Optimization Improvement Of Key Chemical Mechanisms And Simulation Study In The Atmospheric Chemical Model GRAPES＿Meso5.1/CUACE

Atmospheric chemical models are the main pathways in air quality forecasting and effective tools for studying the formation mechanism of air pollution.Describing the chemical mechanisms in the troposphere atmosphere reasonably and comprehensively plays an important role in the accurate prediction of aerosol and gaseous pollutants.In this study,the key chemical mechanisms in the chemical-weather coupling model GRAPES＿Meso5.1/CUACE independently developed by China are optimized and improved.Specifically,nine heterogeneous chemistry reactions involving H₂O₂,HNO₃,HO₂,N₂O₅,NO₂,NO₃,O₃,OH,and SO₂are incorporated into GRAPES＿Meso5.1/CUACE together with various SO₂and NO₂heterogeneous uptake coefficient schemes.The widely used gas-phase chemical scheme CB05CL is coupled online to GRAPES＿Meso5.1/CUACE.Selecting June and December of2016 as representative months of summer and winter,the impacts of heterogeneous chemical reactions and gas-phase chemical mechanisms on simulations of gaseous（SO₂,NO₂,and O₃）and aerosol(sulfate,nitrate,ammonium,and PM_2.5)pollutants in typical regions and cities in China are evaluated to verify the effectiveness of these newly added mechanisms.The impacts of heterogeneous chemistry and Aerosol-Radiation Interaction on haze and combined air pollution are investigated.The interaction mechanisms between the two are proposed.The major conclusions are as follows:The differences in RH or ER uptake coefficients result in obvious differences in sulfate and nitrate concentrations,especially during the severe haze pollution.Compared with the RH-dependent schemes,the ER schemes restrict the excessive production of sulfate and nitrate under high RH effectively by including the self-limitation of heterogeneous reactions.The ER-dependent scheme for SO₂and RH/ER-dependent scheme for NO₂are used to form the improved heterogeneous chemistry in GRAPES＿Meso5.1/CUACE.The model with the improved heterogeneous chemistry shows better performance in capturing the PM_2.5chemical components and mass concentrations in Beijing-Tianjin-Hebei（BTH）and Yangtze-River-Delta（YRD）during winter haze pollution,of which the normalized mean biases（NMB）of sulfate,nitrate,ammonium,and PM_2.5in the megacity Beijing decrease from-26.7%,-28.3%,-58.2%,and-34.0%to 1.0%,-2.2%,-47.2%,and-24.2%,respectively.And the fractions of sulfate,nitrate,ammonium,and organics during the polluted periods change from 13.7%,19.3%,6.9%,and 60.1%to 16.5%,23.0%,7.6%,and 52.9%,which are more consistent with the actual observations（16.0%,23.2%,14.1%,and 46.7%）.The simulations of secondary inorganic aerosol（SNA）and PM_2.5in another megacity Shanghai have similar improvements.This indicates the importance of heterogeneous chemistry for the accurate prediction of haze pollution.The newly introduced gas-phase chemical scheme CB05CL well reproduces the spatio-temporal distributions of SO₂,NO₂,O₃,and PM_2.5over Eastern China both in the summer and winter of 2016.The locations of the high centers of these species are consistent with observations but the values are underestimated in general.The simulation performances of gaseous pollutants modeled by GRAPES＿Meso5.1/CUACE with CB05CL are comparable to the mainstream atmospheric chemical models.NMBs of simulated SO₂,NO₂,and O₃in the BTH in the summer of 2016 are-31.6%,-24.0%,and-5.9%,and those in the YRD are-25.8%,-29.1%,and-8.3%,respectively.Simulations in winter have similar results but O₃concentrations during severe haze pollution in the BTH are overestimated clearly,which may be owing to the significant overestimation of surface temperature and downward short-wave radiation during heavy haze when not considering the two-way feedback between aerosols and meteorology.The total contribution of heterogeneous chemistry（Het）and Aerosol-Radiation Interaction（ARI）to winter PM_2.5in Middle-Eastern China is 29.7%(43.7μg m^-3),of which the contribution of ARI alone is 19.7%,that of Het is 6.1%,and the additional contribution caused by the interaction between the two is 3.9%.With the aggravation of haze pollution,these contributions become increasingly higher but to different degrees.The contribution of Het to PM_2.5concentration rises from 4.0%on clean days to 6.6%under moderate pollution and 7.8%under heavily polluted conditions.The contribution of ARI increases rapidly from9.1%on clean days to 21.0%under moderate pollution and 29.1%on heavy pollution.This indicates that the significant enhancement of ARI is the main reason for the explosive growth of PM_2.5levels during severe haze episodes.ARI directly leads to an increase in PM_2.5concentration,but also further promotes the heterogeneous oxidation of SO₂and NO_xby increasing the surface area concentration of aerosols,accelerating the heterogeneous formation of SNA.This interaction additionally contributes 18.5%of SNA and 7.3%of PM_2.5mass under heavy pollution conditions.The impacts of Het and ARI on the formation of combined air pollution in summer may be relatively less,of which their total contributions to concentrations of PM_2.5and the daily maximum 8-h average of O₃in Beijing are 3.0μg m^-3（7.3%）and-4.2μg m^-3（-2.9%）respectively,which may be related to the overall light PM_2.5pollution in summer.