Terpenes and Their Oxidation Products in the French Landes Forest: Insights from Vocus PTR-TOF Measurements
Li, H., Riva, M., Rantala, P., Heikkinen, L., Daellenbach, K., Krechmer, J. E., Flaud, P.-M., Worsnop, D., Kulmala, M., Villenave, E., Perraudin, E., Ehn, M., and Bianchi, F.
Forests are the single largest contributor of organic carbon to Earth’s atmosphere. Terpenes, emitted by trees, are not only important biological signaling molecules and indicators of local ecosystem health, but also participate in chemical reactions in the atmosphere that influence air quality and climate change. Vocus PTR-TOF is particularly well-suited to study these molecules and their reaction products, because it is a field deployable, direct-analysis instrument with time response fast enough to capture rapidly changing environmental processes.
Measuring Atmospheric Oxidation Processes
In July 2018, Li et al. (2020) deployed a Vocus 2R PTR-TOF instrument at a research station in the Landes Forest, France, a rural site near the Atlantic coast. The main goals of the study were to comprehensively detect and identify volatile organic compounds present at the site, and use this information to explore atmospheric oxidation processes (terpene oxidation mechanisms).
Vocus 2R PTR-TOF was selected for this study for the following reasons:
- New, high-sensitivity ion-molecule reactor design
- No dependence of sensitivity on ambient humidity
- High mass resolving power
- Specially designed inlet to limit wall-loss and memory effects of lower-volatility compounds
Because the physical conditions inside the Vocus ion-molecule reaction chamber maintain a linear relationship between PTR kinetic rate constant and sensitivity, the authors were able to estimate the concentrations of many exotic species not present in their calibration cylinder.
An important accomplishment of the study is a thorough comparison of environmental chamber (laboratory) experiments and the ambient measurements. Laboratory experiments provide detailed understanding of chemical mechanisms, but can only be verified by comparison with real-world studies. This study found more oxygenated compounds and more organic nitrates in the ambient environment, compared to the laboratory study. Li et al. were also able to constrain the relative participation of O3, NO3, and OH in oxidation reactions.
Terpenes consist of one or more linked isoprene monomers. Typically, only isoprene (one unit) and monoterpenes (two isoprene unites) are measured at field sites. Occasionally, sesquiterpene (three units) are reported, but these species are difficult to measure because of their low mixing ratios and lower volatility. Surprisingly, Li et al. were able to detect and quantify not only sequiterpenes, but also diterpenes (four isoprene units), which have never before been reported in ambient measurements from forests. The average mixing ratio of diterpenes was 2 parts-per-trillion. Interestingly, sesquiterpene oxidation products were detected, suggesting that reactions involving larger terpenes are a component of atmospheric chemistry in forested regions.
This study contributes to the large body of research that seeks to understand the relationship between plant biology, ecosystem health, and global processes including climate change and carbon cycling. Progress in this field is driven by the new insights into chemistry offered by innovative instrumental techniques such as the Vocus PTR-TOF.