Composite Simulation Analysis Of Aerosol Direct Radiative Effects On Meteorology And Pollutants During Wintertime Severe Haze Events In The North China Plain

With the rapid economic development,the North China Plain（NCP）has become one of the regions with the most serious air pollution problems,with severe haze events occurring frequently.During the severe haze events,high concentrations of aerosols can absorb and scatter solar radiation and consequently affect the energy balance of the atmosphere.Aerosol direct radiative effects influence meteorology,which in turn affects the concentrations of aerosols by altering the generation and dissipation of aerosols.Aerosols with different optical properties（for example,the scattering or absorbing aerosols）have different effects on meteorology and chemical components.Therefore,it is crucial to study the aerosol-radiation-meteorology interactions during severe haze events.In this study,the WRF-Chem model（Weather Research and Forecasting Model with Chemistry）was used to examine the influence of aerosol direct radiative effects on meteorology and the feedbacks on air pollutants for 5 selected severe haze events in NCP.We aim to identify the common features in all episodes and also to distinguish the roles of absorbing and scattering aerosols.This study mainly includes the following three parts:（1）Previous studies on the impacts of severe haze episode on meteorology were generally focused on single episodes.This study aimed to obtain the common features of the influence of aerosol direct radiative effects on meteorology during severe particulate pollution events by composite analyses on model results from WRF-Chem model.Five heavily polluted events（15～17January 2014,13～15 January 2015,8～10 December 2015,21～23 December 2015 and 29-31 December 2016）in the NCP（35.4～41.2°N,113.3～119.3°E）were selected,which were representative of the most frequently observed weather patterns for severe haze.Model results showed that aerosols in the five heavily polluted events led to strong negative changes in radiative flux of 52.1～86.7 W m^-2,reductions in the 2-m temperature by 0.28～0.97℃,and the reductions in planetary boundary layer heights by 23.1～58.5 m,when averaged over the areas with PM_2.5 concentrations>150μg m^-3.Considering the changes in temperature per unit mass of PM_2.5,values of （35）T2/PM_2.5 averaged over the region with PM_2.5>150μg m^-3 were-0.33,-0.36,-0.26,-0.44,-0.16 10^-2℃m³μg^-1 in the five cases,respectively.The magnitudes of aerosol induced changes in the 2-m temperature were found to be influenced not only by aerosol concentration but also by wind speed,wind direction,and the convergence or divergence of winds.High wind speed diluted the aerosol-induced cooling,the convergence of wind limited the temperature change to local area,and the intrusion of cold air mass near the surface enhanced the aerosol-induced cooling.We also found the common characteristics of aerosol-induced changes in winds.In all the cases,aerosols induced an anti-cyclone at 500 h Pa around NCP,reduced the sea level pressure in the eastern NCP,and anomalous northerlies or northeasterlies at the surface in the eastern NCP.（2）Based on the model results obtained in the first part,we examined the aerosol direct radiative effects on concentrations of PM_2.5 and its compositions.Model results showed that aerosol direct radiative effects in the five severe haze events led to the increases in concentrations of PM_2.5,BC,and OC by 11.38～26.12μg m^-3,1.74～5.16μg m^-3,and 3.24～9.26μg m^-3,respectively.The comparison of the percentage increase in BC and PM_2.5 indicates that physical processes dominated the increase in aerosol concentrations in the five cases,while chemical processes were negative.The chemical processes of aerosol direct radiative effects decreased sulfate concentration by 0.05～2.00μg m^-3.The reduction of sulfate was associated with a decrease in atmospheric oxidation.The reduction of OH weakened the gaseous phase oxidation of sulfate,while the reduction of O₃ and H₂O₂ weakened the liquid phase oxidation process of sulfate.The chemical process of the direct radiative effects of aerosols resulted in the reduction in nitrate and ammonium concentration of 1.42-18.45μg m^-3 and 0.42-6.11μg m^-3.The reduction in nitrate was due to a decrease in atmospheric oxidation,especially O₃ and OH,leading to a decrease in the secondary formation of nitrate.The reduction in ammonium salts was associated with a decrease in sulfate and nitrate.Affected by the direct radiative effect of aerosols,O₃,OH and H₂O₂ decreased by 1.71～4.53μg m^-3,0.26～1.62 10^-6μg m^-3 and0.35～3.45 10^-2μg m^-3.（3）In addition to the model results in the first part,we conducted sensitivity experiments to remove the direct radiative effects of absorbing aerosols and scattering aerosols,respectively,to distinguish the roles of these two types of aerosols.The model results showed that both absorbing and scattering aerosols can reduce shortwave radiation at the surface,increase shortwave radiation flux in the atmosphere,and led to a reduction in the 2-m temperature and planetary boundary layer height.Absorbing aerosols led to a larger warming around 850 h Pa and a smaller cooling at the surface.The feedbacks of aerosol radiative effect on concentrations of PM_2.5 at the surface were simulated to be-11.20 to 9.40μg m^-3 by absorbing aerosols and 8.69～35.69μg m^-3 by scattering aerosols.The increase in BC and OC concentrations by scattering aerosols was associated with a decrease in the planetary boundary layer height,while the decrease in BC and OC concentrations by absorbing aerosols was associated with an increase in wind speed.The comparison of the percentage increases in BC and PM_2.5 indicated that the contribution of physical processes affected by absorbing aerosols was negative in some regions and chemical processes was negative,while the contribution of physical processes affected by scattering aerosols was positive and chemical processes was negative.The chemical process of direct radiative effects of absorbing and scattering aerosols led to reductions in sulfate,nitrate,and ammonium by 0.05～3.57μg m^-3 and 0.03～3.41μg m^-3,2.33～23.02μg m^-3 and0.70～22.29μg m^-3,0.69～8.05μg m^-3 and 0.20～7.77μg m^-3,respectively.The effect of absorbing aerosols was slightly larger than the effect of scattering aerosols.The decrease in sulfate was due to a significant reduction in ozone,which weakened the formation of sulfate in liquid phase oxidation.The reduction in ozone and hydroxyl radical（OH）led to the decrease in the pathway of forming nitrate from nitrogen oxides,resulting in the decrease in nitrate.Both absorbing and scattering aerosols led to the decreases in surface-layer ozone concentrations by 1.23～5.36μg m^-3and 1.70～6.89μg m^-3.The reduction in ozone were associated with the weakening of photochemical reactions as a result of the increase in PM_2.5.