An assessment of the tropospherically accessible photo-initiated ground state chemistry of organic carbonyls

Keiran N Rowell,Meredith J T Jordan,Scott H Kable

doi:10.5194/acp-22-929-2022

Keiran N Rowell, Meredith J T Jordan + Show 1 more

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https://doi.org/10.5194/acp-22-929-2022

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Abstract

Abstract. Carbonyls are among the most abundant volatile organic compounds in the atmosphere. They are central to atmospheric photochemistry as absorption of near-UV radiation by the C=O chromophore can lead to photolysis. If photolysis does not occur on electronic excited states, non-radiative relaxation to the ground state will form carbonyls with extremely high internal energy. These “hot” molecules can access a range of ground state reactions. Up to nine potential ground state reactions are investigated at the B2GP-PLYP-D3/def2-TZVP level of theory for a test set of 20 representative carbonyls. Almost all are energetically accessible under tropospheric conditions. Comparison with experiment suggests the most significant ground state dissociation pathways will be concerted triple fragmentation in saturated aldehydes, Norrish type III dissociation to form another carbonyl, and H2 loss involving the formyl H atom in aldehydes. Tautomerisation, leading to more reactive unsaturated species, is also predicted to be energetically accessible and is likely to be important when there is no low-energy ground state dissociation pathway, for example in α,β-unsaturated carbonyls and some ketones. The concerted triple fragmentation and H2-loss pathways have immediate atmospheric implications for global H2 production, and tautomerisation has implications for the atmospheric production of organic acids.

Highlights

Carbonyls are a class of volatile organic compounds (VOCs) central to atmospheric chemistry
Up to 10 % of carbon initially fixed by plants is subsequently emitted as biological volatile organic compounds (BVOCs), which include directly emitted carbonyls, as well as other volatile species that are subsequently oxidised to carbonyls (Seco et al, 2007)
Internally excited carbonyls could undergo species such as O2, H2O, qOraHpi,dNbOixmaonledcNulHar3.rTeahcetipoonsswibiitlhity of a reaction between O2 and internally hot acetaldehyde, formed following excitation at 248 nm, considerably above the actinic range, was speculated by Morajkar et al (2014), this was not further investigated or resolved. Such bimolecular reactions may lead to radical species, and we suggest they may be responsible for radical quantum yield (QY) observed following photolysis at energies below the Norrish type I reaction (NTI) thresholds in formaldehyde (Horowitz and Calvert, 1978; Moortgat and Warneck, 1979; Valachovic et al, 2000)

Summary

Introduction

Carbonyls are a class of volatile organic compounds (VOCs) central to atmospheric chemistry. They arise, in large quantities, from primary anthropogenic and biogenic emissions and via secondary atmospheric processes (Kesselmeier and Staudt, 1999; Millet et al, 2010; Chen et al, 2014). Small organic carbonyls, such as acetone, formaldehyde and acetaldehyde, which are ranked in the top 25 of all anthropogenically emitted molecules by mass (Simon et al, 2010), are emitted as pollutants (Chen et al, 2014). Atmospheric concentrations of organic carbonyls are in the pptV–ppbV range (Tanner et al, 1996), with high concentrations found at low altitudes and in polluted environments (Lee et al, 1998; Pal et al, 2008; Guo et al, 2014; Menchaca-Torre et al, 2015)

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