Kinetics and pressure-dependent HO yields of the reaction between the Criegee intermediate CH₂OO and HNO₃

The reaction of Criegee intermediates with nitric acid (HNO₃) plays an important role for removal of Criegee intermediates as well as in oxidation of atmospheric HNO₃ because of its fast reaction rate. Theoretical prediction suggests that the product branching ratios of the reaction of the simplest Criegee intermediate CH₂OO with HNO₃ are strongly pressure dependent and the CH₂OO may be catalytically converted to OH and HCO radicals by HNO₃. The direct quantification of HO radicals formed from this reaction is hence crucial to evaluate its atmospheric implications. By employing mid-infrared multifunctional dual-comb spectrometers, the kinetics and product yields of the reaction CH₂OO + HNO₃ are investigated. A pressure independent rate coefficient of (1.9 ± 0.2) × 10^-10 cm³ molecule^-1 s^-1 is obtained under a total pressure of 6.3-58.6 Torr at 296 K. The product branching ratios are derived by simultaneous determination of CH₂OO, formaldehyde (CH₂O), OH and HO₂ radicals. At the total pressure of 12.5 Torr, the yield for the formation of NO₂ + CH₂O + HO₂ is 36% and only 3.2% for OH + CH₂(O)NO₃, whereas the main remainder may be thermalized nitrooxymethyl hydroperoxide (NMHP, NO₃CH₂OOH). Additionally, the fractional yields of both the OH and HO₂ product channels are decreased by a factor of roughly 2 from 12 to 60 Torr, indicating that there is almost no catalytic conversion of CH₂OO to the OH radicals in the presence of HNO₃.