Theoretical Study On Effects Of NH₃ And CO₂ On Atmospheric Nucleation Mechanisms

Atmospheric new particle formation（NPF）,including nucleation and subsequent growth of the nuclei,is a major source for secondary particles,and in turn significantly affects global climate and local air quality.However,the molecular mechanism of NPF,especially effects of atmospheric species on the initial nucleation,is still unclear.This study employed various theoretical methods to investigate the effects of NH₃ and CO₂ on atmospheric NPF.The main contents and conclusions are as follows:（1）Quantum chemical calculation,transition state theory and atmospheric cluster dynamics code were adopted to investigate thermodynamics and kinetics of nucleation between iodine species and NH₃.Thermodynamic analysis shows that NH₃ can promote the nucleation of iodine species in two ways:（a）assisting I₂O₅ hydrolysis to form HIO₃ clusters,and（b）stabilizing HIO₃ clusters through acid-base reaction.Further kinetics analysis shows that in the coastal areas with heavy NH₃ pollution,the cluster formation rate for nucleation between iodine species and NH₃ is 1～10⁶ times as high as that without NH₃.The NH₃-mediated mechanism is of importance to explaining the NPF events in coastal areas with heavy NH₃ pollution.（2）Quantum chemical calculation,atmospheric cluster dynamics code,and multiple linear regression analysis were performed to investigate clustering pathways,kinetics,and effects of NH₃ on ion-induced nucleation of organic acids.Results suggested that the stability of ion-contained clusters depends on energy of the lowest unoccupied molecular orbital,polarizability of clusters,and the number of molecules in the cluster.When the number of molecules in ion-contained clusters increases to 2～3,it is easy to recombine with clusters（or ions）with opposite charge,and then form neutral nano-clusters.NH₃ concentration can significantly affect cluster formation rates of ion-induced nucleation involving organic acids.When NH₃ concentration increases from 2.6×10⁸ molecules·cm^-3（clean conditions）to 2.6×10¹¹ molecules·cm^-3（polluted conditions）,the cluster formation rates increase by 10³ times.The mechanism of ion-induced nucleation of organic acids and NH₃ is important to explain the NPF events in the areaes with heavy NH₃ pollution.（3）Born-Oppenheimer molecular dynamics,metadynamics,and thermodynamic integration were adopted to invesitigate heterogeneous hydrolysis of NO₂ in the presence of CO₂ to form HONO.Results show that CO₂ can catalyze the heterogeneous hydrolysis of NO₂.The unveiled reaction pathway is that CO₂ hydrolyzes to produce HCO₃^-,followed by reaction between HCO₃^-and NO₂ dimer on water surface to form OC（O）ONO^-intermediates,NO₃^-,and H₃O⁺;and subsequently,the OC（O）ONO^-intermediates decompose into CO₂ and ONO^-that obtains an H⁺from H₃O⁺to produce HONO,completing the catalytic cycle.Further thermodynamic analysis shows that the catalytic reaction can occur easily,with the free energy barrier being negative relative to the separate reactants and 1.2 kcal·mol^-1 relative to the pre-reactive complex.The HONO produced by the new mechanism can be photolyzed to form·OH that oxidizes SO₂,I₂ and volatile organic compounds into nucleation precursors（e.g.,H₂SO₄,HIO₃,I₂O₅,and organic acids）.Therefore,the new mechanism can indirectly affect atmospheric nucleation.With continuous increase in global atmospheric CO₂ concentration,this mechanism will be more and more important.This study uneviled the direct promotion mechanism and kinetics of NH₃ on iodine nucleation and organic acid ion-induced nucleation,the direct effect of CO₂ on HONO formation,and the indirect effect of CO₂ on atmospheric nucleation,which expands the theory on secondary particle formation mechanisms.Under the situation of“two decrease”（SO₂ and sulfate concentrations decrease）,“two increase”（CO₂ and nitrate concentrations increase）and“one high”（NH₃ concentration remains high）,the new mechanisms revealed in this study will inevitably provide a theoretical base for controling pollution of atmospheric secondary particle.