Measurements of the OH plus Octanal and OH, Cl, and Br plus Nonanal Reaction Rate Coefficients and the Nonanal Aldehydic Site Branching Ratio

In this study, rate coefficients, k(T), for the gas-phase OH radical reaction with octanal, k₁(273–370 K), CH₃(CH₂)₆CHO, and nonanal, CH₃(CH₂)₇CHO, k₂(296–370 K), were measured using a pulsed laser photolysis–laser-induced fluorescence absolute method. A relative rate (RR) method, with Fourier transform infrared (FTIR) detection, was used to measure the rate coefficient for the Cl-atom, k₃(298 K), and Br-atom, k₄(298 K), + nonanal reaction, and the Br-atom + pentanal, k₅(298 K) reaction. The obtained room-temperature rate coefficients and Arrhenius expressions are (cm³ molecule^–1 s^–1): k₁(296 K) = (3.33 ± 0.30) × 10^–11, k₁(T) = (1.62 ± 0.25) × 10^–11 exp (+(217 ± 40)/T); k₂(296 K) = (3.84 ± 0.35) × 10^–11, k₂(T) = (1.97 ± 0.25) × 10^–11 exp (+(198 ± 30)/T), where the quoted uncertainties are 2σ and the k(296 K) and A-factor in the Arrhenius fit include estimated systematic errors. The Cl-atom and Br-atom reaction rate coefficients were measured relative to trans-2-butene, propene, and diethyl ether (Cl-atom reaction) and trans-2-butene (Br-atom reactions) to be (cm³ molecule^–1 s^–1): k₃(295 K) = (5.2 ± 0.7) × 10^–10; k₄(295 K) = (1.9 ± 0.3) × 10^–11; k₅(295 K) = (1.1 ± 0.2) × 10^–11, where the quoted uncertainty includes estimated systematic errors. OH radical initiated oxidation experiments performed in the presence of excess NO₂ were used to derive a nonanal aldehydic H-atom abstraction site-specific branching ratio of 66 ± 8% at 295 K. Results from this study are discussed in terms of the OH + n-aldehyde reaction mechanism, reactivity trends of n-aldehydes, and structure activity relationship (SAR) parametrizations to be used in air quality modeling.