UAV-based sampling systems to analyse greenhouse gases and volatile organic compounds encompassing compound-specific stable isotope analysis

Abstract

Abstract. The study herein reports on the development and testing of sampling systems (and subsequent analytical setups) that were deployed on an unmanned aerial vehicle (UAV) for the purpose of analysing greenhouse gases (GHGs) and volatile organic compounds (VOCs) in the lower atmospheric boundary layer. Two sampling devices, both of which can be mounted to an UAV with a payload capability greater than 1 kg, were tested for respective sampling and analysis of specific GHGs (carbon dioxide, CO2, and methane, CH4) and VOCs (chlorinated ethenes, CEs). The gas analyses included measurements of the molar amounts and the respective stable carbon isotope ratios. In addition to compound calibration in the laboratory, the functionality of the samplers and the UAV-based sampling was tested in the field. Atmospheric air was either flushed through sorbent tubes for VOC sampling or collected and sampled in glass vials for GHG analysis. The measurement setup for the sorbent tubes achieved analyte mass recovery rates of 63 %–100 % (more favourable for lower chlorinated ethenes), when prepared from gaseous or liquid calibration standards, and reached a precision (2σ) better than 0.7 ‰ for δ13C values in the range of 0.35–4.45 nmol. The UAV-equipped samplers were tested over two field sampling campaigns designed to (1) compare manual and UAV-collected samples taken up a vertical profile at a forest site and (2) identify potential emissions of CO2, CH4 or VOC from a former domestic waste dump. The precision of CO2 measurements from whole air samples was ≤7.3 µmol mol−1 and ≤0.3 ‰ for δ13C values and ≤0.03 µmol mol−1 and ≤0.2 ‰ for CH4 working gas standards. The results of the whole air sample analyses for CO2 and CH4 were sufficiently accurate to detect and localise potential landfill gas emissions from a secured former domestic waste dump using level flight. Vertical CO2 profiles from a forest location showed a causally comprehensive pattern in the molar ratios and stable carbon isotope ratios but also the potential falsification of the positional accuracy of a UAV-assisted air sample due to the influence of the rotor downwash. The results demonstrate that the UAV sampling systems presented here represent a viable tool for atmospheric background monitoring, as well as for evaluating and identifying emission sources. By expanding the part of the lower atmosphere that can be practicably sampled over horizontal and vertical axes, the presented UAV-capable sampling systems, which also allow for compound-specific stable isotope analysis (CSIA), may facilitate an improved understanding of surface–atmosphere fluxes of trace gas.

Stable isotope analysis of atmospheric CO2 using a Gasbench II‐Cold Trap‐IRMS setting

“…The autosampler was equipped with a heating unit (25°C) and a sample tray for 96 sample vials containing 12 mL Exetainer glass vials (Labco Limited, Lampeter, UK) and was only used to manoeuvre the double needle, which directs the sample gas from the sample vial to the GB at ~0.8 mL min −1 . Alternatively, the set‐up could also be used with 20 mL headspace vials with crimped polytetrafluoroethylene (PTFE)‐coated butyl rubber septa, as presented in Leitner et al (2020) 5,6 . At the GB (helium [He] inlet pressure of 80 kPa), the sample gas was first dried by passing through a Nafion™ membrane (purged continuously with a counter current stream of He), was then sent through a 250 μL sample loop and eventually vented from the system.…”

“…Whole air samples were prepared by flushing the Exetainer vials with tank air similar to the setting as that for SA‐flushing, but after the vials had been flushed with SA initially. For whole air sampling of atmospheric air, we refer to the sampling procedure presented in the recent publication of Leitner et al (2020 and 2023) 5,6 …”

“…Alternatively, the set‐up could also be used with 20 mL headspace vials with crimped polytetrafluoroethylene (PTFE)‐coated butyl rubber septa, as presented in Leitner et al (2020). 5 , 6 At the GB (helium [He] inlet pressure of 80 kPa), the sample gas was first dried by passing through a Nafion™ membrane (purged continuously with a counter current stream of He), was then sent through a 250 μL sample loop and eventually vented from the system. At an initial transfer time of 160 s, the sample loop was further flushed for 60 s before switching the eight‐port valve from the ‘Load’ position to the ‘Inject’ position to transfer the sample loop volume to the Cold Trap, which had been immersed in a 3‐L liquid nitrogen (LN 2 ) dewar already 10 s ahead.…”

“…For whole air sampling of atmospheric air, we refer to the sampling procedure presented in the recent publication of Leitner et al (2020 and 2023). 5 , 6 …”