Forest Isoprene Emissions: New Insights from a Novel Field Instrument
Isoprene is an important biogenic volatile organic compound (VOC) in natural systems. Its emission by certain species of plants depends on solar radiation levels and on temperature, as well as other factors. Once in the atmosphere, it is rapidly transported and oxidised, and the subsequent chemistry of isoprene plays a significant part in determining the oxidising capacity of the troposphere, and therefore atmospheric composition and aerosol formation. The emission of isoprene is also important to the global carbon cycle and the carbon budget of the biosphere. Measurements have highlighted the great spatial and temporal variation of isoprene concentrations within forests and across different forest types, presenting a challenge for understanding overall emissions. Current understanding of isoprene is limited by the lack of availability of suitable instrumentation for deployment in key field sites. In this thesis the development of a novel portable gas chromatography instrument suitable for challenging field environments, the iDirac, is described. Current estimates for global isoprene emission attribute 70% to tropical forests, but these are under-sampled. This thesis describes a tropical field campaign with measurements of isoprene from tropical trees. The distribution of isoprene within a canopy is poorly understood and existing models do not capture effectively the forest vertical isoprene gradient. The vertical concentration gradient is investigated in a novel measurement campaign over summer 2018 in a temperate forest. Measurements at different canopy heights were taken with maximum isoprene concentrations of 8 ppb. Insolation at the top of the canopy drives the vertical gradient. The forest experienced stress as a result of higher temperatures and low rainfall in the 2018 heatwave and showed elevated isoprene levels. A new simple conceptual model is described, CamCan, which simulates isoprene concentration at different levels in the forest canopy. The model is used to calculate fluxes from the forest and an annual emission of 2.0 Mg is estimated for the 2018 season.