Theoretical Study of the Gas-Phase Reactions of NO₃ Radical with a Series of <i>trans</i>-2-Unsaturated Aldehydes: From Acrolein to <i>trans</i>-2-Octenal

The density functional theory with the BH&HLYP functional has been used in this work to clarify discrepancies found in the literature about the effect of the increasing carbon chain on the reactivity of <i>trans</i>-2-alkenals from acrolein (C₃) to <i>trans</i>-2-octenal (C₈) with nitrate radical. In this work, it was found that (i) the alkyl chain length of the unsaturated aldehydes has little or no influence on the NO₃ reaction rate coefficients (ii) the abstraction of the aldehydic hydrogen from the alkenal is always dominant (83% for <i>trans</i>-2-butanal to <i>trans</i>-2-octenal). The addition channel, which mainly concerns the β addition, has a small influence (17% of the total reaction for the whole series). These results are in good agreement with the experimental studies performed by Zhao et al. in 2011 and by Kerdouci et al. in 2012. All these findings will be useful to complete or improve structure–activity relationships developed to predict the reactivity of NO₃ radicals with organic compounds

An Experimental Study of the Gas-Phase Reactions of NO₃ Radicals with a Series of Unsaturated Aldehydes: <i>trans</i>-2-Hexenal, <i>trans</i>-2-Heptenal, and <i>trans</i>-2-Octenal

Rate constants for the gas-phase reactions of the NO₃ radical with a series of unsaturated aldehydes, <i>trans</i>-2-hexenal, <i>trans</i>-2-heptenal, and <i>trans</i>-2-octenal, have been measured using absolute rate method at 294 ± 3 K and atmospheric pressure. This work was performed to clarify discrepancies found in the literature and thus led to a clearer view of the effect of the increasing carbon chain length on the reactivity of <i>trans</i>-2-alkenals. The rate constants were determined to be (4.7 ± 1.5) × 10^–15, (5.3 ± 1.6) × 10^–15, and (5.6 ± 2.3) × 10^–15 cm³ molecule^–1 s^–1 for <i>trans</i>-2-hexenal, <i>trans</i>-2-heptenal, and <i>trans</i>-2-octenal, respectively. These results clearly indicate that the carbon chain lengthening of the <i>trans</i>-2-alkenals does not significantly affect the rate constant. In addition, the mechanism for the reaction of NO₃ with these unsaturated aldehydes was also investigated. Unsaturated peroxynitrate-type compounds that are exclusively formed through the abstraction channel were observed as the main products

High-NO_<i>x</i> Photooxidation of <i>n</i>‑Dodecane: Temperature Dependence of SOA Formation

The temperature and concentration dependence of secondary organic aerosol (SOA) yields has been investigated for the first time for the photooxidation of <i>n</i>-dodecane (C₁₂H₂₆) in the presence of NO_<i>x</i> in the CESAM chamber (French acronym for “Chamber for Atmospheric Multiphase Experimental Simulation”). Experiments were performed with and without seed aerosol between 283 and 304.5 K. In order to quantify the SOA yields, a new parametrization is proposed to account for organic vapor loss to the chamber walls. Deposition processes were found to impact the aerosol yields by a factor from 1.3 to 1.8 between the lowest and the highest value. As with other photooxidation systems, experiments performed without seed and at low concentration of oxidant showed a lower SOA yield than other seeded experiments. Temperature did not significantly influence SOA formation in this study. This unforeseen behavior indicates that the SOA is dominated by sufficiently low volatility products for which a change in their partitioning due to temperature would not significantly affect the condensed quantities