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

Unsaturated 1,6-dicarbonyls like 2,4-hexadienedial are ring opening product s in the OH initiated photo-oxidation of aromatic hydrocarbons. In the pres ent study, the photolysis of E,Z- and E,E-2,4-hexadienedial has been invest igated under natural sunlight conditions in a large volume outdoor reaction chamber. In the case of the E,Z-isomer, an extremely rapid isomerization i nto the E,E-form was observed. The photoisomerization frequency. relative t o that of NO2, was found to be J(E,Z-2,4-hexadienedial)/J(NO2) = (0.148 +/- 0.012). A more complex photolysis behavior was observed for E,E-2,4-hexadi enedial. Here, a fast equilibrium preceded a comparably slow photolysis. Fo r the equilibrium reaction, relative frequencies of J(E,E-2,4-hexadienedial --> EQUI)/J(NO2) = (0.113 +/- 0.009) and J(EQUI --> E,E-2,4-hexadienedial) //(NO2) = (0.192 +/- 0.016) were obtained, giving an equilibrium constant o f K = (0.59 +/- 0.07). For the photolysis frequencies, ratios of J(E,E-2,4- hexadienedial --> products)/J(NO2) = J(EQUI --> products)/J(NO2) = (1.22 +/ - 0.45) . 10(-2) were determined. Qualitative aerosol measurements during t he 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 wer e determined, values of (7.4 +/- 1.9) . 10(-11) and (7.6 +/- 0.8) . 10(-11) cm(3) s(-1) were obtained for E,Z- and E,E-2,4-hexadienedial, 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, bo th photolysis and OH radical reaction will be important sinks. (C) 1999 Joh n Wiley & Sons, Inc.