Computational Studies Of The Substituent Effects Of Small Nitrogen-containing Molecules And Carboxylic Acids On Atmospheric Nucleation Mechanism

Atmospheric aerosols could greatly affect global climate and human health.New particle formation(NPF)contributes significantly to atmospheric aerosols.Nucleation is the initial stages of NPF and determines the formation rates of new particles.Nitrogen-containing compounds such as amines together with carboxylic acids are important nucleation precursors.The identification of the nucleation ability of nitrogen-containing compounds and carboxylic acids is of great significance.Considering that the organic structure directly affect its properties and the variety of organics in atmosphere,it is impossible to study the nucleation mechanism of different organics one by one from molecular level using experimental methods.Quantum chemical calculations based on the micro-structure of clusters could be used to investigate the nucleation mechanism of various compounds and the properties of clusters.This study investigated the substituent effects on the nucleation ability and cluster property of nitrogen-containing compounds including amino acids,amines and amides together with organic carboxylic acids including acetic acid,glycolic acid and dicarboxylic acid by using density functional theory.The main research contents and results are as follows:(1)The interaction of amino acids composed of hydroxyl group with sulfuric acid and the cluster properties were studied.Results show that hydroxyl group of serine and threonine have great influence on the structure and proton transfer of clusters.Besides,serine may play more important role in stabilizing sulfuric acid to promote NPF in initial steps compared with threonine,glycine and alanine,which indicating that hydroxyl group may enhance the stabilizing effect of amino acids towards sulfuric acid.Hydrated(NH2CH(CH2OH)COOH)(H2SO4)and(NH2CH(OHCHCH3)COOH)(H2SO4)clusters could retain water even at low relative humidity,which indicate that hydroxyl group may enhance the water affinity of amino acid clusters.The dipole moment of(NH2CH(OHCHCH3)COOH)(H2SO4)is higher than that of(NHZCH(CHZOH)COOH)(HZSO4)5 indicating that methyl substitution may enhance the collision efficiency in ion-induced nucleation.The stronger Rayleigh light scattering intensities of(NHZCH(OHCHCH3)COOH)(H2SO4)(H2O)n(Mn=0-3)over(NH2CH(CH2OH)COOH)(H2S O4)(H2O)n(n=0-3)and(NH2CH(CH2OH)COOH)(H2SO4)(H2O)n(n=0-3)over(NH2(CH2)2 OH)(H2SO4)(H2O)n(n=0-3)suggests the enhancement effect of carboxylic substituent and methyl substitution in the scattering intensities of clusters.(2)The roles of monoethanolamine composed of hydroxyl group and alkylamines in nucleation process were compared.Results show that(N(CH3)3)(H2SO4)clusters mainly exists in unhydrated form while(NH2(CH2)2OH)(H2SO4)mainly in hydrated form.When the concentrations of monoethanolamine/trimethylamine/dimethylamine is comparable,the concentrations of dominant clusters are comparable between three amines clusters,indicative of the high ability of monoethanolamine to participate in sulfuric acid-based nucleation process.The performance of amines may be different with change in humidity.Monoethanolamine-enhanced nucleation may play an important role in environments with high humidity while trimethylamine may be the dominant species in dry conditions.Furthermore,the formation of monoethanolamine hydrate in cold area or in cold weather such as winter is easier than that of alkylamine hydrate.The collision efficiency of sulfuric acid_based alkylamine dusters in ion-induced nucleation may be higher than that of alkanolamine while alkanolamine-hydrated clusters may have a better performance than alkylamine-hydrated clusters.(3)The interaction of amides with amine substituent with sulfuric acid and the cluster properties were studied.Results show that amine substituents could influence the structure and proton transfer of amide clusters.The stronger interaction of(CO(NH2)2)(H2SO4)over(HCONH2)(H2SO4)indicates that amine substituent could enhance the sulfuric acid-stabilizing ability of anides.The higher distribution of hydrated formamide dusters over urea suggests that amine substituent may weaken the water affinity of amine clusters.The higher dipole moment of(CO(NH2)2)(H2SO4)over(HCONH2)(H2SO4)are indicative of the enhancement effect of amine substituent on the collision efficiency of amide clusters with ions in ion-induced nucleation process.The stronger Rayleigh light scattering intensities of urea clusters over formamide shows that amine substituent could enhance the scattering intensities of amide clusters.(4)The role of organic carboxylic acids with hydroxyl group in methanesulfonic acid-based nucleation and cluster properties were studied.Results show that the interaction strength between the three carboxylic acid and methanesulfonic acid follows dihydroxylacetic acid<glycolic acid<acetic acid,indicating that hydroxyl group may weaken the interaction between carboxylic acid and methanesulfonic acid.The number of methanesulfonic acid molecules and type of organic carboxylic acid could affect the collision efficiency of organic carboxylic acid-methanesulfonic acid clusters in ion-induced nucleation,However,the effect of hydroxyl group on the Rayleigh light scattering intensities of different organic carboxylic acid-methanesulfonic acid clusters is minimal.