Temperature‐Dependence of the Rates of Reaction of Trifluoroacetic Acid with Criegee Intermediates

Abstract

Abstract:The rate coefficients for gas-phase reaction of trifluoroacetic acid (TFA) with two Criegee intermediates, formaldehyde oxide and acetone oxide,decrease with increasing temperature in the range 240-340 K. The rate coefficients k(CH 2 OO + CF 3 COOH) = (3.4 AE 0. Halogenated organic acids such as trifluoroacetic acid (TFA) form in the Earthst roposphere by oxidation of anthropogenically produced hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluoro-olefins (HFOs), [1] and also have natural sources.[2] They react only slowly with hydroxyl radicals and do not photolyse at actinic wavelengths.[3] Current atmospheric models therefore incorporate surface deposition and rain-out as their main loss processes.[1b, 2] However,r ecent evidence from laboratory studies indicates that organic acids,a nd other trace atmospheric molecules,r eact with Criegee intermediates with room-temperature rate coefficients that approach (or exceed) the expected gas-kinetic limits predicted by collision rates.[4] Barrierless reaction pathways have been identified computationally, [5] corroborating the experimental measurements.These reactions might therefore represent asignificant chemical loss mechanism for halogenated organic acids from the troposphere.Here,w ee xamine the temperature dependence of the reactions of CH 2 OO and (CH 3 ) 2 COO with TFA, which we selected as representative of Criegee intermediate reactions with halogenated organic acids.W ep resent rate coefficients measured over ar ange of temperatures spanning those encountered in the lower troposphere.B imolecular rate coefficients were determined by the pseudo-first-order kinetic method for CH 2 OO + CF 3 COOH (k 1 ), CH 2 OO + CF 3 COOD (k 2 )and (CH 3 ) 2 COO + CF 3 COOH (k 3 )reactions for temperatures from 240 to 340 Kand pressures from 10 to 100 torr. Them easurements used cavity ring-down spectroscopy methods described previously [6] and in Supporting Information (SI).Complementary quantum chemistry calculations provided energies and structures along the reaction pathways to aid interpretation of the kinetic measurements,a nd to guide predictions of rates of as-yet unstudied reactions. Stationary points involved in the reactions of CH 2 OO, (CH 3 ) 2 COO, anti-C((trans-CH 3 )=CH 2 )-CHOO (anti-methacrolein oxide) and syn-CH 3 -trans-(CH=CH 2 )COO (synmethyl vinyl ketone oxide) with CF 3 COOH were calculated at the DF-HF//DF-LCCSD(T)-F12a/aug-cc-pVTZ//B3LY P/ 6-31 + G(d) level of theory.The former two reactants serve as model systems,w hereas the latter two were selected as possible Criegee intermediate products of the ozonolysis at each of the C=Cbonds of isoprene,animportant tropospheric constituent with biogenic sources. [7] Their structures are shown in the SI. Similarities between the calculated reaction paths allow predictions of rates of reaction of TFAwith the Criegee intermediates from isoprene ozonolysis which we incorporate into atmospheric chemistry models. Figure 1s hows an example of the method for determination of k 2 f...

Investigating the Tropospheric Chemistry of Acetic Acid Using the Global 3‐D Chemistry Transport Model, STOCHEM‐CRI

“…The high concentrations of CH 3 COOH in the Amazon rain forest area coincide with high concentrations of sCI, and their reactions could significantly reduce the atmospheric concentration of CH 3 COOH. Chhantyal‐Pun et al () found that these reactions show a relatively small temperature dependence over the temperature range found in the lower troposphere. Incorporating these reactions of Criegee intermediates with CH 3 COOH in the model results in reduction of CH 3 COOH in the Amazon rain forest area.…”

“…Recent studies suggested that the reaction of Criegee intermediates with organic acids may be of importance for the tropospheric chemistry of CH 3 COOH (Chhantyal‐Pun et al, ; Welz et al, ). An effort has been made to establish the scale of the potential contribution of these reactions to the tropospheric levels of CH 3 COOH.…”

“…Precise estimates for dry and wet deposition rates of CH 3 COOH are of importance as its tropospheric lifetime is strongly controlled by the deposition loss (Chebbi & Carlier, ). However, recent studies (Chhantyal‐Pun et al, ; Welz et al, ) suggested a very rapid chemical reaction of Criegee intermediates with carboxylic acids. Therefore, it is possible that the role of gas phase loss processes has been underestimated by previous studies.…”

Criegee Intermediate Reactions with Carboxylic Acids: A Potential Source of Secondary Organic Aerosol in the Atmosphere

“…[11][12][13][14] Recent laboratory studies showed that their reactions with certain carboxylic acids approach or exceed the gas-kinetic collisional limit. [15][16] Complementary global modelling studies indicated that inclusion of these reactions in chemical schemes significantly lowers the predicted lifetimes and steady state concentrations of these acids in the troposphere. [15][16] The wide range of biogenic and anthropogenic carboxylic acids emitted to the troposphere, and the variety of Criegee intermediates produced by ozonolysis of alkenes (including isoprene and terpenes), mean there are many possible combinations of reactions of atmospheric Criegee intermediates and carboxylic acids.…”

“…[15][16] Complementary global modelling studies indicated that inclusion of these reactions in chemical schemes significantly lowers the predicted lifetimes and steady state concentrations of these acids in the troposphere. [15][16] The wide range of biogenic and anthropogenic carboxylic acids emitted to the troposphere, and the variety of Criegee intermediates produced by ozonolysis of alkenes (including isoprene and terpenes), mean there are many possible combinations of reactions of atmospheric Criegee intermediates and carboxylic acids. This complexity necessitates development of structure-activity relationships (SARs) to account fully for the suite of reaction rates in atmospheric chemistry models.…”