Environ. Sci. Technol. 1998, 32, 3515-3521
Atmospheric Degradation of Glycidaldehyde: Photolysis and Reaction with OH Radicals S. MA, I. BARNES,* AND K. H. BECKER Physikalische Chemie, Fachbereich 9, Bergische Universita¨t, Gaussstrasse 20, D-42097 Wuppertal, Germany
Epoxide aldehydes have recently been detected among the oxidation products of aromatic hydrocarbons. Many epoxides are toxic and very little is known about their atmospheric fate. The products and kinetics of the atmospheric oxidation, OH radical reaction, and photolysis of glycidaldehyde have been investigated in a large volume reactor at 298 K using in situ long-path FT-IR spectroscopy for the analysis. A rate coefficient of k ) (1.69 ( 0.04) × 10-11 cm3 molecule-1 s-1 has been determined for the reaction of glycidaldehyde with the OH radical using the relative kinetic technique. The UV absorption spectrum of glycidaldehyde was measured in the range 220380 nm from which upper limits of its photolysis frequencies in the troposphere have been deduced, e.g., J (hν) ∼ 1.0 × 10-4 s-1 (for July 1, noon, and 50° N). The OH radical initiated photooxidation of glycidaldehyde yields CO, CO2, formic acid, formic acid anhydride, formaldehyde, and hydroperoxymethyl formate as major products. A reaction mechanism is postulated to account for the product formation.
Introduction Glycidaldehyde is the simplest epoxide aldehyde and has been classified as an animal carcinogen substance based on long-term in vitro animal carcinogenicity studies and inhalation studies in humans (1). Recently glycidaldehyde along with other epoxides has been identified by means of GC-MS as products in the OH radical initiated degradation of benzene and toluene (2). Although the yields were not determined and may not be particularly high, their importance in atmospheric chemistry arises principally from their high toxicity. The atmospheric chemistry of epoxides has received very little attention and to our knowledge nothing is presently known about the atmospheric fate of glycidaldehyde. In this study glycidaldehyde has been synthesized and the kinetics and products of its OH radical initiated degradation under atmospheric conditions investigated. The UV spectrum of glycidaldehyde has also been recorded in an attempt to access the relative importance of its loss via reaction with OH radicals compared to photolysis.
Experimental Section Synthesis of Glycidaldehyde. Glycidaldehyde was prepared on the basis of a method described in the literature (3) through aqueous oxidation of acrolein by H2O2 under strictly controlled pH conditions (8.0-8.5) at a temperature between 25 and 30 °C: 10.1021/es9804159 CCC: $15.00 Published on Web 09/18/1998
1998 American Chemical Society
The method has been simplified on two points. First, the reaction mixture was lyophilized overnight to reduce the amount of water instead of using a circulating evaporator. Second, the product was extracted in dichloromethane, and the organic phase was distilled under reduced pressure. This variation replaced the extraction with warm cyclohexanone and the time-consuming rectification on an Oldershaw column. The NMR and infrared spectra showed that the product was essentially pure. A GC-MS determination of the purity was unsuccessful. The density of glycidaldehyde was determined at room temperatue by weighing different volumes of the substance in a calibrated injection syringe and making a linear regression. The measurements gave a value of d20°C ) (1.07 ( 0.05) g cm-3. OH-Kinetic and Product Studies. The experiments were performed in a 1080-L quartz chamber surrounded with 32 low-pressure mercury vapor lamps (Philips TUV/40W; Λmax ) 254 nm) and 32 fluorescent lamps (Philips TL05/40W; Λmax ) 360 nm). The reactor was equipped with a multiple reflection mirror system (White system) interfaced to a BRUKER IFS88 FTIR-spectrometer (total optical path length 579 m). A relative kinetic technique was applied to determine the OH reaction rate constant at 1000 mbar total pressure of synthetic air at 298 ( 2 K. Ethene with a rate coefficent of kOH ) 8.5 × 10-12 cm3 molecule-1 s-1(4) was used as the reference substance. Details of this method can be found elsewhere (5). The method depends on reaction with OH radicals being the only loss process for the reactants: k1
OH + glycidaldehyde 98 products k2
OH + ethene 98 products The initial concentrations of glycidaldehyde and ethene were in the range (3-6) × 1013 molecules cm-3 and the photolysis of ∼1014 molecules cm-3 of H2O2 with the UV lamps was employed as the OH radical source:
H2O2 + hν (λ ) 254 nm) f 2OH Mixtures of glycidaldehyde/ethene/H2O2/air were photolyzed for approximately 30 min, and the degradation of the compounds was monitored in situ using FTIR spectrometry with a resolution of 1 cm-1. Experiments were carried out to test for possible contributions to the measured glycidaldehyde decay rate under the experimental conditions employed through wall loss and photolysis, such losses for ethene are known to be negligible. The wall loss of glycidaldehyde was very low (
