Abstract
Abstract. The iDirac is a new instrument to measure selected hydrocarbons in the remote atmosphere. 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. The iDirac is a dual-column isothermal oven gas chromatograph with photoionisation detection (GC-PID). The instrument is designed and built in-house. It features a modular design, with the novel use of open-source technology for accurate instrument control. Currently configured to measure biogenic isoprene, the system is suitable for a range of compounds. For isoprene measurements in the field, the instrument precision (relative standard deviation) is ±10 %, with a limit of detection down to 38 pmol mol−1 (or ppt). The instrument was first tested in the field in 2015 during a ground-based campaign, and has since shown itself suitable for deployment in a variety of environments and platforms. This paper describes the instrument design, operation and performance based on laboratory tests in a controlled environment as well as during deployments in forests in Malaysian Borneo and central England.
Highlights
Isoprene (C5H8) is one of the most important non-methane biogenic volatile organic compounds (BVOC) emitted into the atmosphere
The iDirac is a portable gas chromatograph equipped with a photoionisation detector (GC-PID): the VOCs in an air sample are separated on chromatographic columns and sequentially detected by the PID
The general pneumatic design of the instrument is built around two phases in the analysis cycle which are represented schematically in Fig. 2: a loading phase, in which the analyte of interest is pre-concentrated on an adsorbent trap, and an injection phase, in which the analyte is desorbed from the trap and directed into the oven for separation and, eventually, detection
Summary
Isoprene (C5H8) is one of the most important non-methane biogenic volatile organic compounds (BVOC) emitted into the atmosphere. These techniques differentiate between VOCs either by separation (gas chromatography) or by identification of their molecular ions based on mass-tocharge ratios (mass spectrometry) These instruments, while offering high precision and stability, are not built to withstand field conditions for long periods of time due to their need for power, temperature-controlled environments and speciality carrier gases. This is especially true in under-sampled regions of high isoprene emissions, which are typically in remote or challenging environments (e.g. tropical forests). In these locations, instrument size, portability, autonomy, power demand and gas consumption severely limit the length of a deployment. Results on the impact of herbivory on canopy photosynthesis and isoprene emissions in a UK woodland (Visakorpi et al, 2018) and on isoprene concentrations near the Antarctic Peninsula (Nadzir et al, 2019) have already been published
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