A181-0022
Shining a New Light on Formaldehyde Photochemistry

Tuesday, 15 December 2020
Poster
Blair Welsh1, Maggie Corrigan2, Emmanuel Assaf3, Meredith Jordan2, Christa M Fittschen3 and Scott Kable1, (1)University of New South Wales, School of Chemistry, Sydney, NSW, Australia, (2)University of Sydney, Sydney, Australia, (3)University of Lille 1, Villeneuve d'Ascq, France
Abstract:
Understanding the chemistry of our atmosphere is a key part of evaluating our influence on this planet. Before 1950, formaldehyde (H2CO) and methane (CH4) were the only confirmed atmospheric volatile organic compounds (VOCs)1. Approximately 105 atmospherically relevant VOCs have since been identified, representing a massive increase in scope of the field of atmospheric chemistry. Formaldehyde, while one of the earliest VOCs to be discovered, remains one of the most important and well-studied, due to its biogenic and anthropogenic prevalence. For over forty years, however, there has been a discrepancy in its photochemistry, with radical photolysis products HCO and H observed over 2000 cm-1 below the corresponding, well-characterised energetic threshold2. This has historically been attributed to the excitation of vibrationally excited molecules of H2CO, or ‘hot-bands’. We have discovered that a new mechanism we have termed atmospheric photothermal oxidation (APTO) is responsible for this discrepancy. The APTO mechanism involves an O2 molecule abstracting a hydrogen atom from a photoexcited formaldehyde molecule before it can dissociate or dissipate its energy. Since APTO is effectively a reaction of a VOC with O2, this process emulates reactions studied in combustion chemistry, with the normally prohibitive energetic barrier to this process being overcome by the energy from the absorbed photon. The primary products of formaldehyde APTO are the hydroperoxyl (HO2) radical, and the formyl (HCO) radical, the latter of which will react with O2 to produce yet another HO2 radical under atmospheric conditions. APTO may therefore represent a significant and as-yet unaccounted for source of HO2 radicals, one of the most important radicals in the atmosphere and one that has proven difficult for the atmospheric chemistry community to model effectively3. APTO is also showing promise as a viable pathway for many other atmospherically relevant molecules beyond formaldehyde, paving the way for future research.

1. Mader, P. P.; MacPhee, R. D.; Lofberg, R. T.; Larson, G. P. Ind. Eng. Chem. 1952, 44 (6), 1352–1355.

2. Terentis, A. C.; Kable, S. H., Chem. Phys. Lett. 1996, 258 (5–6), 626–632

3. Stone, D.; Whalley, L. K.; Heard, D. E., Soc. Rev. 2012, 41 (19), 6348–6404.

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