The combination of two well-established methods, of quadrocopter-borne air sampling and methane isotopic analyses, is applied to determine the
source process of methane at different altitudes and to study mixing processes. A proof-of-concept study was performed to demonstrate the
capabilities of quadrocopter air sampling for subsequently analysing the methane isotopic composition
Methane's (
Isotopic composition allows different source categories to be distinguished. Biologically produced methane has a typical
The source process of methane and the importance of different natural sources is under discussion for various locations worldwide. Known sources in
northern latitudes are permafrost areas
In the Arctic, inter-annual shifts in the sea ice drift patterns generate an inter-annually patchy methane excess in polar surface water and methane
efflux
The need to improve understanding of the heterogeneous methane sources and the transition from the surface into the atmosphere in the Arctic motivated the development of a flexible airborne sampling system, which provides information on atmospheric stability. In this context, unmanned aerial systems (UASs) fill an observational gap for methane mixing processes. They are able to sample small scales with a typical horizontal distance of 1 km, if they are required to be operated in the line of sight, and they reach the top of the atmospheric boundary layer, with a maximum altitude of typically around 1 km. UAS can be operated in remote areas, requiring less infrastructure in comparison with permanent measurement stations, and they can be used more flexibly than manned aircraft, enabling fast reactions to environmental events like changes of emissions through rain, drought, construction or fire.
The first applications of measuring the methane concentration with UAS have been demonstrated: the air sampling inlet integrated into multirotor systems
is either directly connected to the ground-based methane analyser via a sampling line
The goal of the study is a proof of concept for the experimental set-up of the quadrocopter-borne sampling system, and subsequent laboratory analyses, to identify vertical layers of different isotopic composition.
In order to test the system's capabilities of providing reliable vertical profiles of the isotopic composition, measurements were performed at
a rewetted peatland site, Polder Zarnekow
In the following, the quadrocopter ALICE (airborne tool for methane isotopic composition and polar meteorological experiments) as the carrier system, the payload consisting of the air sampling subsystem and the meteorological sensors, and data acquisition are described. Further, the laboratory air analysis procedures and the measurement site for system tests are introduced. For evaluating the whole measurement chain, a local source of methane of particular isotopic composition and atmospheric conditions that first inhibit and then enforce vertical mixing processes (morning transition) were required. These conditions were met at Polder Zarnekow on 23 May 2018. For confirming the results, the same measurement strategy was applied to the same site on 5 September 2018.
The quadrocopter ALICE was designed as a platform to carry meteorological sensors and 12 glass bottles for air sampling
(Fig.
The quadrocopter ALICE before take-off in Zarnekow on 5 September 2018.
The quadrocopter is constructed with a thrust-to-weight ratio of
As the system was intended for operations in the polar regions, the design point of the system is
Air temperature is recorded with various temperature sensors of different behaviour. Fast fine-wire temperature sensors, manufactured at the Institute
of Flight Guidance
The air sampling system consists of 12 glass flasks (sample containers) of 100 mL volume, which are evacuated before take-off. Their arrangement with
respect to the copter can be seen in Fig.
ALICE vital components. (1) Overall view of the system. (2) Gas sampling payload consisting of 12 evacuated glass flasks which can be filled by opening an electromagnetic valve during the flight. The positions of the two samples taken simultaneously are indicated. (3) Principle of air sampling with manual and electromagnetic valves, showing the configurations of the valves during flight, sampling and transport.
The onboard data are downlinked to a ground station and displayed to the operator. Depending on the atmospheric structure, the operator decides on the altitude of taking samples during the descent, e.g. above/below the temperature inversion, or within altitudes of enhanced humidity, as required for the scientific question. The whole mission can be flown automatically by a Pixhawk autopilot. It is supervised by a safety pilot and a scientific operator who is controlling the system and performance as well as the measurements. Two small cameras, one pointing downwards (GoPro HERO5 Black, 12 megapixels), one looking to the side (GoPro HERO Session Actionkamera, 8 megapixels), were integrated. The captured video of the downward-pointing camera was transmitted to the operator with 720-pixel resolution and 60 Hz. There are different telemetry connections: a 2.4 GHz link is used for the remote control. A 868 MHz connection serves to send commands to the autopilot, and an 868 MHz link is used for activating the safety parachute trigger. Further, a 5.8 GHz video link is established. Scientific data are transmitted via a 433 MHz connection.
