RESUMEN
Nitrite (NO2-) is one of the common salts in aqueous aerosols, and its photolytic products, nitric oxide (NO) and hydroxyl radical (OH), have potential for use in the oxidation of organic matter, such as dissolved formaldehyde, methanediol (CH2(OH)2), which is regarded as the precursor of atmospheric formic acid. In this work, the simulation of UVA irradiation in an aqueous mixture of NaNO2/CH2(OH)2 was carried out via continuous exposure with a 365 nm LED lamp, and the reaction evolutions were probed by in situ and real-time infrared and Raman spectroscopy, which provided multiplexity in the identification of the relevant species and the corresponding reaction evolution. Although performing infrared absorption measurements in aqueous solution seemed impracticable due to the strong interference of water, the multiplexity of the vibrational bands of parents and products in the non-interfered infrared regimes and the conjunction with Raman spectroscopy still make it possible to perform in situ and real-time characterization of the photolytic reaction in the aqueous phase, supplementary to chromatographic approaches. During the 365 nm irradiation, NO2- and CH2(OH)2 gradually decreased, concomitant with the formation of nitrous oxide (N2O) and formate (HCOO-) in the early period and carbonate (CO32-) in the late period, as revealed by the vibrational spectra. The losses or the gains of the aforementioned species increased with increases in the concentration of CH2(OH)2 and the irradiation flux of the 365 nm UV light. The ionic product HCOO- was also confirmed by ion chromatography, but oxalate (C2O42-) was absent in the vibrational spectra and ion chromatogram. The reaction mechanism is reasonably proposed on the basis of the evolutions of the aforementioned species and the predicted thermodynamic favorableness.
RESUMEN
Ultraviolet irradiation of aqueous nitrite (NO2-) quickly generates hydroxyl (OH) and nitric oxide (NO) radicals, which are subsequently consumed concomitantly with the generation of NxOy species. Recently, dissolved formaldehyde (CH2O) in aqueous solution (e.g., methanediol, CH2(OH)2), has been regarded as the precursor in the formation of atmospheric formic acid (HCOOH) via the reaction with OH and cascading processes ( Nature2021, 593, 233-237). In this work, a step-scan Fourier transform interferometer was utilized to monitor the time-resolved difference infrared spectra of NaNO2 aqueous solution in the presences of methanediol and methanol upon pulsed irradiation at 355 nm. The fates of the dinitrogen trioxide (N2O3), generated via a series of reactions of OH, NO, and NO2-, differed in the presences of CH2(OH)2 and CH3OH. The decay of N2O3 via hydrolysis with H2O was retarded more by CH3OH than by CH2(OH)2. The monohydroxyl group of CH3OH does not behave like the hydroxyl group of water, whereas the two hydroxyl groups of CH2(OH)2 can be treated as a water reservoir via the quick equilibrium between H2O and CH2O, further facilitating the hydrolytic solvation reaction of N2O3. The observed difference in reaction kinetics involving hydration should be thoroughly taken into consideration in formaldehyde-related aqueous reactions.