Introduction
Since the 18th-century industrial revolution, greenhouse gas (GHG) mole
fractions have been increasing due to anthropogenic activity. Rapid increases
in carbon dioxide (CO2) and methane (CH4) have occurred since the
1950s, contributing to global climate change .
Understanding and quantifying both natural and anthropogenic fluxes of the
two major GHGs, namely CO2 and CH4, is vital to predict future mole
fraction levels and to help monitor the effectiveness of the emissions
reduction efforts.
Both CO2 and CH4 are naturally occurring greenhouses gases in our
atmosphere, with CO2 the more abundant of the two. Today, natural
production of CO2 happens mainly through decay of organic matter and
respiration by aerobic organisms. Besides the natural sources of atmospheric
CO2, there are additional anthropogenic contributions to the total
atmospheric CO2 mole fractions, mainly from burning of fossil fuels. In
recent years the mole fractions of atmospheric CO2 have been increasing by
∼ 2 ppm (parts per million) per year .
Methane has a shorter lifetime in the atmosphere than that of CO2,
but CH4 is more efficient at trapping radiation. The comparative impact of
CH4 on climate change is 20–30 times greater than that of CO2 over a
100-year period . Methane is naturally
produced and emitted to the atmosphere when organic matter decomposes in
low-oxygen environments, and natural sources include wetlands, swamps, marshes,
termites, and oceans. From 2007 to 2016, the increase of the global methane
mole fractions has been ∼ 7 ppb (parts per billion) per year
. The main contributors to anthropogenic methane
emissions are leakages from coal mining and the oil and gas industry,
ruminant animals, rice agriculture, waste management, and biomass burning
. The quantification of CH4 emissions is
highly important in studying the global methane cycle where vertical
profiling with high resolution provides further information on the
contributions from CH4 sources and sinks .
In 2010, the National Oceanic and Atmospheric Administration (NOAA) developed
the first AirCore, an innovative atmospheric air sampling system
from an idea originally developed and patented by
Pieter Tans . The AirCore consists of long, thin-wall stainless-steel
tubing capable of sampling and preserving atmospheric profile information.
The AirCore is evacuated as it is lifted up to a high altitude
(∼ 30 km) by a balloon, and then during descent after the balloon
bursts it is passively filled with atmospheric air samples due to the
increasing ambient pressure. The samples are analyzed on the ground to
retrieve the GHG vertical profiles. The length and diameter of the tubes and
the time it takes from sampling until analysis ultimately determine the
vertical resolution. Since the first development of the AirCore
, additional augmentations of the AirCore has been
developed and tested. This includes smaller and lighter AirCores developed at
Goethe University Frankfurt and the University of Groningen
, and a high-resolution (HR) AirCore developed at
École Polytechnique, Université Paris-Saclay . Other
applications using the AirCore technique include measurements of
δ13CH4 and C2H6 / CH4 ratios, using the
AirCore to store a rapidly acquired sample and analyze the sample at a lower
flow rate while maintaining the sample integrity .
In recent years, the use of unmanned aerial vehicles (UAVs) has become a new
complementary platform for GHG measurements. Previous studies include the
investigation of temporal and spatial variations of atmospheric CO2 using
a unique CO2 measurement device attached to a small UAV (kite plane)
; atmospheric monitoring of point source fossil fuel CO2
and CH4 from a gas treatment plant using a Helikite
; CO2, CH4, and H2O measurements on board the National Aeronautics and
Space Administration (NASA) Sensor Integrated Environmental Remote Research
Aircraft (SIERRA) UAV ; a small atmospheric sensor
measuring CO2, CH4, and H2O attached to a robotic helicopter
; the quantification of CH4 mole fractions and isotopic
compositions from heights up to 2700 m on Ascension Island using a remotely
piloted octocopter ; and a dedicated CO2
analyzer, COmpact Carbon dioxide analyzer for Airborne Platforms (COCAP),
capable of being flown onboard small UAVs .
For this study, we combine the flexibility and mobility of UAVs, and the
AirCore's ability to capture and preserve the spatial resolution of
atmospheric air samples to design and develop an alternative AirCore version,
named active AirCore. Instead of passively sampling air due to the changing
ambient pressure during flight, the active AirCore pulls atmospheric air
samples through the tube at a certain flow rate using a micropump. This
allows for a highly mobile system that can obtain both vertical and
horizontal profiles with a high spatial resolution. Unlike the original
AirCore and the newer versions that are all made to sample the atmospheric column
including the stratosphere, the active AirCore has been designed to fulfill a
different purpose and does not aim to reach a height well above the
troposphere like its predecessors. The active AirCore provides a powerful
tool to fill the vertical gap of GHG measurements between the surface and the
lowest altitude usually reachable by aircraft. The flexibility and mobility of the system makes it possible to
make GHG observations at locations where tall-tower measurements are not
readily available.
With the capability of sampling horizontal transects, the active AirCore can
help quantify CO2 and CH4 emissions from local areas such as wetlands,
landfills, and other CH4 hot spots, and quantify point sources emissions
such as power plant plumes. It can also provide highly accurate and
precise measurements to be used for validation of measurements of remote-sensing techniques.
The instrument design is presented in the Method section, together with the
experimental setup and the data processing method. The Results section
presents the measurements made by the active AirCore during five flights in a
day. Section 4 discusses the horizontal and vertical resolution. Section 5
presents the conclusions.
Method
The active AirCore, designed to fly with a lightweight UAV (total weight
below 4 kg), consists of ∼ 50 m thin-wall stainless-steel tubing, a
dryer, a micropump, and a data logger. It is placed in a carbon fiber box and
attached to the UAV using two carbon fiber rods. Prior to every flight, the
active AirCore is flushed with a calibrated fill gas that is spiked with
∼ 10 ppm CO, which helps to identify the starting point of ambient air
sampling during later analysis. The active AirCore starts to collect air
samples when the micropump is turned on using a switch located outside the
box shortly before a UAV flight, and the pump is turned off after the UAV
lands. Air samples are collected during the flight and retained within the
active AirCore. The active AirCore samples are then immediately analyzed with
a trace gas analyzer (CRDS, Picarro, Inc., CA, USA, model G2401).
