Kinetic Studies on the Reactions of Hydroxyl Radicals with a Series of Alkoxy Esters S. M. O’Donnell and H. W. Sidebottom* Chemistry Department, UniVersity College Dublin, Dublin 4, Ireland J. C. Wenger* Chemistry Department, UniVersity College Cork, Cork, Ireland A. Mellouki and G. Le Bras Laboratoire de Combustion et Syste ` mes Re ´ actifs, CNRS, 45071 Orle ´ ans, Cedex 2, France ReceiVed: March 18, 2004; In Final Form: June 6, 2004 Rate coefficients for the gas-phase reactions of hydroxyl radicals with a series of alkoxy esters of structure RC(O)O(CH 2 ) n OR′, where R ) H, CH 3 ,R′ ) CH 3 ,C 2 H 5 , and n ) 1-2, have been determined with use of relative and absolute rate methods. Relative rate measurements were performed in a Teflon reaction chamber at 298 ( 2 K and atmospheric pressure. Absolute rate measurements were made with pulsed laser photolysis- laser induced fluorescence over the temperature range 263-372 K at pressures of ∼100 Torr. The kinetic data are used to derive Arrhenius expressions for the reactions and tropospheric lifetimes for the alkoxy esters. The reactivity of the alkoxy esters is discussed in light of the current understanding of the atmospheric chemistry of oxygenated organic compounds. Introduction Multifunctional oxygenated organic compounds, such as alkoxy esters, are increasingly being employed as water-soluble solvents and fuel additives. Ethoxyethyl acetate, CH 3 C(O)OCH 2 - CH 2 OC 2 H 5 , for example, is used in paints and cleaning solutions. 1 The use of these volatile organic compounds (VOCs) can lead to significant emissions into the atmosphere where they are likely to contribute to the formation of ozone and other secondary pollutants. Another source of alkoxy esters in the atmosphere is the tropospheric degradation of diethers which are also employed as solvents and have considerable potential for use as fuel additives. 2-4 To fully assess the environmental impact of alkoxy esters, a detailed understanding of the kinetics and mechanisms for their atmospheric oxidation is required. Gas-phase reaction with hydroxyl (OH) radicals is the principal, if not dominant, atmospheric fate of the vast majority of saturated oxygenated VOCs. As a result there have been many kinetic studies of the reactions of hydroxyl radicals with monofunctional oxygenated compounds such as ethers, alcohols, ketones, and esters. 4 In contrast only a limited number of studies have been performed on the reactions of multifunctional oxygenated compounds such as alkoxy esters. 1,4-7 The aim of this work was to investigate the kinetics of the reactions of hydroxyl radicals with a series of alkoxy esters of structure RC- (O)O(CH 2 ) n OR′, where R ) H, CH 3 ,R′ ) CH 3 ,C 2 H 5 , and n ) 1, 2. The names and formulas of the alkoxy esters are shown in Figure 1. Rate coefficients have been determined at room temperature, using a conventional relative rate technique, and over the temperature range 263-372 K, using the absolute rate technique of pulsed laser photolysis-laser induced fluorescence (PLP-LIF). The kinetic data are used to derive Arrhenius expressions for the reactions and compared to the values obtained for other oxygenated hydrocarbons. The atmospheric implications of the results are also discussed. Experimental Section Relative rate studies were conducted in a Teflon reaction chamber at University College Dublin. Absolute rate measure- ments were made with use of the PLP-LIF technique at LCSR- CNRS Orle ´ans. Both experimental systems have been described in previous papers from these laboratories 8,9 and are only briefly outlined here. Relative Rate Measurements. Reactions were carried out at 298 ( 2 K and atmospheric pressure in a collapsible 50 L FEP (fluorine-ethene-propene) Teflon reaction chamber. The chamber was surrounded by 10 Philips TUV 15 W germicidal lamps which provided a source of irradiation at 254 nm for OH radical generation from the photolysis of O 3 in the presence of water vapor. The substrate and reference compounds were introduced into the reactor by flushing measured amounts from calibrated Pyrex bulbs with a stream of high-purity synthetic air (Air Products). The reactor was then filled to around 80% capacity with synthetic air and the remaining volume filled with synthetic air containing 1-5% v/v ozone, produced by passing synthetic air through an ozone generator. Water (triply distilled, 0.5 mL) was injected directly into the chamber. The initial concentrations of ozone and water vapor were 50-100 and 1000-10000 ppm, respectively (1 ppm ) 2.46 × 10 13 molecule cm -3 at 298 K and 1 atm of pressure). The initial concentrations of substrate and reference compounds were in the range 15- 95 ppm. The reactions were initiated by switching on the lamps to photolyze ozone. The gas mixtures were sampled at several stages during the reactions and analyzed by using gas chroma- tography with flame ionization detection (Shimadzu 8A). Chromatographic separation was achieved by using a 15 m * To whom correspondence should be addressed. J.C.W.: e-mail j.wenger@ucc.ie; fax +353 21 4903014; phone: +353 21 4902454. H.W.S.: e-mail howard.sidebottom@ucd.ie; fax +353 1 7062127; phone +353 1 7062293. 7386 J. Phys. Chem. A 2004, 108, 7386-7392 10.1021/jp048782w CCC: $27.50 © 2004 American Chemical Society Published on Web 08/13/2004