The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal

Abstract

The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. E-2-Hexenal and E,E-2,4-hexadienal were found to undergo rapid isomerization to produce Z-2-hexenal and a ketene-type compound (probably E-hexa-1,3-dien-1-one), respectively. Both isomerization processes were reversible with formation of the reactant slightly favoured. Values of j(E-2-hexenal)/j(NO₂) = (1.80 ± 0.18) × 10⁻² and j(E,E-2,4-hexadienal)/j(NO₂) = (2.60 ± 0.26) × 10⁻² were determined. The gas phase UV absorption cross-sections of E-2-hexenal and E,E-2,4-hexadienal were measured and used to derive effective quantum yields for photoisomerization of 0.36 ± 0.04 for E-2-hexenal and 0.23 ± 0.03 for E,E-2,4-hexadienal. Although photolysis appears to be an important atmospheric degradation pathway for E-2-hexenal and E,E-2,4-hexadienal, the reversible nature of the photolytic process means that gas phase reactions with OH and NO₃ radicals are ultimately responsible for the atmospheric removal of these compounds. Atmospheric photolysis of Z-3-hexenal produced CO, with a molar yield of 0.34 ± 0.03, and 2-pentenal via a Norrish type I process. A value of j(Z-3-hexenal)/j(NO₂) = (0.4 ± 0.04) × 10⁻² was determined. The results suggest that photolysis is likely to be a minor atmospheric removal process for Z-3-hexenal.