Atmospheric Photochemical Degradation of 1,4-Unsaturated DicarbonylsXIAOYU LIU, HARVEY E. JEFFRIES*, and KENNETH G. SEXTONDepartment of Environmental Science and Engineering,CB7400, Rosenau Hall,The University of North Carolina at Chapel Hill,Chapel Hill, North Carolina 27599-7400*Author to whom correspondence should be addressed. E-mail: [email protected]: 919-966-7312, fax: 919-933-2393IntroductionThe elucidation of details of photochemical reaction mechanisms for the oxidation ofaromatic volatile organic compounds remains a major problem (1). This is because there areapparently a large variety of photooxidation products formed in the reactions. Each of theseproducts further reacts apparently in ways that are different from the better-understood alkaneand simple alkene monofuctional products. The total reacted carbon balance is still poor in thearomatic systems, mostly because of lack of appropriate analytical methods for detecting theproduct mixtures of those photochemical reactions.One of the most important types of products generated in the reaction of monocyclicaromatic hydrocarbons with the hydroxyl radical under tropospheric conditions is 1,4-unsaturated dicarbonyls (2-4). Formation of butenedial from toluene and o-xylene, 4-oxo-2-pentenal from toluene, o- and m-xylene, and 3-hexene-2, 5-dione from p-xylene and 1,2,4-trimethylbenzene have all been experimentally observed and are postulated as ring-cleavagecarbonyl products during the OH-initiated atmospheric degradation of those aromatichydrocarbons (2,5, 6). These species are expected to be very reactive after they are formed in theatmosphere, undergoing reactions with OH and O3 as well as directly photolyzing (2-4,7). Thus,2they may act as important sources of free radicals, promote organic aerosols, and serve asprecursors of carboxylic acids, hydroperoxides, and oxidants such as O3, PAN andperoxycarbocyclic acids (8). Detailed studies of the fates of these highly reactive compounds,however, are lacking. A thorough investigation of atmospheric photooxidation of these reactiveintermediates, which appear to be major secondary products, is required if we are to havepredictive knowledge of the fates of their precursors and clearly understand photochemicalreaction mechanisms for aromatics.A few investigations of the atmospheric chemistry of unsaturated 1,4-dicarbonyls (3, 4,7) have been reported. Using Fourier transform infrared (FTIR) absorption spectrometry, Tuazonet al. (4) investigated the photolysis of cis- and trans-3-hexene-2, 5-dione and reactions of theseisomers with OH and O3 at room temperature. Methylglyoxal and formaldehyde were found tobe the major products of the reaction with O3, but methylglyoxal was not detected as a product ofthe reaction of the 3-hexene-2, 5-dione with OH radicals. They also reported that the rateconstants of the cis and trans isomers of 3-hexene-2, 5-dione were (1.8±0.2)x10-18 and(8.3±1.2)x10-18 cm3molecule-1s-1 for the reaction with O3 at 298±2 K, and (6.3±0.6)x10-11 and(5.3±0.5)x10-11 cm3molecule-1s-1 for the reaction with OH. Another compound of this category,3,4-dihydroxy-3-hexene-2, 5-dione, has recently been examined by Wiesen et al. (7) with long-path FTIR absorption spectroscopy. These authors argued that a hydrated vicinal polyketone,3,3-dihydroxyhexane-2, 4, 5-trione, was probably the major product during the reaction of OHradicals with the above enediol form of acetylformoin. They also conducted a series of reactionsof OH radicals in a 1080-L quartz-glass chamber with butenedial, 4-oxo-2-pentenal, 3-hexene-2,5-dione, among others (3), concluding that the reaction with OH radicals and photolysis aretwo major atmospheric sinks for these species. Rate constants of OH radical reactions (in unit of310-11 cm3molecule-1s-1) are: cis-butenedial, 5.2±0.1, trans-butenedial, ≥2.41±0.79, cis/trans 4-oxo-2-pentenal, 5.58±0.21, cis-3-hexene-2,5-dione, 6.9±2.1, and trans-3-hexene-2,5-dione,4.0±0.4. Rate constants of these compounds with O3 were estimated from structure reactivity. Tothe best of our knowledge, there is no experimental report on rate constants of O3 with butenedialand 4-oxo-2-pentenal in the literature.In this paper, we report a study in the UNC outdoor smog chamber of both OH- andozone-initiated photooxidation of three unsaturated 1,4-dicarbonyl compounds: butenedial, 4-oxo-2-pentenal, and 3-hexene-2, 5-dione. Carbonyl products and intermediates produced fromreactions of these compounds with OH radicals and O3 were measured, as were time series of afew carbonyl products. Rate constants for O3 consumption by these compounds were alsodetermined. Carbonyl products were analyzed by the O- (2,3,4,5,6-pentafluorobenzyl)-hydroxylamine (PFBHA) derivative method coupled with the gas chromatography (GC) /ion trapmass spectrometry (MS) separation and detection. This method has been successful inidentifying multifunctional carbonyls in other studies (9, 10). In daytime outdoor experiments,we began with oxides of nitrogen and butenedial, or 4-oxo-2-pentenal, or 3-hexene-2, 5-dione inthe chamber. In nighttime experiments, ozone was injected at a constant rate into each side of thechamber for the duration of the experiment. One side was filled with an initial amount of thecompound to be studied and about 100-150 ppmV cyclohexane as the OH radical scavenger. Theother side served as a reference for O3 concentration.By comparison with their corresponding standards, four categories of products are foundin the OH and O3 initiated reactions, namely, simple aldehydes, dicarbonyls, unsaturatedcarbonyls, and hydroxy carbonyls. Many of these carbonyls are on the EPA’s hazardous airpollutants (HAPs) list or are known to be toxic. We also observed a few compounds whose4molecular weights were obtained by mass spectra, but whose identities could not unambiguouslybe determined because of lack of standards. Their possible structures are discussed in the text.Rate constants of these 1,4-unsaturated dicarbonyls with O3 were measured. We proposephotooxidation schemes for the three unsaturated 1,4-dicarbonyls.Experimental SectionCarbonyl Reactants Preparation. Butenedial, 4-oxo-2-pentenal, and 3-hexene-2, 5-dione weresynthesized by a collaborating organic chemist from commercially available compounds, such asfuran, 2-methylfuran, and 2,5-dimethylfuran. Details of these synthesis were described elsewhere(11). The purity of these compounds as measured by NMR is greater than 90%.Chamber