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

ABSTRACT

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 HOx 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.