In order to quantify the effect of the vertical flow induced by the quadrocopter, numerical simulations were performed with the software ANSYS CFX. The
simulations were transient in nature using a Reynolds-averaged Navier–Stokes (RANS) approach with the shear-stress transport (SST) turbulence model
Simulation of the relative vertical flow velocity (
Assuming that in the worst case the sampling takes place within the downwash of the rotor blades of not more than
A more realistic estimate of the altitude interval uncertainty assumes the vertical velocities directly next to the rotor blade tips (see
Fig.
Following the quadrocopter mission, the sample containers (SCs) were transported to the laboratory at the Alfred Wegener Institute in Bremerhaven,
Germany, for analysis of the isotopic composition. The
The shallow Polder Zarnekow with a water depth of less than 1 m belongs to a large area of rewetted peatlands in the Peene River valley in
north-eastern Germany (53
It is equipped with state-of-the-art eddy covariance (EC) instrumentation recording the wind vector; temperature; and the concentration of water
vapour,
The restoration of the peatland area towards a net sink of the greenhouse gas
For identifying the reason for small-scale inhomogeneities of the atmospheric methane isotopic composition, the methane source located within the
surface water was sampled at different locations on 5 September 2018. Six water samples were taken with Kemmerer glass bottles of 50 mL at the
locations Z-1 to Z-6 indicated in Fig.
Aerial picture of Polder Zarnekow obtained with the quadrocopter ALICE on 5 September 2018. Almost the whole polder dried out after the extremely warm and dry summer 2018. The sites where water samples were taken are indicated with Z1 to Z6. The location of the EC station is indicated with a red circle.
The aerial measurement strategy consists of an automatic climb flight up to 1 km altitude and down again with real-time data transmission of selected parameters at 1 Hz. For the missions presented here, a permission from the nature protection agency and coordination with the German Civil Aviation Agency (DFS, Deutsche Flugsicherung) were required. Flights were permitted between sunrise and sunset. As it takes some hours until the nocturnal temperature inversion is replaced by a well-mixed boundary layer with increasing solar radiation, it was possible to take the air samples during the transition from nocturnal stable boundary layer to the convectively mixed boundary layer.
On 23 May 2018, three consecutive measurement flights of around 10–11 min duration were performed over a time period of 3 h in the morning, with take-off times of 06:04, 07:30 and 08:29 UTC, corresponding to a local time between 08:04 and 10:29 LT. Sunrise was at 04:55 LT.
Each flight consisted of manual take-off and climb up to around 50 m altitude, then handing over the system to the autopilot. The flight pattern
followed three waypoints at 50 m altitude leading to a position directly above the open water fraction of the polder. There, a vertical ascent with
a mean vertical speed of 5
On 5 September 2018, five consecutive measurement flights of around 12–13 min duration were performed over a time period of 5 h in the morning,
with take-off times of 06:04, 07:15, 09:12, 10:05 and 10:57 UTC, corresponding to a local time between 08:04 and 12:57 LT. Sunrise was at 06:23 LT.
The same flight strategy as described above was applied. The ascents reached an altitude of 1000 m, but the sampling altitudes remained the same. The
vertical ascents were done with a mean vertical speed of 6.5
The quadrocopter data are available at the data centre PANGAEA
On 23 May 2018, the synoptic situation was characterized by a pronounced high-pressure system above Scandinavia and the Baltic Sea, leading to
conditions of low wind speed below 5
Diurnal course of the main meteorological parameters air temperature, global radiation, wind speed, wind direction and methane concentration (closed-path Los Gatos sensor) recorded at the meteorological mast at Zarnekow on 23 May 2018. The times of the quadrocopter air sampling are indicated by vertical boxes.
On 5 September 2018, the synoptic situation was determined by two strong high-pressure systems located above the Atlantic Ocean and above northern
Russia, and a low-pressure system above southern Europe. This resulted in low wind speed below 4
In the morning, until 05:30 UTC, around 30 min before the first flight, fog was observed, which was denser towards the east. Starting around
09:00 UTC, before flight 3, shallow convective cumulus clouds were present. This is also evident in the high variability of the global radiation
(Fig.
