Measurements of the OH + Octanal and OH, Cl, and Br + 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.