# T- and pH-Dependent Hydroxyl-Radical Reaction Kinetics of Lactic Acid, Glyceric Acid, and Methylmalonic Acid in the Aqueous Phase

## TL;DR

This study measures how quickly lactic, glyceric, and methylmalonic acids react with hydroxyl radicals in water, which helps understand their removal in the atmosphere.

## Contribution

New pH- and temperature-dependent rate constants for •OH radical oxidation of three carboxylic acids in aqueous phase.

## Key findings

- Rate constants for •OH radical reactions with lactic, glyceric, and methylmalonic acids were determined using thiocyanate competition kinetics.
- The reaction rate trends showed kA2– > kHA– > kH2A for the investigated acids.
- Atmospheric lifetimes and partitioning of the acids were calculated using the measured rate constants.

## Abstract

Carboxylic acids
are a common class of compounds found in atmospheric
aerosols and cloud droplets. In this study, the oxidation kinetics
of several carboxylic acids in the aqueous phase by the atmospherically
relevant •OH radical were investigated to better
understand the loss processes for this class of compounds. The rate
constants for the reactions of the •OH radical were
determined using the thiocyanate competition kinetics method for lactic
acid, glyceric acid, and methylmalonic acid as a function of temperature
and pH. The Arrhenius equations for oxidation by the •OH radical are as follows (unit in L mol–1 s–1): For lactic acid: k(T, HA) = (1.3 ± 0.1) × 1010 × exp[(−910
± 160 K)/T] and k(T, A–) = (1.3 ± 0.1) × 1010 × exp[(−800 ± 80 K)/T]; for glyceric
acid: k(T, HA) = (6.0 ± 0.2)
× 1010 × exp[(−1100 ± 170 K)/T] and k(T, HA±) = (3.6 ± 0.1) × 1010 × exp[(−1500
± 100 K)/T]; and for methylmalonic acid: k(T, H2A) = (5.5 ± 0.1)
× 1010 × exp[(−1760 ± 100 K)/T], k(T, HA–) = (1.4 ± 0.1) × 109 × exp[(−530
± 80 K)/T] and k(T, A2–) = (9.6 ± 0.4) × 1010 × exp[(−1530 ± 270 K)/T]. The
general trend of the •OH rate constant was observed kA2–> kHA–> kH2A. The energy barriers of the •OH
radical
reaction and thus the most probable site of H atom abstraction were
calculated using density functional theory simulations in Gaussian with the M06-2X method and the 6-311++G(3df,2p)
basis set. Kinetic data predicted from structure–activity relationships
were compared to the measured •OH radical rate constants. •OH radical oxidation in the aqueous phase could serve
as an important sink for carboxylic acids, and the pH- and T-dependent
rate constants of •OH radical reactions provide
a better description of the aqueous-phase sink processes. Hence, the
atmospheric lifetime as well as the partitioning of the investigated
carboxylic acids was calculated.