Atmospheric photochemical degradation of 1,4-unsaturated dicarbonyls

To better understand fates of aromatics hydrocarbon species in the atmosphere, we have investigated the transformation chemistry of butenedial (CHOCH=CHCHO), 4-oxo-2-pentenal (CH3COCH=CHCHO), and 3-hexene-2, 5-dione (CH3COCH=CHCOCH3). These 1,4-unsaturated dicarbonyls are known to be products of aromatic photochemical oxidation. Both hydroxyl radical (OH) and ozone (O-3) initiated smog chamber experiments under atmospheric conditions were conducted in the University of North Carolina outdoor smog chamber. Carbonyl intermediates and products were measured using the O-(2,3,4,5,6-pentafluorobenzyl)hydroxyl a mine derivatization method followed by gas chromatography/ion trap mass spectrometry analysis. Carbonyl products detected and identified by comparison with standards in the OH-initiated photooxidation of butenedial include formaldehyde, acrolein, glycolaldehyde, glyoxal, and malonaldehyde (CHOCH2CHO). For 4-oxo-2-pentenal, the carbonyl products were formaldehyde, methyl vinyl ketone, glycolaldehyde, hydroxyacetone, glyoxal, methylglyoxal, and malonaldehyde. For 3-hexene-2,5-dione the products were formaldehyde, acetaldehyde, acetone, hydroxyacetone, and methylglyoxal. Carbonyl products detected in the O-3-initiated experiments with cyclohexane as the OH scavenger were formaldehyde and glyoxal in butenedial; formaldehyde, glyoxal, methylglyoxal, and malonaldehyde in 4-oxo-2-pentenal; and formaldehyde and methylglyoxal in 3-hexene-2,5-dione. Rate constants of reactions of O-3 with these unsaturated dicarbonyls were computed from O-3 loss in these systems with OH radicals suppression. The rate constant computed for O-3 with butenedial is (1.6 +/- 0.1) x 10(-18) cm(3) molecule(-1) s(-1) at 294-298 K, with 4-oxo-2-pentenal is (4.8 +/- 0.8) x 10(-18) cm(3) molecule(-1) s(-1) at 293-297 K, and with 3-hexene-2,5-dione is (3.6 +/- 0.3) x 10(-18) cm(3) molecule(-1) s(-1) at 295-297 K. Also reported are time series of some carbonyl products. Reaction mechanisms are proposed and discussed in the last part of the paper.