Kinetic study of the gas-phase photolysis and OH radical reaction of E,Z- and E,E-2,4-Hexadienedial

✍ Scribed by Björn Klotz; Ian Barnes; Karl-Heinz Becker

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 174 KB
• Volume: 31
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/(sici)1097-4601(1999)31:10<689::aid-jck2>3.0.co;2-p

✦ Synopsis

Unsaturated 1,6-dicarbonyls like 2,4-hexadienedial are ring opening products in the OH initiated photo-oxidation of aromatic hydrocarbons. In the present study, the photolysis of E,Z-and E,E-2,4-hexadienedial has been investigated under natural sunlight conditions in a large volume outdoor reaction chamber. In the case of the E,Z-isomer, an extremely rapid isomerization into the E,E-form was observed. The photoisomerization frequency, relative to that of NO 2 , was found to be J(E,Z-2,4-hexadienedial)/J(NO 2 ) ϭ (0.148 Ϯ 0.012). A more complex photolysis behavior was observed for E,E-2,4-hexadienedial. Here, a fast equilibrium preceded a comparably slow photolysis. For the equilibrium reaction, relative frequencies of J(E,E-2,4-hexadienedial : EQUI)/J(NO 2 ) ϭ (0.113 Ϯ 0.009) and J(EQUI : E,E-2,4-hexadienedial)/J(NO 2 ) ϭ (0.192 Ϯ 0.016) were obtained, giving an equilibrium constant of K ϭ (0.59 Ϯ 0.07). For the photolysis frequencies, ratios of J(E,E-2,4-hexadienedial : products)/J(NO 2 ) ϭ J(EQUI : products)/J(NO 2 ) ϭ (1.22 Ϯ 0.45) • 10 Ϫ2 were determined. Qualitative aerosol measurements during the experiments showed that the photolysis of 2,4-hexadienedials is a source of secondary organic aerosol.

In addition to the photolysis study, OH radical reaction rate constants were determined, values of (7.4 Ϯ 1.9) • 10 Ϫ11 and (7.6 Ϯ 0.8) • 10 Ϫ11 cm 3 s Ϫ1 were obtained for E,Z-and E,E-2,4hexadienedial, respectively.

The results indicate that the dominant fate of E,Z-2,4-hexadienedial in the atmosphere will be photoisomerization, while for E,E-2,4-hexadienedial, both photolysis and OH radical reaction will be important sinks.