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

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.

Kinetic and product studies of the reactions of NO3 with a series of unsaturated organic compounds

Rate coefficients for the reaction of NO₃ radicals with 6 unsaturated volatile organic compounds (VOCs) in a 7300 L simulation chamber at ambient temperature and pressure have been determined by the relative rate method. The resulting rate coefficients were determined for isoprene, 2-carene, 3-carene, methyl vinyl ketone (MVK), methacrolein (MACR) and crotonaldehyde (CA), as (6.6 ± 0.8) × 10^-13, (1.8 ± 0.6) × 10^-11, (8.7 ± 0.5) × 10^-12, (1.24 ± 1.04) × 10^-16, (3.3 ± 0.9) × 10^-15 and (5.7 ± 1.2) × 10^-15 cm³/(molecule•sec), respectively. The experiments indicate that NO₃ radical reactions with all the studied unsaturated VOCs proceed through addition to the olefinic bond, however, it indicates that the introduction of a carbonyl group into unsaturated VOCs can deactivate the neighboring olefinic bond towards reaction with the NO₃ radical, which is to be expected since the presence of these electron-withdrawing substituents will reduce the electron density in the π orbitals of the alkenes, and will therefore reduce the rate coefficient of these electrophilic addition reactions. In addition, we investigated the product formation from the reactions of 2-carene and 3-carene with the NO₃ radical. Qualitative identification of an epoxide (C₁₀H₁₆OH⁺), caronaldehyde (C₁₀H₁₆O₂H⁺) and nitrooxy-ketone (C₁₀H₁₆O₄NH⁺) was achieved using a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) and a reaction mechanism is proposed.

Mechanism and Product Distribution of the O3-Initiated Degradation of (E)-2-Heptenal, (E)-2-Octenal, and (E)-2-Nonenal

The O₃-molecule initiated degradation of three 2-alkenals (E)-2-heptenal, (E)-2-octenal, and (E)-2-nonenal has been investigated in a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air using in situ FTIR spectroscopy to monitor the reactants and products. The experiments were performed in the absence of an OH scavenger. The molar yields of the primary products formed were glyoxal (49 ± 4) % and pentanal (34 ± 3) % from the reaction of (E)-2-heptenal with O₃, glyoxal (41 ± 3) % and hexanal (39 ± 3) % from the reaction of (E)-2-octenal with O₃, and glyoxal (45 ± 3) % and heptanal (46 ± 3) % from the reaction of (E)-2-nonenal with O₃. The residual bands in the infrared product spectra for each of the studied reactions are attributed to 2-oxoaldehyde compounds. Based on the observed products, a general mechanism for the ozonolysis reaction of long chain unsaturated aldehydes is proposed, and the results are compared with the available literature data.

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO₃) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO₃-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO₃-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO₃ radical, the difficulty of characterizing the spatial distributions of BVOC and NO₃ within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO₃-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO₃-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

Ozonolysis of a series of C7–C9 unsaturated biogenic aldehydes: reactivity study at atmospheric pressure

Rate coefficients for the reactions of ozone with the biogenic aldehydes trans-2-heptenal, trans-2-octenal and trans-2-nonenal have been determined in an environmental chamber at 298 K in 990 mbar air using in situ FTIR spectroscopy to monitor the reactants.

Gas-surface reactions of nitrate radicals with vinyl-terminated self-assembled monolayers

Interfacial reactions between gas-phase nitrate radicals, a key nighttime atmospheric oxidant, and a model unsaturated organic surface have been investigated to determine the reaction kinetics and probable reaction mechanism.