Nocturnal Radical Chemistry of (E)-β-Farnesene in the Atmosphere

Plants that are prone to environmental oxidative and thermal stress exhibit high emission rates of biogenic volatile organic compounds. In this context sesquiterpenes have received an increased interest during the past decade, however, the organic compounds formed from their atmospheric degradation have not been thoroughly investigated, and their contribution to secondary organic aerosol (SOA) remains poorly characterized. Moreover, the nocturnal chemistry of sesquiterpenes has received virtually no attention.

The farnesenes are acyclic sesquiterpenes emitted by plants and agricultural crops and have applications in the biofuel, pharmaceutical, and food industry, and more recently in biotechnology (Sandoval et al., 2014). The (E)-β-farnesene has been studied in reactions with OH radicals and ozone (Kourtchev et al., 2009; 2012) however, the gas-phase chemistry of nitrate radical (NO₃) initiated oxidation of (E)-β-farnesene has not yet been investigated. In particular, the study of nighttime SOA formation from (E)-β-farnesene oxidation is important, because nocturnal chemistry generates preconditions to daytime ozone formation and secondary organic aerosol growth.

Investigations on the gas-phase kinetic and atmospheric chemical degradation of (E)-β-farnesene with NO₃ were performed in the 27 m³ Irish Atmospheric Simulation Chamber IASC (Cork, Ireland) at 295±2 K and 1000±3 mbar of atmospheric pressure. Positive-benzene and negative-iodide mode Chemical Ionization Mass Spectrometry (CIMS) analysis was used to evaluate the chemical composition of the gas mixture. SOA were sampled on PTFE filters (1 µm, 25 mm) and subsequently analyzed via the FIGAERO N₂-thermo-desorption inlet connected to the CIMS instrument. A Scanning Mobility Particle Sizer (SMPS) was employed to measure particle number, size distributions and formation yields.

The reactivity of (E)-β-farnesene toward NO₃ radicals was investigated by a relative rate method with 2-methoxyphenol and 2,5-dimethylfuran as reference compounds. (E)-β-farnesene was found to have a short atmospheric lifetime due to its fast reactions with NO₃ radicals, which efficiently remove it from the atmosphere under nighttime conditions.

The formation of gas-phase organic nitrates and peroxynitrates and highly oxygenated products were identified during the NO₃-initiated oxidation of (E)-β-farnesene. The aerosol composition was also investigated in this study. Compounds observed in both gas and particle phases provide a direct link between gas-phase chemistry and aerosol composition since their volatility decreases through functionalization or accretion reactions. Atmospheric chemical mechanisms for the formation of these oxidation products, both in the gas and particle phase, will complement daytime OH and ozone (E)-β-farnesene oxidation. The degradation of (E)-β-farnesene on the reaction pathways will also be discussed. 

Acknowledgement: This research is part of a Transnational access project that is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRAIA-2020-1, ATMO-ACCESS Grant Agreement number: 101008004. Funding is also provided by Taighde Éireann - Research Ireland (Grant numbers 21/FFP-A/8973 and GOIPD/2025/1260).

References:

Kourtchev, I., Bejan, I., Sodeau, J. R., & Wenger, J. C. (2009) Atmos. Environ., 43, 3182–3190.

Kourtchev, I., Bejan, I., Sodeau, J. R., & Wenger, J. C. (2012) Atmos. Environ. 46, 338–345.

Sandoval, C. M., Ayson, M., Moss, N., Lieu, B., Jackson, P., Gaucher, S. P., Horning, T., Dahl, R. H., Denery,