Diurnal course of the main meteorological parameters air temperature, global radiation, wind speed, wind direction and methane concentration (closed-path Los Gatos sensor) recorded at the meteorological mast at Zarnekow on 5 September 2018. The times of the quadrocopter air sampling are indicated by vertical boxes.
On 23 May 2018, the near-surface temperature increased from around 13–14
Profiles of potential temperature and water vapour mixing ratio obtained on 23 May 2018. The times of the five flights are given in UTC. The horizontal box represents the height interval of the temperature inversion, which is also visible in the large changes of the water vapour mixing ratio.
On 5 September 2018 during the first flight, the profiles show a mixed layer up to 50 m altitude and a stable stratification above
(Fig.
Profiles of potential temperature and water vapour mixing ratio obtained on 5 September 2018. The times of the three flights are given in UTC. The horizontal box represents the height interval of the temperature inversion, which is also visible in the large changes of the water vapour mixing ratio.
For both observation days, the methane concentration recorded at the eddy covariance tower at 2.6 m altitude above the surface shows a high
variability during the night and the morning, and a relatively constant concentration during the day. During the time period of the first flight on
23 May 2018, the methane concentration was still slightly enhanced compared to the background value during the day
(Fig.
The isotopic composition of the air samples was different during the first flight of each day at the altitudes located within the stable
stratification. On 23 May 2018, the delta value was depleted at 10 m and 100 m altitude with a value of
Profiles of
Profiles of
Aerial pictures obtained during the measurement flights for the two case studies show the difference in water level: on 23 May 2018, the lake was
filled with water (Fig.
Aerial picture of Polder Zarnekow obtained with the quadrocopter ALICE on 23 May 2018. The location of the EC station is indicated with a red circle.
The water samples taken on 5 September 2018 within a radius of 100 m revealed highly different
Water samples on 5 September 2018: location and water depth of sampling, colour, concentration and delta value.
Methane flux data indicate that the observation site was a source of methane during both measurement days but had much higher emissions on 23 May
2018 (not shown). During the first flights in the early morning, under stable stratification, an enhanced methane concentration representing local
emissions close to the observation site can be expected
The difference in delta values obtained for air samples near the ground and above the temperature inversion during stable stratification is around
1.5 ‰, thus significantly higher than the uncertainties (flight 1 for each measurement day). This shows that the observed systematic vertical
differences are not caused by measurement uncertainty but are due to local emissions in combination with limited mixing due to stable stratification.
Other methane sources in the surroundings of the polder are presumably biological source processes as well; they may include larger areas of the
rewetted peatland and ruminants, with similar isotopic composition
Assuming that the parts of the surface with high methane concentrations, like samples Z-3, Z-4 and Z-6, act as a methane source, with a delta value of
around
The isotopic composition of the two air samples taken on 5 September 2018 simultaneously but with a constant horizontal distance of 13 cm agree
within 0.1 ‰ at the lowest altitude of 10 m for flights 1 to 4 (Fig.
The delta values are significantly more negative in the morning before vertical mixing starts, as long as a temperature inversion is present (first flight on 23 May 2018 below 150 m, and first flight on 5 September 2018 below 70 m). The difference between delta values below and above the temperature inversion is larger than the uncertainty. This is in agreement with methane from biologic processes emitted from the surface that is not mixed across the inversion.
Two aspects can be highlighted:
The differences in delta values may indicate natural inhomogeneity. However, the very dynamic circulation process around the quadrocopter and
the sampling time of 1.3 s have to be taken into account. Besides the vertical turbulent mixing, the natural spatial inhomogeneity of delta values is not known for the measurement site. Small-scale
horizontal variability can be induced by inhomogeneous sources. Such variability of methane emissions at the field site as well as of the potential
upwind
Altogether, a clear transition in the vertical distribution of the delta values can be seen.
