UAV-based gas monitoring systems for the underpinning of urban, agricultural and industrial emission roadmaps – a methodological approach

Currently sampling of the atmosphere for gas emission measurements involves building towers or hiring airplanes - capital-intensive methods. Easy access to unmanned aerial vehicles (UAV) has opened-up new opportunities for remote gas sampling. The project Iso-2-Drone aims to develop and produce a modular UAV-based gas monitoring system for emission measurements to substitute current technologies. A key feature of the UAV-attached gas sampler design was the ready-to-use nature of the system. This meant that the system was designed to mesh with commonly available equipment, using collection vessels which can be easily and immediately measured by common continuous flow - isotope ratio mass spectrometer (CF-IRMS) instrumentation. The target compounds comprise the three major natural greenhouse gases CH₄, CO₂ and N₂O to be measured at natural isotopic abundance and ambient levels.

We use 20 mL headspace vials for CH₄ and CO₂ sampling. Vials can be conditioned on-sight with our sample preparation prototype using repeatedly evacuating and synthetic air refilling cycles to prevent ambient air contamination. On the UAV-attached sampler atmospheric air is sampled passively by pressure compensation of the vacuum. N₂O is sampled actively via adsorption tubes, filled with Molecular Sieve 5Å and conditioned in the lab. Both a prototype device and two UAV-attached samplers have been designed, built and are currently tested.

The measurement setup in the lab comprises of two autosamplers, a purge & trap system (VSP 4000, IMT Innovative Maschinentechnik GmbH) and a headspace sampler (CTC CombiPal, Chromtech GmbH) in order to switch from ppb range necessary for CH₄ and N₂O to a ppm range for CO₂. For CO₂ measurements the CTC injects 600 µl of sampled air to a Restek Micropacked Column (Shin Carbon ST 100/120, 2m x 1mm ID and 1/16” OD) within a Thermo Scientific Trace GC Ultra heated up from 40°C to 110°C, maintained for 5 min, before heating up to 180°C by 12°C per minute. Thereby CO₂ is properly separated from the potentially interfering N₂O. For CH₄ the residual air sample is cryo-focused at -140°C in a HayeSep D filled trap, transferred to the GC and targeted with a Poraplot Q (30m x 0.32mm) held at 35°C. Using the similar GC method and autosampler N₂O is desorbed after switching the autosampler to thermal desorption mode. All three analytes pass an oxidation/reduction reactor (1030°C) before they are introduced into the IRMS (Thermo Scientific DeltaV Advantage) via a universal gas interface (Thermo Scientific Conflo IV). The IRMS continuously scans the intensity of the mass-to-charge ratios of mass 44, 45, 46 for CH₄ and CO₂ and 28, 29 for N₂0 converted to N₂. δ¹³C and δ¹⁵N are referenced against calibrated laboratory reference gases.

We are currently tuning the methods and testing the prototypes and will present the lasted results and open questions at the conference.