A team of atmospheric chemists led by Dr. Jim Jr-Min Lin, a Research Fellow at the Institute of Atomic and Molecular Sciences, Academia Sinica, has discovered that a carbonyl oxide (a Criegee intermediate) may be an efficient oxidant of atmospheric sulfur dioxide (SO₂).

Oxidation of SO₂ produces SO₃ and subsequently sulfuric acid (H₂SO₄), an important constituent of aerosols and acid rain, and is therefore an important topic of research in atmospheric chemistry. The research results were published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) on August 13, 2015.

These findings may explain discrepancies found between the observed and modeled concentrations of atmospheric H₂SO₄ and suggest that Criegee intermediates play a greater role in atmospheric chemistry than previously believed. The current research follows on from the team’s discovery published in January, 2015 that another type of Criegee intermediates predominantly reacts with water vapor in the atmosphere, which clarified the fate of the simplest Criegee intermediate (CH₂OO) in the atmosphere [Chao W, et al. (2015) Science 347(6223):751-754].

In this work, the kinetics of the reactions of dimethyl substituted Criegee intermediate (CH₃)₂COO with water vapor and with SO₂ were directly measured via UV absorption of (CH₃)₂COO under near atmospheric conditions. The results indicated, first, that the water reaction with (CH₃)₂COO is not fast enough (k_H2O < 1.5×10^-16 cm³s^-1) to consume atmospheric (CH₃)₂COO significantly, and second, that (CH₃)₂COO reacts with SO₂ at a near gas-kinetic-limit rate (k_SO2 = 1.3×10^-10 cm³s^-1). These observations imply a significant fraction of atmospheric (CH₃)₂COO may survive under humid conditions and react with SO₂, a very different scenario from the case of the simplest Criegee intermediate CH₂OO, in which the reaction with the water dimer predominates in the CH₂OO decay under typical tropospheric conditions. In addition, a significant pressure dependence was observed for the reaction of (CH₃)₂COO with SO₂, suggesting the use of low pressure rate may underestimate the impact of this reaction. This work demonstrates that the reactivity of a Criegee intermediate towards water vapor strongly depends on its structure, which will influence the main decay pathways and steady-state concentrations for various Criegee intermediates in the atmosphere.

This study clearly demonstrates that water vapor does not react with dimethyl substituted Criegee intermediate (CH₃)₂COO, at least not fast enough to significantly consume (CH₃)₂COO in the troposphere. On the other hand, (CH₃)₂COO reacts with SO₂ three times faster than CH₂OO does, indicating Criegee intermediates of a structure similar to (CH₃)₂COO are potential candidates for an efficient oxidant in the atmospheric SO₂ oxidation.

The article, entitled “Kinetics of a Criegee intermediate that would survive high humidity and may oxidize atmospheric SO₂” can be found at : http://www.pnas.org/content/early/2015/08/13/1513149112.full.pdf

The full list of authors is: Hao-Li Huang, Wen Chao and Jim Jr-Min Lin.