According to the simulations, undisturbed air sampling with the multicopter is possible with a sideways pointing inlet that reaches 25 cm beyond the
rotor or a tube that reaches 50 cm above the rotors for hover and climb. Air sampling during descent experiences more additional disturbance by the
rotor blades and therefore should be compared with air sampling during climb. The initial operation idea was to observe at first the atmospheric
stratification in climb and determine the sampling altitudes for subsequent descent based on the altitude of the temperature inversion. However, the
first simulation results quantify the difference in additional vertical velocities induced by the measurement system, which are much higher during
descent (Fig.
Due to efficiency reasons, the vertical climb speed is higher than the descent speed for the current ALICE system. The impact of the climb speed has to be taken into account for the temporal resolution of the sensors. In order to more closely constrain the altitude interval of the sampled air, measurements during hover or slow climb flight in combination with an inlet tube of the dimensions mentioned above would be preferable for continuous sampling. However, for the presented sampling system with small volume, the air volume contained in the tubes is not exchanged continuously and would further induce uncertainties.
Sampling during slow ascent or hover requires adjusting the battery capacities or the flight mission, e.g. the maximum flight altitude. Further, simulations of the whole multicopter system including the payload are required to quantify the flow field and find the optimal sensor location.
For a systematic comparison with the eddy flux measurements, temporally and spatially integrated measurements would be adequate. However, averaging is
only suitable for sufficiently homogeneous surface conditions and emissions. This is not the case here, as already indicated by the different methane
concentrations and isotopic compositions for the water samples. Therefore, the instantaneous point samples cannot be compared directly with the
classical micrometeorological methods like 30 min averaged EC analyses. For further investigations of small-scale inhomogeneities, new methods for
observations and analyses are required: simultaneous profiles of the methane concentration and isotopic composition upwind and downwind of an EC tower
could be combined with wavelet analyses instead of EC covariance analyses, as suggested by
The measurements serve both as a proof of concept for the system and show the vertical mixing of methane in the atmospheric boundary layer by means of its isotopic composition. With ALICE air samples and subsequent laboratory analyses, it is possible to determine vertical differences in the methane isotopic composition caused by atmospheric stability.
In summary, the first application of ALICE and the analyses of the air samples show potential for improvement for future missions:
Air sampling during climb and hover is much less influenced by rotor-induced turbulence than during descent. Double sampling is highly recommended for system assessment. The punctual air sampling for delta value determination should be complemented with fast and accurate simultaneous onboard methane concentration
measurements. Lightweight instruments with sufficient accuracy and temporal resolution might be operational in the near future.
The differences in delta values of water and air, the differences in delta values between both flight days, and the development during each day
emphasize the highly complex and inhomogeneous nature of methane processes on horizontal scales below 1 km in sediments, at the sediment–water and
the water–atmosphere interfaces. Therefore, a suitable method is required for quantifying small-scale inhomogeneous methane sources. Vertical layering
of air masses with different methane properties strongly depends on atmospheric stability, both concerning concentration as well as the isotopic
composition. A holistic approach is needed to investigate methane processes from sediments to the atmospheric boundary layer, including dedicated
measurements of the isotopic fractionation. Despite some points that can be improved, the first applications of ALICE for air sampling and methane
isotopic analyses show the potential to contribute substantially to investigations of the layering and mixing processes of atmospheric methane of different
sources. The use of the multicopter represents an advantage over air sampling at tall towers, as it is much more flexible and easier to apply.
The data of the flight are available at PANGAEA
AL wrote the paper. FP developed the quadrocopter payload. TK developed the quadrocopter. FP, TK, TR, AL and MA conducted the measurement campaigns. FP and ED performed the laboratory methane isotope analyses. CW and TS performed the methane flux measurements. LL performed the quality check of the copter measurements. DG, DSZ and SB performed the simulations. All authors contributed to and commented on the paper.
The authors declare that they have no conflict of interest.
The authors would like to thank Barbara Altstädter and two anonymous referees for critically reading the manuscript. Further, we would like to thank the associate editor Christof Ammann for his valuable comments.
This research has been supported by the German Research Foundation (DFG) priority programme “Antarctic Research with comparative investigations in Arctic ice areas” (grant no. LA2907/8-1, DA1569/1-2). This open-access publication was funded by Technische Universität Braunschweig.
This paper was edited by Christof Ammann and reviewed by two anonymous referees.