A Study Of Aqueous Phase Oxidation Of Carboxylic Acids With OH Radicals,SO₄^- Radicals And NO₃ Radicals Using Laser Equipment

Carboxylic acids are important components of secondary aerosols(SOA),and their presence has been proven in several studies of atmospheric particles.Due to their low vapor pressure and high hygroscopicity,carboxylic acids can be accumulated in the particle phase and reduce the surface tension of cloud condensation nodules.They reduce visibility by altering the cloud formation process image radiation budget and affect the acidity of cloud water,which in turn affects the acidity of precipitation.Therefore,carboxylic acids have an important impact on the environment and human health.On the other hand,as an important oxidant in the atmosphere,free radicals have high reactivity and they can oxidatively degrade various pollutants,among which the oxidative degradation of carboxylic acids by free radicals is an important way to remove carboxylic acids in the atmospheric aqueous phase.In the early 1990s,by conducting radical kinetic studies using radicals as oxidants in the atmospheric aqueous phase,Herrmann et al.found that the reactions of carboxylic acids with radicals are ubiquitous and highly reactive.In research up to now,it has been shown that the reaction products of carboxylic acids with free radicals may be a potential source of oligomers and SO A.Therefore,conducting kinetic and mechanistic studies of reactions of free radicals and carboxylic acids can help to gain insight into the formation of SOA in the atmospheric aqueous phase.In this study,a laser flash photolysis long path absorption experiment was used to determine the aqueous phase chemical reactions of rate constants of hydroxyl radicals(OH)with glyceric and methylmalonic acids,sulfate radicals(SO4-)and nitrate radicals(NO3)with butyric acid,2-hydroxybutyric acid and 3-hydroxybutyric acid.The reactions of rate constants of OH radicals with carboxylic acids were calculated using competitive kinetics,using hydrogen peroxide(H2O2)as the radical precursor and potassium thiocyanide(KSCN)as the reference,and the values were corrected for internal absorption effects.In contrast,direct measurements were used for SO4-and NO3 radicals.The pH conditions(different forms of carboxylic acids:monocarboxylic acids(MACs)in protonation and deprotonation,dicarboxylic acids(DACs)in protonation,monoanionic and deprotonation)were determined from the dissociation constants of carboxylic acids.Both the temperature(278 K,288 K,298 K,308 K and 318 K)and pH dependence of the reactions of rate constants of OH radicals,SO4-and NO3 radicals with carboxylic acids can be expressed according to Arrhenius plots and equations.The reaction rate constants obtained from the experiments at 298 K are summarized as follows:OH radicals with glyceric acid,k(HA)=(1.4±0.1)× 109 L·mol-1·s-1、k(A-)=(2.4±0.4)×109 L·mol-1·s-1;OH radicals with methylmalonic acid,k(T,H2A)=(1.6±0.1)× 108 L·mol-1·s-1、k(T,HA-)=(2.6 ± 0.1)× 108 L·mol-1·s-1、k(T,A2-)=(6.2±0.4)×108 L·mol-1·s-1;SO4-radicals with butyric acid,k(T,HA)=(7.7 ± 0.6)× 106 L·mol-1·s-1、k(T,A-)=(9.7 ± 1.5)× 106 L·mol-1·s-1;SO4-radicals with 2-hydroxybutyric acid,k(T,HA)=(2.1 ± 0.3)× 107 L·mol-1·s-1、k(T,A-)=(2.5 ± 0.4)× 107 L·mol-1·s-1;SO4-radicals with 3-hydroxybutyric acid,k(T,HA)=(1.7±0.2)× 107 L·mol-1·s-1、k(T,A-)=(1.1±0.1)× 107 L·mol-1·s-1;NO3 radicals with butyric acid,k(T,HA)=(4.1±0.6)×105 L·mol-1·s-1 k(T,A-)=(6.6± 1.0)×106 L·mol-1·s-1;NO3 radicals with 2-hydroxybutyric acid,k(T,HA)=(5.3 ± 0.6)×106 L·mol-1·s-1、k(T,A-)=(1.3 ± 0.3)× 107 L·mol-1·s-1;NO3 radicals with 3-hydroxybutyric acid,k(T,HA)=(8.5±1.3)×105 L·mol-1·s-1、k(T,A-)=(5.7 ± 1.2)× 106 L·mol-1·s-1.From the above data,it can be seen that the reactions of rate constants of free radicals with MACs satisfy that k(A-)>k(HA),and DACs have similar conclusions:k(A2-)>k(HA-)>k(H2A).Combing the previous studies,the rate constants of the three radicals with carboxylic acids can be derived in the rule:k(OH)>>k(SO4-)>k(NO3).In addition,the expansion of the scope of temperature research shows that with the increase of temperature,the reaction rate constants of carboxylic acids with free radicals gradually accelerates,filling the gap in temperature research.From the reaction mechanism,k(A-)>k(HA)may be due to the fact that hydrogen abstraction reactions of the three radicals with the carboxylic acids occur slower than an electron transfer.In addition,the possibility of electron transfer when the carboxylic acids are present as A-.The results for NO3 radicals and SO4-radicals with butyric acid,2-hydroxybutyric acid and 3-hydroxybutyric acid are shown that the presence of hydroxy functional groups facilitates the reactions,probably because the hydroxyl functional group reduces the hydrocarbon bonding energy of the carbon atoms to which they are attached.After simultaneous calculation of the diffusion reaction rate constants and activation parameters,comparison of the experimental measurements shows that the carboxylic acids with the three radicals are mainly controlled by chemical reactions.The chemical lifetimes of the carboxylic acids were calculated from the radical concentrations in the cloud and particle phases obtained by the CAPRAM 3.0i multiphase model.It can be concluded that compared with SO4-radicals,NO3 radicals with carboxylic acids have a longer chemical life,while OH radicals and carboxylic acids have a shorter chemical life,basically measured in minutes.The kinetic data obtained in this study can be further applied to atmospheric aqueous phase chemical models,combined with density functional theory to calculate the difference of reaction rate constants and study the reaction products,so as to improve the aqueous phase kinetics research.And then it could describe the reaction mechanism and lifetimes of carboxylic acids and predict the atmosphere changes.