Active AirCore
The dimensions of the active AirCore, along with some key parameters, are
given in Table .
As the thin-wall tubing is very fragile, we have used custom-made
stainless-steel connectors to reinforce the connection with the coiled tube
and Swagelok fittings at both ends. These connectors have an inner diameter
(ID) of 3.275 mm on one end and an outer diameter (OD) of 3.175 mm on the
other. The 3.175 mm ID of the connector is inserted onto the thin-walled
AirCore tubing and fastened using glue for ceramics, leaving the 3.175 mm OD
side open and usable by Swagelok fittings. To obtain a constant flow through
the AirCore, an orifice (OD 1/4 in., orifice diameter 45 ± 10 % µm,
Lenox laser Inc.) is placed between the pump and the coiled tube. The
upstream pressure of the orifice is close to ambient, or more accurately the
ambient pressure minus a small pressure drop across the whole coiled tube,
while the downstream pressure of the orifice is mainly determined by the
pumping capacity and was measured at 380 hPa with the pump (KNF micropump,
model 020L). Thus, the flow across the coiled tube is expected to be critical
as long as the upstream pressure is above ∼ 760 hPa (2 × 380 hPa), or
below ∼ 2.4 km above the sea level, and was measured to be 21.5 sccm
(standard cubic centimeters per minute) in the laboratory. The pressure
between the orifice and the pump is constantly monitored through a
stainless-steel Swagelok tee junction (Honeywell TruStability High-accuracy Silicon Ceramic (HSC) Series). The pump
and the tee junction are connected via flexible fluorinated ethylene
propylene (Tygon) tubing (1/8 in. ID). This same type of tubing is also
connected to the outlet of the pump and leads to a hole on the side of the
AirCore box, venting the pump exhaust outside of the box. Air samples are
dried with a 7.5 cm long stainless-steel tube (1/4 in. OD) filled with
magnesium perchlorate before they are sampled into the coiled tube. The inlet
of the active AirCore system is placed at the bottom of the carbon fiber
AirCore box and is attached through a hole to the dryer tube with a small
piece of flexible 1/4 in. ID nylon tubing.
(a) Schematic design of the UAV AirCore system.
(b) Image of the UAV AirCore system.
The dimensions and key parameters of the active AirCore.
Length
49.1 m
Tubing
304-grade stainless steel
Outer diameter (OD)
3.175 mm (1/8 in.)
Wall thickness
0.127 mm (0.005 in.)
Coating
SilcoNert 1000, by Restek Inc.
AirCore tubing weight
431 g
AirCore volume
358 mL
Total payload weight
1131 g
Vertical spatial resolution
24.1 to 27.5 m/24.7 to 29.3 m
CO2 / CH4 (1.5 m s-1)
Horizontal spatial resolution
40.3 to 46.0 m/41.2 to 48.9 m
CO2 / CH4 (2.5 m s-1)
The AirCore box itself is made from 0.5 mm thick carbon fiber plate with a
density of 1600 kg m-3, providing a sturdy and lightweight box to house the
active AirCore system. The AirCore box has a length of 34 cm, a width of 19.5 cm, and a height of 12 cm. The total weight of the active AirCore system,
including the AirCore box, is 1.1 kg. Figure a shows a
schematic design of the active AirCore system, while Fig. b shows a photo of the prototype product.
The data logger is made using an Arduino MEGA 2650 board that records
meteorological data via two pressure sensors, five temperature sensors, a
relative humidity sensor, and a GPS (Global Positioning System) receiver. The
pressure sensors are silicon pressure sensors of the model Honeywell
TruStability HSC. One pressure sensor monitors the pressure between the pump
and the orifice, while the other measures the outside ambient pressure
through a flexible nylon tube going through the bottom of the box. These
sensors have an accuracy of ±0.25 % in the range of 67–1034 hPa (1–15 psi). The relative humidity is a model DHT22, which measures in the range
of 0–100 % with an uncertainty of 2.5 %. The temperature sensor embedded
in the relative humidity sensor can measure in a range of -40 to 125 ∘C with an uncertainty of 0.5 ∘C. During the day of this
study, however, we did not have relative humidity measurements, due to the
sensor being placed inside the enclosed AirCore box. This has been resolved
in the latest version of the active AirCore system, where the relative
humidity sensor is now placed underneath the AirCore box. The external
temperature sensors are all PT100 elements from Innovative Sensor Technology
and have an uncertainty of 0.15 ∘C. The GPS coordinates and time are
measured using a GPS model ATM2.5 NEO-6M module with EEPROM built-in
activity.
The data logger is powered by one 9 V battery, while the micropump is powered
by 12 V, four 3 V batteries connected in series. The micropump was
controlled via an on–off switch mounted on the outside of the carbon fiber
box for easy use before takeoff.
The trace gas analyzer
All mole fraction analyses of air samples from the active AirCore are
conducted using a cavity ring-down spectrometer (CRDS, Picarro, Inc., CA,
USA, model G2401) , situated close to the landing site of the
UAV. The cavity of the analyzer is strictly maintained at a pressure of
∼ 186 hPa (140 Torr) and a temperature of 45 ∘C to achieve a
precision (1σ, 0.5 Hz) better than 0.03 ppm for CO2, 0.5 ppb for
CH4, and 7 ppb for CO, based on cylinder measurements before and after
analysis of the AirCore. We control the sample flow of the analyzer operating
in the inlet valve control mode at a constant rate using a needle valve
between the analyzer and the vacuum pump. We set the flow rate during all the
analyses of active AirCore samples at ∼ 20.5 sccm. The flow rate was
monitored using an Alicat MB-100SCCM-D/5M flowmeter located at the exhaust of
the pump and was noted down at the beginning of the analysis and assumed
constant throughout the analysis of the AirCore. After each analysis, the
analyzer is switched to measure fill gas through the active AirCore at a
higher flow rate of ∼ 120 sccm by fully opening the needle valve. In
this way, we are able to shorten the time interval between one flight to the next
to 50 min.
