The Application Of Laser Equipment For Free Radical Kinetics Research Of Carboxylic Acid In The Aqueous Phase

As important organic components of atmospheric particulate matter and clouds,carboxylic acids are prevalent in the atmospheric aqueous phase(rain,cloud droplets,fog,haze and hygroscopic particles containing 'aerosol liquid water(ALW)')and play important roles in atmospheric aqueous phase reactions,which have important influences on the hygroscopicity,acidity and composition of atmospheric particulate matter,and human health.The kinetic information of the pH-and temperature-dependent degradation rate constants of carboxylic acids obtained in the present study will help to better explain the chemical transformation processes in atmospheric aqueous aerosol and cloud chemistry.A laser flash photolysis-laser long-path absorption set-up is newly installed and used to measure the rate constants of aqueous-phase chemical reactions of five organic compounds,including three monocarboxylic acids(n-butyric acid,2-hydroxybutyric acid,3-hydroxybutyric acid)and two dicarboxylic acids(tartaric acid,mucic acid)with OH radicals.A pulsed laser of wavelength ?=248 nm is used to photolyze hydrogen peroxide generating OH radicals in the measurement cell.Simultaneously,a continuous wave(cw)laser of wavelength ?=561 nm is used to measure the change in absorbance in the measurement cell due to the absorption products formed.The method of competitive kinetics is used because OH radicals absorb only weakly in the deep UV range and its absorption is often superimposed by the absorption of the subsequent species(alkyl or peroxyl radicals).Potassium thiocyanide is used as the OH radical scavenger in this work,which forms the(SCN)2·-radical anion absorbing strongly in the visible range of the spectrum after its reaction with the OH radicals.The rate constants are determined by keeping the radical scavenger concentration(SCN-)and the radical precursor concentration(H2O2)constant and gradually increasing the concentration of the organic reactant in the individual experiments.The resulting change in concentration of(SCN)2·-is determined by the change in the absorbance and is then used to calculate the rate constant for the reaction of the OH radical with the added organic compound.An inner filter effect of organic compounds can occur and influences the measured rate constant.It is therefore treated for tartaric acid which absorbs light at ?=248 nm and accordingly the rate constants of tartaric acid with OH radicals are corrected for this effect.Different pH(depending on the form of carboxylic acids)and temperature(278,288,298,308,318 K)are set for the reactions of carboxylic acids with OH radicals based on the Arrhenius equation,obtaining Arrhenius plots and expressions for the temperature dependence of different forms of carboxylic acids.The Arrhenius expressions(in units of L mol-1 s-1)for the different compositions of the five carboxylic acids(monocarboxylic acids in protonated and deprotonated forms,and dicarboxylic acids in protonated,monoanionic and deprotonated forms)have been obtained:butyric acid,k(T,HA)=(1.7±0.1)× 1011 × exp[(-1400±240K)/T],k(T,A-)=(1.8±0.1)×1011× exp[(-1340 ± 190K)/T];2-hydroxybutyric acid,k(T,HA)=(3.1±0.1)× 1010 × exp[(-1060±190K)/T],k(T,A-)=(5.7±0.1)×1010 × exp[(-1170±120K)/r];3-hydroxybutyric acid,k(T,HA)=(3.9±0.1)× 1010 × exp[(-1210 ± 140 K)/T],k(T,A-)=(4.9±0.1)× 1010 × exp[(-1230 ± 140K)/T];tartaric acid corrected for inner filter effect,k(T,H2A)=(3.3 ± 0.1)×1010 ×exp[(-1350±110K)/T],k(T,HA-)=(3.6 ± 0.1)× 1010 × exp[(-580 ±110K)/T],k(T,A2-)=(3.3±0.1)× 1010 × exp[(-1190 ± 170 K)/T];mucic acid,k(T,H2A)=(2.2±0.1)× 1010 × exp[(-1140 ± 150K)/T],k(T,HA-)=(4.8±0.1)× 1010 × exp[(-1280±170 K)/T],k(T,A2-)=(2.1±0.1)× 1010 × exp[(-970 ± 70 K)/T].Overall,the rate constants for the reactions of OH radicals with monocarboxylic acids in their protonated forms are smaller than those for the deprotonated forms(k(/HA)<k(A-)),and a similar behavior is observed for the dicarboxylic acids(k(T,H2A)<k(T,HA-)<k(T,A2-)).Mechanistically,the OH radical reacts with the saturated carboxylic acids through a hydrogen-atom abstraction.Because of the C-H bond strength,the following order-CH,-CH2,-CH3,-OH towards higher bond strength and smaller rate constant is obtained.The diffusion limit is calculated for the aforementioned reactions,from this calculation it is evident that the reactions are not fast enough to be diffusion controlled,but are mainly chemically controlled.Comparing the second-order rate constants of carboxylic acids with OH radicals,including adipic acid with mucic acid,butyric acid with 2-hydroxybutyric acid,and 2-hydroxy butyric acid with 3-hydroxy butyric acid,the addition of OH functional group to-CH2 may reduce the second-order rate constants of H-atom abstraction reactions in similar form of carboxylic acid in the same protonated form and the second-order rate constant of OH functional group in ?-C(3-hydroxybutyric acid)is smaller than in ?-C(2-hydroxybutyric acid).Comparing the measured rate constants with calculated rate constants from structure-activity relationships,rate constants are the same order of magnitude as the observed ones can be calculated,with the calculated values being in better agreement for the monocarboxylic acids than for the dicarboxylic acids.The chemical lifetimes of five measured carboxylic acids in the aqueous phase of the atmosphere ranges from a few minutes in the particulate phase to over ten days in the cloud,depending on the detailed scenarios chosen.These kinetics data determined in the present study can be applied in chemical models to describe the transformations and lifetimes of carboxylic acids in the atmospheric aqueous phase.The results from this analysis contribute to a more detailed understanding of the chemical transformation of carboxylic acids in the troposphere.