Study of the tropospheric degradation of 2-methylbutanal initiated by OH radicals, Cl atoms and sunlight

The biogenic oxygenated volatile compound 2-methylbutanal (2MB) is emitted into the low atmosphere from several natural sources such as microbiological processes, wildland fires, or emissions from vegetation¹. Moreover, some industrial operations also generate 2MB². During the day, the oxidation of 2MB can be initiated by sunlight, hydroxyl (OH) radicals or chlorine (Cl) atoms in marine atmospheres. Up to date, gas-phase kinetics of OH with 2MB has only been studied at room temperature³. The photolysis rate coefficients (J) of 2MB initiated by sunlight have also been reported⁴. However, there is no available data for the reaction of Cl atoms with 2MB and the photolysis products.

In this work, the photolysis rate coefficient (J) of 2MB has been measured using a solar simulator in a Pyrex cell coupled to a Fourier Transform Infrared (FTIR) spectrometer to monitor the loss of 2MB. Moreover, the gas-phase kinetics of the reaction of 2MB with Cl (k_Cl) and OH (k_OH) have been investigated to evaluate the contribution of these homogeneous degradation routes to the total loss of 2MB in the atmosphere. All the kinetic experiments were carried out under free-NO_x conditions (simulating a clean atmosphere). Regarding the relative kinetic study on the Cl-reaction, an atmospheric simulation chamber coupled to a FTIR spectrometer was used at 298 K and 760 Torr ⁵ of air, whereas for the absolute kinetics of the OH-reaction, k_OH was determined as a function of temperature and pressure (T = 263-353 K and P = 50-600 Torr of helium) by using a pulsed laser photolysis-laser induced fluorescence system⁶. Finally, in addition to FTIR, gas chromatography coupled to mass spectrometry and proton transfer time-of-flight mass spectrometry were used to detect the gas-phase reaction products when 2MB was exposed to Cl and sunlight. The atmospheric implications will be discussed in terms of lifetimes and reactions products.

REFERENCES: 1. Szwajkowska-Michale, L., Busko, M., Lakomy, P., and Perkowski, J.: Determination of profiles of volatile metabolites produced by Trametes versicolor isolates antagonistic towards Armillaria spp. Sylwan. 2018, 162, 499–508. 2. Kolar, P.; Kastner, J. R. Low-Temperature Catalytic Oxidation of Aldehyde Mixtures Using Wood Fly Ash: Kinetics, Mechanism, and Effect of Ozone. Chemosphere. 2010, 78 (9), 1110–1115. 3. D’Anna, B.; Andresen, O.; Gefen, Z. and Nielsen, C.J.: Kinetic study of OH and NO₃ radical reactions with 14 aliphatic aldehydes. Phys.Chem.Chem.Phys. 2001, 3, 3057-3063. 4. Wenger, J.C.: Chamber Studies on the Photolysis of Aldehydes. Environmental Simulation Chambers: Application to Atmospheric Chemical Processes. 2006. Nato Science Series: IV: Earth and Environmental Science, vol 62. Springer, Dordrecht. 5. Antiñolo, M.; Asensio, M.; Albadalejo, J. and Jiménez E.: Gas-Phase Reaction of trans-2-methyl-2-butenal with Cl: Kinetics, Gaseous Products, and SOA Formation. Atmosphere 2020, 11 (7), 715. 6. Blázquez, S.; Antiñolo, M.; Nielsen, O. J.; Albadalejo, J. and Jiménez, E.: Reaction kinetics of (CF₃)₂CFCN with OH radicals as a function of temperature (278-358 K): A good replacement for greenhouse SF₆? Chem.Phys.Lett. 2017, 687, 297-302.