Schematic of the roof air test setup in the laboratory. The blue
lines indicate the time at which both the Picarro and the AirCore sample the
roof air, while the red lines indicate the time at which the Picarro analyzes
the sampled air from the AirCore.
Measured mole fractions of CO2, CH4, and CO for
both the direct roof air measurement and the AirCore sampled air.
Laboratory tests
Prior to the flights, we validated the active AirCore measurements in
laboratory experiments against in situ mole fraction measurements of
CO2, CH4, and CO using a CRDS analyzer.
Figure shows a schematic of the experimental
setup. During the experiments, the CRDS analyzer and the active AirCore were
set up to sample the roof air through the same inlet via a tee junction. The
roof air was partially dried, having a water vapor content of
∼ 0.1 %. The water vapor effects were corrected based on
and for CO2 and CH4, and
for CO to obtain dry-mole fractions of CO2, CH4,
and CO, respectively. Both the analyzer and the AirCore were flushed with dry
cylinder air prior to the start of the test, until the measured water vapor
level was below 0.005 %. Once the active AirCore was fully sampled, the
micropump was turned off and a shutoff valve was switched to close the
inlet. This was followed by the analysis of the active AirCore samples using
the same CRDS analyzer. A three-way valve at the end of the active AirCore
was also turned so that the sample was chased by dry cylinder air with known
mole fractions. The flow rate through the CRDS analyzer during analysis was
19.2 sccm, while the air samples were collected into the active AirCore at a
flow rate of 21.5 sccm. Once the test was complete, the active AirCore data
were processed as described in Sect. . Three
experiments were performed to verify the consistency of the results, and we
observed a strong correlation between the direct CRDS analyzer measurements
and the sampled active AirCore mole fraction values. The R2 values were
0.99, 0.97, and 0.97, with the mean differences of 0.04 ± 0.21 ppm,
0.58 ± 0.67 ppb, and 0.86 ± 27.37 ppb for CO2, CH4,
and CO, respectively. Figure shows the time series of one of
the experiments; the mole fractions during the three tests ranged from 394 to
417 ppm for CO2, 2009 to 2120 ppb for CH4, and 118 to 1657 ppb
for CO. The large standard deviation in CO is due to a sharp spike of several
hundred parts per billion during three experiments, as seen in Fig. c.
During the roof air tests, the data logger tracked the inside pressure,
outside pressure, and the temperature of the AirCore, which are the essential
parameters that go into the processing. From Fig. a and c, a
small time lag between the AirCore measurement and the direct measurement can
be seen. This is believed to be due to water vapor effects, as the air was
not fully dried. Figure b also shows a small CH4 spike
around 14:39 UTC. This is likely due to
metal-to-metal friction, generated by touching the stainless-steel tubing
during analysis.
The UAV
The active AirCore system has been flown aboard a small quadcopter UAV (model
DJI Inspire 1 Pro). The UAV (including battery and propellers) weighs ∼ 2.9 kg, has a maximum flight time of approximate 15 min, and is capable
of flying at wind speeds up to 10 m s-1. With zero wind, the UAV is capable of
ascending with a speed up to 5 m s-1 and descending with a speed up to 4 m s-1, and
it has a maximum horizontal speed of up to 22 m s-1. When carrying the active
AirCore as payload, the UAV system weighed ∼ 4 kg and was able to make a
∼ 12 min flight. The payload was attached to the bottom of the UAV,
so that the inlet was facing downwards towards the ground, using two 10 mm carbon fiber rods that were fixed to the UAV using zip ties and duct tape.
A schematic of the analysis setup.
The analysis box
We constructed an analysis box to simplify the
analysis of the air samples from the active AirCore and to reduce the
potential contamination of the sample from non-sampled air. A schematic of
the analysis box is shown in Fig. . Two female
Swagelok quick connectors (QC series) for the reference and the fill gas are
placed on the left side of the box. One of the two cylinders is selected via
a Fluid Automation Systems solenoid valve (model CH-1290) by the software of
the Picarro CRDS analyzer. A Swagelok metering valve (model SS-SS2) and an
excess flow path are situated between the solenoid and the six-port Vici
rotary valve (model EUDB-26UWE). The metering valve is used to restrict the
total airflow that is set slightly larger than the flow rate through the CRDS
analyzer, with the rest venting through the excess flow path. The rotary
valve provides two positions, namely position A (analysis) and position B
(bypass). The position is controlled via buttons outside the analysis box.
Two 1/8 in. Swagelok bulkhead connectors are fixed to the middle of the box
where the active AirCore is connected. On the right side of the analysis box
is the outlet, which is connected directly to the CRDS analyzer.
The analysis of CO2 (a), CH4 (b),
CO (c), and H2O (d) for the second flight on
13 September 2016. The red and green dots indicate the start and end point of
the sample, respectively.
