Event Details

A better understanding of the emissions and global distribution of volatile organic compounds (VOCs) is crucial for an accurate characterisation of the composition and oxidising capacity of the atmosphere,especially as global change over the next decades is expected to profoundly affect the drivers of VOC emissions. However a number of key regions on the planet remain under sampled as they are logistically challenging for the long-term deployment of traditional field instrumentation.

The iDirac is a portable gas chromatograph with photo-ionisation detector (GC-PID) designed for long-term measurements of isoprene and selected VOCs in remote and challenging environments. A robust design is central to its specifications, with portability, power efficiency, low gas consumption and autonomy as the other driving factors in the instrument development. Designed and built in-house, the iDirac features a modular design, with novel use of open-source technology for accurate instrument control. For isoprene measurements in the field, the instrument precision (relative standard deviation) is ±11%,with a limit of detection down to 38 pmol mol⁻¹ (or ppt).

We used the iDirac to monitor isoprene concentrations in Wytham Woods (Oxfordshire) throughout the summers of 2018 and 2019. Wytham Woods is Oxford University’s research forest. The pedunculate oak (Quercus robur), one of the main isoprene emitters in mid-latitudes, is abundant in Wytham woods. Using the treetop walkway, isoprene abundances were measured continuously at four heights within, below and above the canopy from May-Oct 2018 with selected ancillary variables. These observations were complemented with occasional leaf gas exchange measurement and whole air samples, and satellite retrievals of normalised difference vegetation index (NDVI) and photochemical reflectance index (PRI). A limited number of measurements including isoprene were made in 2019 (Apr-Sept).

In 2018 there was a long and uninterrupted heatwave in the UK (22/06-08/08/18). We show the expected strong correlation of isoprene with temperature and PAR for most of the summer, with daily peak isoprene concentrations during the heatwave higher than those before or after by up to a factor of 5. However, current models of isoprene emissions could not account for the full enhancement observed during the heatwave-drought period. The lack of precipitation during the heatwave allows an assessment of the effects of moderate drought on isoprene emissions, using NDVI as an indicator of ecosystem health. Constrained canopy models require an adjustment to the current isoprene emission algorithm to reproduce the observations, which suggest more isoprene is emitted in response to stresses induced by low soil moisture.

Preliminary results from 2019(in which a shorter, but more intense, heatwave took place while the soil was fully moist) will be presented, as well as a selection of results from other field deployments of the iDirac.

Valerio Ferracci¹, Conor G. Bolas²,Andrew D. Robinson², Fred Otu-Larbi³,Thomas King³, John E. Beale¹, Toby W. Waine¹,Kirsti Ashworth³, Roderic L.Jones² and Neil R.P. Harris¹

1. Cranfield University, Cranfield, MK43 0AL, UK
2. University of Cambridge, Cambridge, CB2 1EW, UK
3. Lancaster University, Lancaster, LA1 4YQ, United Kingdom

All students, staff and colleagues are encouraged to attend.

Join the conversation #LECSeminar!