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.