Data processing
One of the major advantages of the UAV-based active AirCore is that, in
contrast to a free balloon-based AirCore, the UAV normally lands next to the
operator. This allows for immediate analysis of the air samples after landing
and thus minimizes the spatial resolution degradation due to molecular
diffusion of air samples in the tube. During flight, the CRDS analyzer
runs a reference gas through a bypass path so that once the active AirCore
is connected the analysis can begin immediately. Switching from bypass to
analysis makes the reference gas “push” the active AirCore sample, while the
analyzer drags the sample with a constant flow rate of 20.5 sccm. The sample
is in fact analyzed in reverse, with the first measured mole fractions linked
to the landing of the UAV. The spiked CO mole fractions are seen towards the
end of the analysis until finally the reference gas mole fractions are seen
on the analyzer. This leads to a well-defined sample between the two cylinder
gas mole fraction values, seen as a “plug” between the reference gas mole
fraction values. Since the active AirCore is open on both ends, a small
contamination from water vapor and ambient air is seen at the ends of each
sample. Table shows the mole fractions
for the reference and fill gas, calibrated with respect to the WMO 2007,
2004A, and 2004A scales for CO2, CH4, and CO, respectively.
During the processing of the data the measured mole fraction values are
corrected for water vapor as stated in Sect. . A
pre-determined calibration curve is applied to the measured dry-mole
fractions to correct for drift in the linear calibration curve, and finally
the mole fractions are corrected with a single bias between the measured and
calibrated values of the reference gas. Figure shows the
analysis of CO2 (a), CH4 (b), CO (c), and H2O (d) for the
second flight made on 13 September 2016. The green and red dots indicate the
start and the end point of the sample, respectively. The start point was
selected as three-fourths of the way into the water vapor increase, where the analysis goes
from dried cylinder air to AirCore, while the end point was selected as the
last point before the mole fractions goes above 2000 ppb CO, a little into
the CO-spiked fill gas. These points were empirically determined from the
fifth flight, where the maximum correlation between the active AirCore and
the 60 m continuous measurements was found. These points were consistently
selected for all the flights.
The calibrated mole fraction values of the reference and fill gas.
CO2 [ppm]
CH4 [ppb]
CO [ppb]
Reference gas
390.8 ± 0.1
2010.9 ± 0.9
156 ± 1
Fill gas
411.4 ± 0.1
2027.7 ± 1.3
9376 ± 23
The air entering the tube will quickly equilibrate with the mean active
AirCore temperature. The pump creates a low pressure of ∼ 380 hPa at the
downstream end of the active AirCore, which is more than 2 times lower than
the ambient upstream pressure, forming a critical flow through the orifice.
The length and the diameter of the active AirCore remain constant, and thus
the only parameters that influence the sampling flow rate are the ambient
pressure and the temperature of the AirCore and the orifice. Based on this
and the ideal gas law, we estimate the number of moles of air (Δn)
that flows into the active AirCore within a time step Δt at any given
time as the sum of the change of the number of moles of total air in the
active AirCore and the number of moles of air flowing out of the AirCore:
Δnt=VRΔPtTt-PtΔTtT2t+Pt⋅Δt⋅ftRTt,
where Δn is the number of moles of air sampled into the active
AirCore, P is the ambient pressure, V is the total volume of the active
AirCore, R is the universal gas constant, T is the temperature of the active
AirCore, t is the time, and f is the volumetric flow rate given by
ft=Cd⋅ARTM,
where Cd is the dimensionless discharge coefficient that can be
empirically determined, A is the area of the orifice, R is the universal
gas constant, T is the temperature of the orifice in kelvin, and M is the
molar mass of air in kilograms per mole. During the analysis of the air samples by the
CRDS analyzer, the flow is set at a constant rate. Therefore, the number of
moles of air analyzed within a time step Δt′ at any given time t′
can be expressed as
Δn′t=P′f′t′Δt′RT′,
where f′ is the analysis flow rate, and P′ and T′ are the ambient pressure
and temperature in the laboratory, respectively. The number of moles of air
samples that entered into the active AirCore during flight and the equal
number of moles of air samples analyzed by the CRDS analyzer are used to
establish the link between the time it took to collect the sample and the
time it took to analyze it.
The analysis of CO2 (a), CH4 (b), and
CO (c) for the second flight on 13 September 2016 with its original
analysis time, the data-logger-linked time series, and the shifted
data-logger-linked time series of the analysis. The red and the green dots
represent the time when the drone took off and landed, respectively.
Using Eqs. (), (),
and (), an approximated flight-linked analysis
time can be obtained, having effectively linked the number of moles going
into the sample with the analysis time. The measured mole fractions can then be
directly linked to the time series of the data logger. Figure shows the CRDS analyzer analysis with the original
analysis time vs. the flight-linked analysis time.
Atmospheric station
The atmospheric measurement station Lutjewad was established in the year 2000
by the Centre for Isotope Research (CIO), University of Groningen, and an
aerial photograph is shown in Fig. . The
station is located 30 km from the city of Groningen, is easily accessible
via roads, and is located on the northern coast of the Netherlands
(6.3529∘ E, 53.4037∘ N, 1 m a.s.l.) situated roughly 50 m behind the
Wadden Sea dike. In analyzing wind direction data for the years 2006 to 2014,
it was found that the station received 16 % of the time northerlies (315–45∘ sector), 34 % southerlies (135–225∘ sector), 22 %
easterlies, and 28 % westerlies. Hence, about half of the time the station
receives relatively polluted continental air masses. On the seaside,
sporadically flooded salt marshes next to the dike pass into the Wadden Sea
with its tidal flats. It stretches about 6 km to the north, where
the island Schiermonnikoog marks the transition to the North Sea. The
observatory itself is surrounded by low shrubs and grass. The rural landscape
to the south consists mainly of pasture and cropland with patches of forested
land. The livestock in the area is dominated by dairy cows and sheep. The
nearest large town is the city of Groningen (200 000 inhabitants) at a
distance of about 30 km in the east-southeast (ESE) direction. The annual frequency of ESE
winds, which could carry pollution from the city directly, is usually less
than 1 % .
Some of the common characteristics for the five different flights.
