Kinetics of the Reactions of NO₃ Radical with Methacrylate Esters

Two different experimental methods (relative rate and absolute rate methods) were used to measure the rate coefficients for the reactions of NO₃ radical with six methacrylate esters: methyl methacrylate (MMA, k₁), ethyl methacrylate (EMA, k₂), propyl methacrylate (PMA, k₃), isopropyl methacrylate (IPMA, k₄), butyl methacrylate (BMA, k₅), and isobutyl methacrylate (IBMA, k₆). In the relative rate method, the loss of the esters relative to that of a reference compound was followed in a 7300 L Teflon-walled chamber at 298 ± 2 K and 1000 ± 5 hpa. In the absolute method, the temporal profiles of NO₃ and N₂O₅ were followed by using a dual channel cavity ring-down spectrometer in the presence of an excess of ester in the 7300 L chamber. The rate coefficients from these two methods (weighted averages) in the units of 10^-15 cm³ molecule^-1 s^-1 at 298 K are k₁ = 2.98 ± 0.35, k₂ = 4.67 ± 0.49, k₃ = 5.23 ± 0.60, k₄ = 7.9₁ ± 1.0₀, k₅ = 5.91 ± 0.58, and k₆ = 6.24 ± 0.66. The quoted uncertainties are at the 2σ level and include estimated systematic errors. Unweighted averages are also reported. In addition, the rate coefficient k₇ for the reaction of NO₃ radical with deuterated methyl methacrylate (MMA-d₈) was measured by using the relative rate method to be essentially the same as k₁. The trends in the measured rate coefficient with the length and nature of the alkyl group, along with the equivalence of k₁ and k₇, strongly suggest that the reaction of NO₃ with the methacrylate esters proceeds via addition to the double bond on the methacrylate group. The present results are compared with those from previous studies. Using the measured values of the rate coefficients, along with those for reactions of these esters with OH, O₃, and chlorine atoms, we calculated the atmospheric lifetimes of methacrylate esters. We suggest that NO₃ radicals do contribute to the atmospheric loss of these unsaturated esters, but to a lesser extent than their reactions with OH and O₃.