Direct Measurements of Conformer-Dependent Reactivity of the Criegee Intermediate CH ₃ CHOO

Abstract

Although carbonyl oxides, "Criegee intermediates," have long been implicated in tropospheric oxidation, there have been few direct measurements of their kinetics, and only for the simplest compound in the class, CH2OO. Here, we report production and reaction kinetics of the next larger Criegee intermediate, CH3CHOO. Moreover, we independently probed the two distinct CH3CHOO conformers, syn- and anti-, both of which react readily with SO2 and with NO2. We demonstrate that anti-CH3CHOO is substantially more reactive toward water and SO2 than is syn-CH3CHOO. Reaction with water may dominate tropospheric removal of Criegee intermediates and determine their atmospheric concentration. An upper limit is obtained for the reaction of syn-CH3CHOO with water, and the rate constant for reaction of anti-CH3CHOO with water is measured as 1.0 × 10(-14) ± 0.4 × 10(-14) centimeter(3) second(-1).

Kinetics of a Criegee intermediate that would survive high humidity and may oxidize atmospheric SO 2

“…Experimental results (Table 1) show that CH 2 OO and anti-CH 3 CHOO react with water vapor very quickly (3,(22)(23)(24). Thus, their steady-state concentrations would be too low to have a significant impact in SO 2 oxidation under typical atmospheric conditions, as shown in modeling results (15,16).…”

“…Although it has been shown that CIs may react very fast with SO 2 , NO 2 , and organic acids (2,3,14), the reactions of CIs with atmospheric water vapor would still strongly influence the fates and concentrations of atmospheric CIs (see Fig. 1 for a simplified schematic).…”

“…However, the impact of such oxidation reactions is affected by the reactions of Criegee intermediates with water vapor, because of high water concentrations in the troposphere. In this work, the kinetics of the reactions of dimethyl substituted Criegee intermediate (CH 3 ) 2 COO with water vapor and with SO 2 were directly measured via UV absorption of (CH 3 ) 2 COO under near-atmospheric conditions. The results indicate that (i) the water reaction with (CH 3 ) 2 COO is not fast enough (k H2O < 1.5 × 10 −16 cm 3 s −1 ) to consume atmospheric (CH 3 ) 2 COO significantly and (ii) (CH 3 ) 2 COO reacts with SO 2 at a near-gas-kinetic-limit rate (k SO2 = 1.3 × 10 −10 cm 3 s −1 ).…”

“…Criegee intermediates are thought to play a role in atmospheric chemistry, in particular, the oxidation of SO 2 , which produces SO 3 and subsequently H 2 SO 4 , an important constituent of aerosols and acid rain. However, the impact of such oxidation reactions is affected by the reactions of Criegee intermediates with water vapor, because of high water concentrations in the troposphere.…”

“…This work demonstrates that the reactivity of a Criegee intermediate toward 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. Ozonolysis of unsaturated hydrocarbons produces highly reactive Criegee intermediates (CIs) (1), which may (i) decompose to radical species like OH radicals or (ii) react with a number of atmospheric species, for example, with SO 2 to form SO 3 and with NO 2 to form NO 3 (2,3). The SO 2 oxidation by CIs has gained special attentions because the SO 3 product would be converted into H 2 SO 4 , an important constituent of aerosols and acid rain (4)(5)(6)(7)(8).…”