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

Conformational diversity and environmental implications of trans-2-pentenal

This study investigates the conformational intricacies of trans-2-pentenal (trans-2PA), a significant biogenic volatile organic compound. To unveil its potential implications in atmospheric chemistry and environmental pollution, we employ advanced infrared resonant vacuum ultraviolet mass-analysed threshold ionisation spectroscopy. Through this method, we identify the major conformers within trans-2PA, encompassing trans-s-trans (tt-) and trans-s-cis (tc-) structures with planar (cis) and non-planar (gauche) configurations introduced by a methyl group. In a pioneering spectroscopic examination, we analyze trans-2PA in both the neutral and cationic states. This approach allows us to gain a comprehensive understanding of its molecular behavior. Our conformer-specific vibrational spectra not only reveal the relative populations of the main conformers, notably tt-cis and tt-gauche conformers, but also shed light on atmospheric oxidation processes and lower tropospheric organic aerosol formation mechanisms. Our findings expand the understanding of the role of trans-2PA in environmental and biological contexts. Additionally, they contribute to a broader understanding of its influence on air quality, climate, and atmospheric dynamics. The collaboration between advanced experimental techniques and computational methods fortifies the scientific underpinning of this study, opening doors to further exploration in the realms of atmospheric chemistry and environmental science.

Kinetic and Products Study of the Atmospheric Degradation of trans-2-Hexenal with Cl Atoms

The gas-phase reaction between trans-2-hexenal (T2H) and chlorine atoms (Cl) was studied using three complementary experimental setups at atmospheric pressure and room temperature. In this work, we studied the rate constant for the titled oxidation reaction as well as the formation of the gas-phase products and secondary organic aerosols (SOAs). The rate constant of the T2H + Cl reaction was determined using the relative method in a simulation chamber using proton-transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) to monitor the loss of T2H and the reference compound. An average reaction rate constant of (3.17 ± 0.72) × 10^-10 cm³ molecule^-1 s^-1 was obtained. From this, the atmospheric lifetime of T2H due to Cl reaction was estimated to be 9 h for coastal regions. HCl, CO, and butanal were identified as primary products using Fourier transform infrared spectroscopy (FTIR). The molar yield of butanal was (6.4 ± 0.3)%. Formic acid was identified as a secondary product by FTIR. In addition, butanal, 2-chlorohexenal, and 2-hexenoic acid were identified as products by gas chromatography coupled to mass spectrometry but not quantified. A reaction mechanism is proposed based on the observed products. SOA formation was observed by using a fast mobility particle sizer spectrometer. The measured SOA yields reached maximum values of about 38% at high particle mass concentrations. This work exhibits for the first time that T2H can be a source of SOA in coastal atmospheres, where Cl concentrations can be high at dawn, or in industrial areas, such as ceramic industries, where Cl precursors may be present.

Theoretical investigation of the mechanism, kinetics and subsequent degradation products of the NO3 radical initiated oxidation of 4-hydroxy-3-hexanone

The oxidation mechanism of 4-hydroxy-3-hexanone (CH3CH2C(O)CH(OH)CH2CH3) initiated by NO3 radicals in the nighttime is investigated systematically by applying quantum theoretical methods. According to thermodynamic research, the process of H-abstraction on the -CH- group adjacent to the hydroxyl group is the most dominant pathway with the lowest activation energy. The analysis of Mulliken charge charts and molecular electrostatic potential maps illustrate that C-H bonds are the active sites of the reaction, and the calculated C-H bond dissociation energy of the CH3CH2C(O)CH(OH)CH2CH3 molecule further confirms that α-CH is the most easily activated. Individual rate constants for five H-abstraction pathways are calculated by canonical variational theory coupled with small curvature tunneling method over the temperature range of 260-330 K, and the branching ratios are also evaluated. A total rate constant of 1.18 × 10-15 cm3 per molecule per s is obtained at 298 K, which is in good agreement with the reported experimental value. A negative temperature dependence is observed in the titular reaction. The subsequent degradation processes of the advantageous product alkyl radical (CH3CH2C˙(OH)COCH2CH3) are carried out in a NO-rich environment, and propionic acid, NO2 and ozone are obtained as the major final products. The nighttime atmospheric lifetime of 4-hydroxy-3-hexanone is estimated to be around 19 days, indicating that it has impact at night. The titular reaction rate constants are fitted to a three-parameter Arrhenius formula.

Atmospheric chemistry of alkyl iodides: theoretical studies on the mechanisms and kinetics of CH3I/C2H5I + NO3 reactions

Mechanisms and kinetics of the reactions of the NO₃ radical with CH₃I and C₂H₅I have been investigated from a sound theoretical basis.