Flight no. 1
Flight no. 2
Flight no. 3
Flight no. 4
Flight no. 5
Flight duration
00:12:00
00:10:49
00:10:27
00:10:57
00:11:00
Takeoff time
05:15:59 UTC
06:17:00 UTC
07:17:16 UTC
08:21:48 UTC
09:18:00 UTC
Landing time
05:27:59 UTC
06:27:49 UTC
07:27:43 UTC
08:32:51 UTC
09:29:00 UTC
Time between flights
–
00:49:00
00:49:27
00:54:05
00:45:09
Takeoff location
6.3523∘ E
6.3523∘ E,
6.3519∘ E,
6.3518∘ E,
6.3525∘ E,
53.4039∘ N
53.4039∘ N,
53.4038∘ N
53.4041∘ N,
53.4039∘ N,
2.3 m a.s.l
2.3 m a.s.l
6.1 m a.s.l
2.3 m a.s.l
2.3 m a.s.l
Landing location
6.3523∘ E
6.3521∘ E,
6.3519∘ E,
6.3518∘ E,
6.3520∘ E,
53.4039∘ N
53.4039∘ N,
53.4038∘ N
53.4041∘ N,
53.4038∘ N,
2.3 m a.s.l
2.3 m a.s.l
6.1 m a.s.l
2.3 m a.s.l
2.3 m a.s.l
Google Maps image of the atmospheric station Lutjewad and its
surroundings.
CO2 and CH4 were continuously monitored at 60 m a.g.l. via humid-air
analysis from a Picarro CRDS system model 2301, while measurements of CO2,
CH4, and CO at 7 m were similarly measured using a Picarro CRDS system
model 2401. The Picarro CRDS measurements at the 7 m inlet were started a
week prior to this campaign. The atmospheric station maintains continuous
temperature, relative humidity, and atmospheric pressure measurements at 7,
40, and 60 m. At 7 and 40 m the wind speed is also measured, and at 60 m,
the wind speed and wind direction. However, during the day of this study the
wind speed and wind direction measurements at 60 m malfunctioned and were
not recorded.
Results
Flight trajectories
All flights conducted for this study were performed on 13 September
2016. The first three flights aimed to obtain vertical profile measurements
of CO2, CH4, and CO. Information regarding the flight duration, time
between flights, takeoff location, landing location, and mean speeds can be
found in Table . The first flight took
place at 06:15 UTC. The sunrise occurred at 06:05 UTC. The UAV ascended
up to 210 m and hovered at this altitude for 45 s before ascending up
to 500 m. The UAV hovered at this altitude for 20 s before descending
back down to the landing zone. During the second flight, the UAV ascended up
to an altitude of 300 m and, upon reaching this altitude, immediately started
its descent towards an altitude of 60 m. Once this altitude was reached, it
ascended back up to 180 m before starting its final descent towards the
landing zone. The third flight trajectory was similar to the first flight,
ascending from the takeoff zone up to 500 m at a steady pace before
descending back down to the landing zone. The data logger malfunctioned during
this flight, causing the micro SD card to appear empty upon retrieval. This
led to no stored temperature, relative humidity, or pressure readings during
this flight. For the processing of this flight, ambient pressure readings
from the first flight were used to approximate similar altitude pressures.
The temperature profile from the first flight was used as the measured active
AirCore temperature but adjusted according to measured temperature profiles
from the atmospheric station. The time series from the UAV flight log was
used together with noted-down times of when the pump was running to link with
the analysis time. The GPS coordinates and altitude were also obtained from
the UAV log.
The area between the northern dike and the coastal sea is covered with
wetlands, and flight no. 4 measured the CO2 and CH4
enhancement by flying from the dike to the sea. The takeoff zone was located
on the dike, having an elevation of 6.1 m. The UAV started at the takeoff
zone and ascended to an altitude of 22 m before flying horizontally over the
wetlands towards the sea (northwestern direction). The horizontal speed was
averaging at 12 m s-1 for this leg of the flight. Once the UAV reached
the sea, it descended to an altitude of 10 m and flew along the coastline
(southwestern direction) at an average speed of 4 m s-1. Right before
the UAV reached a critical battery level beyond the point of no return, it
changed its direction and headed back towards the landing zone, cruising at
an average speed of 5 m s-1 at an altitude of 10 m. At the landing
site, the UAV hovered for 2 min before landing.
The continuous CO2 (a), CH4 (b), and
CO (c) measurements from the atmospheric tower at 7 m (black) and
60 m (red), along with the mole fractions measured with the active AirCore
(blue). The highlighted areas indicate the time span for each of the flights,
approximately spaced 1 h apart. The altitude covered during the flights
was 485, 301, 478, and 23 m for flight nos. 1, 2, 3, and 4,
respectively, transecting both the 7 and 60 m altitudes. Flight no. 5 hovered
at 60 m close to the 60 m tower inlet.
The fifth and final flight was a verification flight for the active AirCore
system. The UAV hovered close to the 60 m tower inlet at the atmospheric
station, sampling with the active AirCore while air at the 60 m inlet was
pumped down to be analyzed by a CRDS analyzer in the ground station.
Ascending to an altitude of 60 m, the UAV positioned itself next to the tower
and hovered for 9 min before starting its descent towards the landing
zone.
The meteorological data measured at the atmospheric station during
13 September 2016. Panel (a) shows the relative humidity,
(b) the temperature, and (c) the wind speed. The black curve
indicates measurements at 7 m, the blue curve at 40 m, and the red curve at
60 m. The highlighted areas indicate the times of the five flights.
Tower measurements
Figure a, b, and c show the continuous measurements
of CO2, CH4, and CO, respectively, on the full day of 13 September
2016. The 7 m inlet measurements are indicated with the black
curves, while the 60 m inlet measurements are indicated by the red curves.
