OH-Initiated Degradation of Unsaturated Esters in the Atmosphere: Kinetics in the Temperature Range of 287−313 K

The kinetics of the gas-phase reactions of hydroxyl radicals (OH) with methyl methacrylate (k₁), butyl methacrylate (k₂), butyl acrylate (k₃), and vinyl acetate (k₄) have been investigated for the first time as a function of temperature using the relative technique. The experiments were performed in a 1080 L quartz glass photoreactor over the temperature range (T = 287−313 K) at a total pressure of 760 ± 10 Torr synthetic air using in situ FTIR absorption spectroscopy to monitor the concentration−time behaviors of reactants. OH radicals were produced by the 254 nm photolysis of hydrogen peroxide (H₂O₂). The following Arrhenius expressions (in units of cm³ molecule⁻¹ s⁻¹) adequately describe the measured rate coefficients as a function of temperature: k₁ = (1.97 ± 0.95) × 10⁻¹² exp[(921 ± 52)/T], k₂ = (1.65 ± 1.05) × 10⁻¹¹ exp[(413 ± 34)/T], k₃ = (4.4 ± 2.5) × 10⁻¹³ exp[(1117 ± 105)/T], and k₄ = (4.06 ± 2.02) × 10⁻¹² exp[(540 ± 49)/T]. All of the rate coefficients display a negative temperature dependence and low pre-exponential factor, which supports an addition mechanism for the reactions involving reversible OH-adduct formation. The rate coefficients (in units of cm³ molecule⁻¹ s⁻¹) determined at room temperature (298 K) were as follows: k₁ = (4.30 ± 0.98) × 10⁻¹¹, k₂ = (6.63 ± 1.42) × 10⁻¹¹, k₃ = (2.17 ± 0.48) × 10⁻¹¹, and k₄ = (2.48 ± 0.61) × 10⁻¹¹. The results are compared with previous values of the rate coefficients reported in the literature, which were mainly measured at room temperature. The reactivity of the various unsaturated esters toward the OH radical is discussed in terms of structure activity relationships and parallels are drawn with the OH-radical activities of structurally similar compounds. Using the kinetic parameters determined in this work, residence times of the esters in the atmosphere with respect to their reaction with OH have been determined and are compared with other possible degradation pathways. Possible atmospheric implications of the various degradation pathways studied are discussed.