Measurements of secondary plant metabolites, particularly isoprene and monoterpenes, in a temperate forest

Hyacinth, Anthony (2025). Measurements of secondary plant metabolites, particularly isoprene and monoterpenes, in a temperate forest. University of Birmingham. M.Phil.

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Abstract

The need to monitor and measure more accurately the biogenic contributions to the atmospheric hydrocarbon budget — especially the contribution from forested landscapes — is becoming more imperative since the biogenic hydrocarbon source to the atmosphere has been identified to be even more significant than the anthropogenic. Isoprene (C\(_5\)H\(_8\)) is the most abundant biogenic volatile organic compound (BVOC) in temperate and tropical forest ecosystems. It is also very reactive in the atmosphere, with several potential degradation pathways that produce different oxidation products, including ozone and secondary organic aerosol. Other BVOCs are present in lower quantities in temperate forest air, but can be both photochemically and ecologically active, i.e., contribute to the production of atmospheric oxidants and perform signaling functions between individuals of the same or different species. A holistic approach to monitoring isoprene and other secondary metabolites is therefore highly desirable. This thesis reports results from two measurement campaigns that were carried out between August and September of 2015 and 2016 at Mill Haft forest in central England, the site of the Birmingham Institute of Forest Research (BIFoR) research facility. Measurements used a proton transfer reaction time of flight mass spectrometer (PTR-ToF-MS). The measurements were divided into periods 1 and 2 for2015, and periods 1 to 5 for 2016.The measurements formed part of the baseline characterisation of the Mill Haft forest, which is dominated by mature (170 year-old, 25 m tall) English Oak (Quercus robur). Air sampling for P1 (2015) was at a height of about 2 metres, while P2 was at 15 and 30metres and all of 2016 were at 30 metres, close to the top of the canopy.
Results from 2015 suggest that Isoprene, and its oxidation products MVK/MACR, also detected by PTR-ToF-MS, do not always show a diurnal pattern when their mixing ratios are low. The isoprene concentrations in period 2 (2015, at inlet heights of 15m and 30m), are more representative of isoprene concentrations in forested landscapes — i.e., in temperate regions like Mill Haft — than the concentrations in period 1. The presence of isoprene and its oxidation products; MVK/MACR, was confirmed in the 2016 sample at the canopy height of about 30 metres. The daily mixing ratios and diurnal patterns vary based on prevailing environmental, physical and climatic conditions; but show median and mean values; that agree with results from similar forests in temperate regions. VOC ‘fingerprints’ (i.e., full mass spectra tentatively assigned) of the forest environment using the PTR-MS is shown to be a fast and reliable method for identifying stable and recurring compounds. Overall, the results show that isoprene, MVK/MACR, and other secondary plant metabolites in these baseline samples have been successfully identified using PTR-MS. A more accurate quantification of identified compounds will require additional methods, such as gas chromatography with mass spectrometry (GC-MS). A better interpretation of observed patterns would be possible if more correlative environmental factors were used in the investigation: e.g., relative humidity; photosynthetic active radiation (PAR); wind speed and direction; and ozone (O\(_3\)) and nitrogen oxides (NOx) mixing ratios, where possible.

Type of Work:

Thesis (Masters by Research > M.Phil.)

Award Type:

Masters by Research > M.Phil.

Supervisor(s):