The vertical shaded lines represent the time interval of each of the five
flights, and the blue curves indicate the sampled mole fractions during each
flight. As shown in Fig. a and b, the CO2 and
CH4 mole fractions deviated strongly from each other at the times of the
first and second flights. During the third flight the 7 and 60 m
measurements were almost identical, indicating that the boundary layer below
60 m was well mixed. At the time of the third, fourth, and fifth flight, a
clear well-mixed boundary layer had formed. The third flight took place at
07:17:16 UTC, which was 09:17:16 LT.
Vertical profiles of (a) CO2, (b) CH4,
and (c) CO for flight nos. 1–3. Panels (a)
and (b) include a dotted line indicating 60 m and shows measured
trace gas mole fractions from the Lutjewad atmospheric station at this
height. Panel (b) also includes a dotted line to indicate 7 m height
and the corresponding CH4 values obtained from the atmospheric station
at this height. The square points represent the mole fractions measured at
the time of the UAV ascent, and the circular points represent the mole
fractions measured during the UAV descent. The color of the markers
represents its respective flight. The CO mole fractions shown in
panel (c) have been averaged by every fifth data point. The ambient
temperature and relative humidity are not shown due to the sensors only being
placed inside the box, as discussed in Sect. .
As mentioned in Sect. , the atmospheric
station maintains continuous measurements of temperature, relative humidity
and wind speed at 7, 40, and 60 m. The time series during 13 September
2016 are shown in Fig. .
The vertical profiles of CO2, CH4, and CO
Figure a, b, and c show the measured mole
fractions of CO2, CH4, and CO against altitude for the first three
flights, respectively. Flight no. 1 is indicated by the red curve, the second
flight the green curve, and the third flight the blue curve. The solid lines
indicate the ascending profiles, while the dotted lines indicate the
descending profiles. The figures also show the measured tower mole fractions
at 60 and 7 m at the same time the drone was at these altitudes. Tower
measurements for CO2 are shown at 60 m in Fig. a, tower measurements for CH4 at both 60 m
and 7 m are shown in Fig. b, and tower
measurements of CO at 7 m are shown in Fig. c.
The vertical CO2 profiles seen in Fig. a
show how CO2 mole fractions change throughout the morning hours. The
vertical mixing of the boundary layer can be seen from the temporal change of
CO2 mole fractions that decrease at ground level from flight nos. 1 to 2,
and further from flight nos. 2 to 3, coupled with a simultaneous growth of the
CO2 mole fractions between the flights at 60 m. This mole fraction growth
at 60 m is also reflected in the CH4 profiles shown in
b. However, a decrease in CH4 between flight nos. 1 and 2 is not observed at ground level, which suggests an enhancement of
methane has taken place between flight nos. 1 and 2. The enhancement in CH4
between flight nos. 1 and 2 is confirmed by the observed CH4 mole fractions
at 7 and 60 m from the Lutjewad tower (Fig. b). The
enhancement is 470 and 450 ppb for CH4 at 7 and 60 m, respectively.
These suggest a strong local surface source, likely from ruminants and
wetlands from the land surrounding the Lutjewad area. As seen in Fig. , a strong decoupling between 7 and 40 m CO2 and
CH4 until about 08:00 UTC+1 indicated a very shallow nocturnal boundary
layer responsible for the high near-ground mole fractions associated with the
local emission sources.
The differences between the measured active AirCore profiles and the
trace gas mole fractions measured at the atmospheric station at 60 and 7 m.
An average mole fraction from the AirCore profile between 50 and 70 m is
compared to an average mole fraction of the 60 m tower measurements within
the same time frame. Similarly, the average mole fraction from the AirCore
profile between 0 and 20 m is compared to average a mole fraction of the 7 m
tower measurements within the same time span.
50–70 m
0–20 m
Trajectory
Horizontal distance
ΔCO2 [ppm]
ΔCH4 [ppb]
ΔCH4 [ppb]
between UAV and
tower
Flight no. 1
Ascending
44 m
12.869 ± 4.446
40.1 ± 28.8
19.5 ± 29.2
Descending
6.162 ± 3.969
-13.2 ± 33.7
57.7 ± 48.9
Flight no. 2
Ascending
43 m
-7.930 ± 7.544
-75.4 ± 79.4
-46.8 ± 12.4
Mid-point
-5.826 ± 3.896
-87.1 ± 63.0
–
Descending
-0.076 ± 9.559
-10.5 ± 30.3
-37.3 ± 35.1
Flight no. 3
Ascending
45 m
-0.223 ± 1.565
-20.0 ± 25.6
-1.4 ± 45.6
Descending
0.146 ± 2.761
13.6 ± 19.3
-20.0 ± 5.9
Above 200 m, the mole fractions of both CO2 and CH4 are nearly
constant, with the exception of the CO2 profile of flight no. 1. This
suggests a stable boundary layer with a height of 200 m. However, we do not
have a good explanation for the observed large variability of CO2 seen in
the descending profile of flight no. 1. Compared to CO2 and CH4, there is
less variability in the mole fractions of CO, as seen in Fig. c. The enhancement in CO in the stable boundary
layer relative to the CO aloft is seen for all the three profiles.
Validation against the atmospheric station measurements
Figure a, b, and c also include the
measured atmospheric station mole fractions of CO2 and CH4 at 60 m, and
CH4 at 7 m. The square markers indicate that the mole fractions was
measured during the time the UAV was ascending, and the round markers
indicate mole fractions measured during descent. The differences between the
flight profiles and the tower measurements can be seen in Table , where an average mole fraction from
50 to 70 m has been compared to the average mole fraction from the 60 m
inlet during the same time frame. Similarly, the average 7 m mole fractions
within the given time frame were compared to AirCore mole fractions between 0 and 20 m.
