Numerical Simulation Of The Three-dimensional Structure And Evolution Characteristics Of Secondary Inorganic Aerosols During Cold Front

Secondary aerosol(SO₄^2-,NO₃^-and NH₄⁺,collectively referred to as SNA)is the main component of PM_2.5.The factors behind the formation mechanism of secondary aerosol pollution in China are still quite unclear due to the intricate interactions among pollution sources,meteorology,and atmospheric chemical processes.Using Weather Research and Forecasting-Community Multiscale Air Quality Modeling System（WRF-CMAQ）,with the integrated source apportionment method（ISAM）,our results highlight 3-D distribution,the regional source appointment,and the formation mechanisms of physical and chemical processes of PM_2.5and SNA for two cold front episodes in eastern China.The results are shown as follows:（1）Observations showed a southward movement of the peaks of pollutants along the cold frontal passages and vertical evolution of PM_2.5profiles in Nanjing.PM_2.5and its chemical component SNA presented similar evolution distribution.The high-value pollutants concentrations area showed a belt distribution from north to south over time,characterized by significant short-term variation,short duration of pollution,strong northerly winds near the surface,and sharp temperature decreases.（2）Model simulations reveal that cold fronts can effectively transport PM_2.5from upstream regions to the Yangtze River Delta（YRD）.In YRD,SO₄^2-was mostly imported from upstream sources,up to 48%near the surface,much higher than from local sources（～27%）.The long-range transport contribution dominated as altitude increased,with about 53%contribution from upstream regions within 0.5-1.0km and only 14%contribution from local sources.The local contribution within 1.0-2.0km was below 6%,and the external contribution was 91%.In contrast to（NH₄）₂SO₄,NH₄NO₃is thermally unstable and readily dissociated in the lower boundary layer.Under repeated gas-particle conversion and dry deposition,NO₃^-has a shorter lifetime than SO₄^2-in the lower boundary layer.Therefore,the local source contribution of NO₃^-near the surface was similar to SO₄^2-（around 27%）,and the external source was 15%(～35%lower than SO₄^2-).As altitude increased and temperature decreased,NH₄NO₃stabilized and had a longer lifetime.Within 0.5-1.0km,the local source of NO₃^-contributed 26%(～12%higher than SO₄^2-),whereas the upstream source was 15%(～35%lower than SO₄^2-).Within 1.0-2.0km,the contribution from the local source of NO₃^-was 19%(～13%higher than SO₄^2-),while the external contribution was 11%(～23%lower than SO₄^2-).NH₄⁺was jointly affected by SO₄^2-and NO₃^-,and the local and upstream contributions near the surface were 54%and 38%respectively.The average upstream contribution increased to64%within 0.5-1.0km,while the local YRD contribution was only 20%.There was almost no local YRD contribution to ammonia within 1.0-2.0km.（3）Physical processes（horizontal advection（HADV）,vertical advection（VADV）,and vertical mixing（VMIX））had relatively consistent effects on variations in PM_2.5and SNA concentrations.The PM_2.5chemical process（AERO）formed a greater mass concentration of NH₄NO₃than（NH₄）₂SO₄.The AERO process of（NH₄）₂SO₄always contributed positively,peaking near 1.5km.Relatively,NH₄NO₃was thermally unstable and in a reversible equilibrium of dissociation/condensation at high/low temperatures.The warm and humid air mass was lifted upward in front of the cold front zone and cold environment promoting the NH₄NO₃formation therein,resulting in high values of NH₄NO₃at about 1 km above the surface.