leads to a discrepancy between simulations and observations of sulfate concentrations.Based on laboratory studies and field observations,a new parameterization of uptake coefficients for heterogeneous reactions on multi-component aerosols is developed in this work.The equivalent ratio（ER）of inorganic aerosol is used to establish the quantitative relationship between the heterogeneous uptake coefficients and the composition of aerosols.Incorporating the new ER-dependent scheme and updating the calculation method of aerosol surface area concentrations,the heterogeneous chemistry module of the atmospheric chemistry model CUACE of the China Meteorological Administration has been improved.The WRF-CUACE model has been applied to simulate PM_2.5 and its main chemical components in the Beijing-Tianjin-Hebei region during wintertime.The heterogeneous uptake coefficient is used to quantify the aerosol surface reaction probability,which is related to the characteristics of aerosol chemical compositions.A self-limitation mechanism of the heterogeneous reaction from clean to polluted cases during wintertime in the BTH region is proposed in this study.Heterogeneous reaction greatly promotes the rapid formation of aerosol secondary inorganic components and reduces the aerosol ER from clean to polluted cases.Consequently,the reduced aerosol ER leads to the decrease of the uptake coefficient and the weakened ability of aerosol to absorb the reactant gases,which inhibits the further heterogeneous reaction by slowing down the increase of aerosol surface area and reaction rate constant.There are uncertainties in the simulation of aerosol surface area and reactive gas concentrations,which could introduce a significant impact on the heterogeneous reaction rate and sulfate concentrations.The differences in dry deposition rate of aerosol cause both the differences in aerosol surface area and uptake coefficient.In the winter of the BTH region,the promotive effect of the aerosol surface area caused by the decrease of dry deposition rate is larger than the decreasing effect of the uptake coefficient,which leads to the increase of reaction rate and sulfate concentration.The aerosol surface area concentration calculated by the characteristics of the wet aerosol considering the hygroscopic growth is greater than that of the dry aerosol,which is better to be used to reproduce the high concentration of sulfate in a polluted day with high relative humidity.The overstimulated SO₂ concentrations may cause the overestimation of sulfate heterogeneous formation.By correcting the SO₂anthropogenic emissions,the simulated SO₂ and sulfate concentrations both show a better agreement with observations.After incorporating the heterogeneous reaction of SO₂,the simulated concentrations of PM_2.5,sulfate,and ammonium over the polluted area of the BTH region in the winter of 2017 increased 3-8.5,2.5-6.2,and 1-2.3μg m^-3,respectively.The contributions in the winter of 2018 were even larger,with 4-10.8,3-7.9,1.2-2.9μg m^-3,respectively.SO₂ concentrations were reduced by 0.4-1.2 and 0.5-1.5 ppb in the winter of 2017 and 2018,respectively.The largest contributions are presented in southern BTH and central Tianjin.The contributions of heterogeneous reaction of SO₂to PM_2.5 increase with the enhanced pollution degree,with a mean value of 1,5.3,and13.3μg m^-3 for clean,slightly polluted,and polluted cases,respectively.