The variability of the flights
As seen in Fig. a, the behavior of the first
flight with respect to the mole fractions of CO2 did not follow
expectations, nor did it have the same features as seen in the consecutive
flights, and the features that are observed for CO2 also do not occur in
the CH4 or CO profiles. The correlation between CO2 and CH4 for
flight nos. 2 and 3 is strong, with R2 values of 0.99 for both flights,
while the correlation for the first flight yields an R2 of 0.58. This low
correlation could be due to CO2 emissions from a nearby power plant. The
Eemshaven coal power plant is located 34 km east of Lutjewad and has a stack
of 120 m. If the winds were not steady before sunrise, CO2 emissions from
the power plant may have dispersed to influence our flight profile, seen as
the features in the Fig. a.
The AirCore analysis of the fifth flight and the continuous tower
measurements from 60 m. The plot shows the analysis profiles and the
correlation between these two measurements from both CO2 and CH4.
The differences in CO2 and CH4 between the two measurements are
also shown.
Both the descending and ascending mole fraction profiles during all the
flights compare well with the continuous measurements of CO2, CH4, and
CO at 60 and 7 m. From Table , it
is seen that the best fit between data and atmospheric tower data occurred
during the third flight. A possible explanation for this could be the smaller
variability of mole fractions within the boundary layer. The drop in the
measured mole fractions at higher altitudes with each successive flight
indicates that the boundary layer is transitioning from its nocturnal state
to a mixed boundary layer. This is expected as the sun rises
.
Verification of the active AirCore
Figure a and d show the measured CO2 and
CH4 mole fractions from the fifth flight together with the measured mole
fractions from the 60 m inlet at the time of flight. Figure b and e show the correlation between the measured
flight mole fractions and the 60 m inlet measurements for CO2 and CH4,
respectively. Figure c and f show the mole
fraction difference between the flight analysis and the 60 m inlet
measurements for CO2 and CH4, respectively.
As seen in Fig. a, the measured flight sample and
the 60 m inlet measurements are in very good agreement throughout the time of
the flight. The first 2 min of the flight measure slightly higher
CO2 mole fractions than the continuous tower measurements, averaging 0.5 ppm above. An offset of the same size is also seen towards the end of the
flight. Figure c shows the difference throughout the
flight, having a mean difference of 0.14 ± 0.36 ppm between the active
AirCore and the 60 m tower inlet. Although the trend is similar, sharp peaks
and troughs have been smoothed in the active AirCore compared to the tower
measurements. There is a strong correlation between the active AirCore
analysis and the 60 m tower inlet measurements. This correlation is seen in
Fig. b and yields an R2 of 0.97 for CO2.
As shown in Fig. d, the CH4 analysis from the
active AirCore and the 60 m inlet measurements follow the same trend.
However, there is a consistent offset where the 60 m tower measurements
measure higher mole fractions of CH4. The difference throughout the flight
is shown in Fig. f, having a mean difference of
-5.6 ± 3.9 ppb between the active AirCore and the 60 m tower inlet. The same
smoothed curve as seen in Fig. a is also seen in
Fig. d. The sharp peaks and troughs measured by
the atmospheric station have been smoothed in the active AirCore. A strong
correlation is seen between the CH4 measurements of the active AirCore and
the 60 m inlet analysis, and is shown in Fig. e.
The R2 is 0.95 for the CH4 measurements. Figure c
also shows a slight downward trend in the difference. This can be
explained by contamination of the AirCore sample at both ends, where the end
has been contaminated by a high mole fraction CO2 spike at one end, likely
due to human breath while disconnecting the AirCore and preparing for the
flight, and the other side by the reference gas, which held a lower
concentration of CO2 than the sampled air.
The measured mole fractions of CH4 and CO2 during the
fourth flight. Takeoff for the flight was on the dike, flying out towards
the sea, doing a 90∘ turn, and flying along the coast before heading
back to the takeoff spot. The white/yellow and red/black colors indicate
high and low mole fraction enhancement, respectively.
Methane enhancement from wetlands
Figure a and b show the measured CH4
and CO2 enhancement relative to the background mole fractions measured at
the atmospheric station during the fourth flight, respectively. The
white/yellow color indicates a high enhancement of its respective trace gas,
while the black/red color indicates a low enhancement. The flight took place
over the wetlands, north of the Wadden Sea dike. The wind was from the
southeast with a wind speed of 2.5–3.0 m s-1, which provided upwind
measurements of CO2 and CH4 at the atmospheric station with respect to
the flight. During the time of flight, the upwind measurements had a mean
mole fraction of 2647 ppb with a standard deviation of 24 ppb for CH4, and
460.0 ppm with a standard deviation of 1.6 for CO2. The CH4 mole
fractions were obtained from the 7 m inlet at the atmospheric tower, while
the 60 m inlet provided the CO2 mole fractions due to the low sampling
frequency of CO2 at 7 m. The mean altitude of the UAV during the flight
was 10.4 m. The mean upwind mole fractions were subtracted from the mole
fractions measured during the flight, providing the enhancement seen over the
wetlands for each respective trace gas.
As seen in Fig. a and b, a clear hotspot
for both CO2 and CH4 is seen towards the most northern part of the
wetlands. The enhancement of CO2 was at its peak 4.3 ppm over the
background upwind measurements, and 85 ppb for CH4, with a ratio
ΔCH4 / ΔCO2 of 19.8 ppb/ppm, which suggests that the
emissions are from the local wetlands . The mean enhancement
during the course of the flight was 1.2 ppm for CO2 and 22.5 ppb for
CH4. The hotspot seen in Fig. a and b
were measured as the UAV was close to the coast. As mentioned previously, the
wind was from the southeast, further supporting that the measured hotspot
originated in the wetlands.
Spatial resolution
The spatial resolution has four contributors, namely smear effects of the
analyzer, molecular diffusion, Taylor dispersion, and an innate uncertainty
in the GPS measurements. Each contribution is discussed below.
