ABSTRACT
The reactions of Criegee intermediates with HNO3 are important in the polluted urban atmosphere because of their large rate coefficients and the significant concentration of HNO3. Employing a step-scan Fourier-transform spectrometer, we recorded infrared spectra of transient species and end products in the reaction CH2OO + HNO3 upon irradiation of a flowing mixture of CH2I2/HNO3/N2/O2 at 308 nm. Eight bands at 1686, 1426, 1348, 1294, 1052, 965, 891, and 825 cm-1 were assigned to the absorption of the adduct nitrooxymethyl hydroperoxide (NMHP, NO3CH2OOH). Additional products from two dissociation channels were observed. Four bands at 1709, 1325, 1276, and 886 cm-1 were assigned to H2C(O)ONO2 (with coproduct OH), produced from the fission of the O-O bond of internally hot NMHP (NMHP*). Simultaneous detection of H2CO (1746 cm-1), NO2 (1617 cm-1), and HO2 (1392 and 1098 cm-1) indicated a direct cleavage of the N-OC and C-OO bonds of NMHP*. The relative yields of these three channels in pressure range 10-150 Torr were estimated. At 10 Torr, the absorption of internally excited HNO3 near 885 and 1320 cm-1 was also detected at an early stage of the reaction. We investigated also the rate coefficient of the reaction CH2OO + HNO3 by probing the temporal profiles of the formation of NMHP and NO2 under total pressures of 40 and 70 Torr at 298 K. The rate coefficient kHNO3 = (2.4 ± 0.4) × 10-10 cm3 molecule-1 s-1 is less than half the only literature value, (5.4 ± 1.0) × 10-10 cm3 molecule-1 s-1, reported by Foreman et al. (Angew. Chem. Int. Ed. 2016, 55, 10419-10422).
