Photophysical oxidation of HCHO produces HO2 radicals

Blair A Welsh; Maggie E Corrigan; Emmanuel Assaf; Klaas Nauta; Paolo Sebastianelli; Meredith J T Jordan; Christa Fittschen; Scott H Kable

doi:10.1038/s41557-023-01272-4

Photophysical oxidation of HCHO produces HO₂ radicals

Nat Chem. 2023 Oct;15(10):1350-1357. doi: 10.1038/s41557-023-01272-4. Epub 2023 Jul 6.

Authors

Blair A Welsh^{1

2}, Maggie E Corrigan³, Emmanuel Assaf^{4

5

6}, Klaas Nauta¹, Paolo Sebastianelli^{1

7}, Meredith J T Jordan⁸, Christa Fittschen⁴, Scott H Kable⁹

Affiliations

¹ School of Chemistry, University of New South Wales, Kensington, New South Wales, Australia.
² Combustion Research Facility, Sandia National Laboratories, Livermore, CA, USA.
³ School of Chemistry, University of Sydney, Sydney, New South Wales, Australia.
⁴ Université Lille, CNRS, UMR 8522, PC2A-Physicochimie des Processus de Combustion et de l'Atmosphère, Lille, France.
⁵ Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA.
⁶ Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA.
⁷ School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, New South Wales, Australia.
⁸ School of Chemistry, University of Sydney, Sydney, New South Wales, Australia. Meredith.jordan@sydney.edu.au.
⁹ School of Chemistry, University of New South Wales, Kensington, New South Wales, Australia. s.kable@unsw.edu.au.

PMID: 37414879
DOI: 10.1038/s41557-023-01272-4

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 O₂ to form HO₂. Here we show HCHO has an additional HO₂ formation pathway. At photolysis energies below the energetic threshold for radical formation we directly detect HO₂ at low pressures by cavity ring-down spectroscopy and indirectly detect HO₂ at 1 bar by Fourier-transform infrared spectroscopy end-product analysis. Supported by electronic structure theory and master equation simulations, we attribute this HO₂ 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 O₂. PPO is likely to be a general mechanism in tropospheric chemistry and, unlike photolysis, PPO will increase with increasing O₂ pressure.

Abstract

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