Photophysical oxidation of HCHO produces HO 2 radicals.

Authors :: Welsh BA
Corrigan ME
Assaf E
Nauta K
Sebastianelli P
Jordan MJT
Fittschen C
Kable SH
Source :: Nature chemistry [Nat Chem] 2023 Oct; Vol. 15 (10), pp. 1350-1357. Date of Electronic Publication: 2023 Jul 06.
Publication Year :: 2023
Abstract: Formaldehyde, HCHO, is the highest-volume carbonyl in the atmosphere. It absorbs sunlight at wavelengths shorter than 330 nm and photolyses to form H and HCO radicals, which then react with O <subscript>2</subscript> to form HO <subscript>2</subscript> . Here we show HCHO has an additional HO <subscript>2</subscript> formation pathway. At photolysis energies below the energetic threshold for radical formation we directly detect HO <subscript>2</subscript> at low pressures by cavity ring-down spectroscopy and indirectly detect HO <subscript>2</subscript> at 1 bar by Fourier-transform infrared spectroscopy end-product analysis. Supported by electronic structure theory and master equation simulations, we attribute this HO <subscript>2</subscript> to photophysical oxidation (PPO): photoexcited HCHO relaxes non-radiatively to the ground electronic state where the far-from-equilibrium, vibrationally activated HCHO molecules react with thermal O <subscript>2</subscript> . PPO is likely to be a general mechanism in tropospheric chemistry and, unlike photolysis, PPO will increase with increasing O <subscript>2</subscript> pressure.<br /> (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)