Atmospheric Sampling on Ascension Island Using Multirotor UAVs

Abstract

As part of an NERC-funded project investigating the southern methane anomaly, a team drawn from the Universities of Bristol, Birmingham and Royal Holloway flew small unmanned multirotors from Ascension Island for the purposes of atmospheric sampling. The objective of these flights was to collect air samples from below, within and above a persistent atmospheric feature, the Trade Wind Inversion, in order to characterise methane concentrations and their isotopic composition. These parameters allow the methane in the different air masses to be tied to different source locations, which can be further analysed using back trajectory atmospheric computer modelling. This paper describes the campaigns as a whole including the design of the bespoke eight rotor aircraft and the operational requirements that were needed in order to collect targeted multiple air samples up to 2.5 km above the ground level in under 20 min of flight time. Key features of the system described include real-time feedback of temperature and humidity, as well as system health data. This enabled detailed targeting of the air sampling design to be realised and planned during the flight mission on the downward leg, a capability that is invaluable in the presence of uncertainty in the pre-flight meteorological data. Environmental considerations are also outlined together with the flight plans that were created in order to rapidly fly vertical transects of the atmosphere whilst encountering changing wind conditions. Two sampling campaigns were carried out in September 2014 and July 2015 with over one hundred high altitude sampling missions. Lessons learned are given throughout, including those associated with operating in the testing environment encountered on Ascension Island.

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“…Example of drone‐carried Tedlar bag sampling for methane mole fraction and δ 13 C CH4 (Brownlow et al, ; Greatwood et al, ).…”

“…Specific local enhancements can be detected using GOSAT observations, as shown by Sheng, Jacob, Turner, et al (2018), who used satellite results, together with a gridded Figure 5. Example of drone-carried Tedlar bag sampling for methane mole fraction and δ 13 C CH4 Greatwood et al, 2017). inventory of emissions , to infer trends in North American emissions from different source types (oil and gas, livestock, and wetlands). In an influential study, Kort et al (2014) coupled satellite observation with ground-based validation to identify the largest satellite-detected anomaly in the United States within the Four Corners region in the Southwest United States (particularly coalbed extraction), which they showed to have emissions up to 10% of the total U.S. methane emission inventory.…”

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“…The payload is housed in a Styrofoam package for thermal insulation and to prevent it from damage during landing. Previously the use of a gas pump and Tedlar bags have been 30 successfully used to sample air from a UAV for methane studies (Greatwood et al, 2017 Fig. 1 shows the schematic diagram of the sampler.…”