Analyzer smearing effects
The cell of the analyzer also plays a role in the effective spatial
resolution, in that it applies an additional smearing effect during the
analysis. The sample flow rate through the CRDS analyzer is kept at a
constant flow rate of 21.5 sccm. The volume of the analyzer cavity is 35 cc
but is maintained at 140 Torr (187 hPa) and 45 ∘C, which makes the
effective cavity volume roughly 5.5 cc (at STP).
We use the response time (1/e exchange) to calculate the contribution of
the smearing effect to the total spatial resolution and have determined it to be
15.3 s of the flight time. Considering the smearing effect alone, the
spatial resolution of the active AirCore measurements is determined to be 23.0 m
with a mean ascent or descent speed of 1.5 m s-1, or 38.3 m with a mean speed
of 2.5 m s-1.
GPS uncertainties
While the UAV is at a standstill, the uncertainty of the GPS is given as 0.5 m in the vertical direction and 2.5 m in the horizontal direction.
Diffusion and Taylor dispersion
Molecular diffusion and Taylor dispersion that affects the profiled sample
can be expressed with an effective diffusion coefficient, assuming that the
flow is laminar through the active AirCore during sampling and analysis
. The effective diffusion is expressed as
Deff=D+a2⋅v‾248⋅D,
where D is the molecular diffusivity of the different molecules in the gas
(D is 0.16 cm2 s-1 for CO2 and 0.23 cm2 s-1 for
CH4; ), a is the inner radius of the active AirCore tubing, and
v‾ is the average velocity of the air inside the active AirCore. The
distance of diffusion Xrms is then given as
Xrms=2⋅2⋅Deff⋅t,
where t represents the storage time from the moment the UAV lands and the
analysis is complete. The factor 2 in front of the square root comes from
diffusion in both directions. The effective resolution in the horizontal and
vertical direction can then be expressed in terms of a fraction of distance
traveled in space:
Δddiff=Xrms⋅Af⋅v′,
where Δddiff is the effective resolution due to diffusion and
dispersion, f is the mass flow rate of the CRDS analyzer, A is the area of
the tube, and v′ is the speed of the UAV. Due to the difference in
molecular diffusion for CO2 and CH4, the spatial resolution differs
between the GHGs. When the UAV is flying at an average speed of 1.5 m s-1,
the uncertainties range from 7.6 to 15.2 m for CO2 depending on the
storage time, while for CH4 the uncertainty ranges from 9.1 to 18.2 m
depending on the storage time. Storage time ranges from 10 to 40 min.
Effective spatial resolution
The effective spatial resolution can be calculated as a product of all the
mentioned uncertainties and is given by
Δd=Δddiff2+Δdsmear2+Δdgps2.
Typical spatial resolutions for CO2 are ±40.3 to 46.0 m in the
horizontal direction with a mean speed of 2.5 m s-1 and ±24.1 to 27.5 m
in vertical direction with a mean speed of 1.5 m s-1, with the major
contribution from the Picarro CRDS smearing effect. For CH4 the spatial
resolutions with similar mean speeds are slightly lower, having ±41.2 to
48.9 m in the horizontal direction with a mean speed of 2.5 m s-1 and
±24.7 to 29.3 m in vertical direction with a mean speed of 1.5 m s-1.
Conclusions
In this paper, a UAV-based active AirCore was developed and was tested both
in the laboratory and during flights.
The laboratory test results show that the mean differences between the
measurements of roof air by the active AirCore and a co-located CRDS analyzer
are 0.04 ± 0.21 ppm, 0.58 ± 0.67 ppb, and 0.86 ± 27.37 ppb for
CO2, CH4, and CO, respectively. The direct comparison between the
measurements of atmospheric air samples at 60 m from the active AirCore
during flight and from the tower indicates a mean difference of
0.14 ± 0.36 ppm for CO2 and -5.6 ± 3.9 ppb for CH4.
We demonstrate that the buildup of the boundary layer was clearly observed
with three consecutive vertical profile measurements in the early morning
hours. A clear enhancement in both CO2 and CH4 was captured during a
low-altitude horizontal transect flight and was determined to be caused by
emissions from the wetlands north of the Wadden Sea dike.
The spatial resolution of the active AirCore samples is comprised of four
factors: analyzer smearing effects; GPS uncertainties; and diffusion and Taylor
dispersion, where the analyzer smear effect is the largest contributor. At
typical speeds of 1.5 m s-1 for ascent and descent, and 2.5 m s-1 for horizontal
flying, the effective spatial resolution is determined for CH4 to be 24.7
to 29.3 and 41.2 to 48.9 m, respectively, depending on the storage time.
For CO2, the spatial resolution at the same speeds are 24.1 to 27.5
and 40.3 to 46.0 m, respectively, depending on the storage time. Due to the
small amount of time between sampling and analysis (10–40 min), samples
obtained using the active AirCore experience a low loss of sample resolution
due to molecular diffusion. A modified CRDS analyzer with a reduced cavity
pressure, e.g., 106 or 53 hPa, would greatly enhance the spatial
resolution, since the response time of the CRDS analyzer would go down. Note
that with a cavity pressure of 53 hPa the spatial resolution is determined
mainly by molecular diffusion, instead of the smearing in the analyzer.
The design of the volume, the length of the active AirCore, and the chosen
sampling flow rate provide up to 16 min of flight time. The range of
the flights is largely determined by the performance of the UAV; however, the
spatial resolution of the measurements is compromised by the speed of the
flight.
The light weight of the active AirCore of 1.1 kg, its excellent preservation
of the resolution of atmospheric air samples, and the mobility of a UAV lead
to an effective sampling tool to measure greenhouse gases CO2 and CH4
mole fractions and a related tracer CO. This study shows the active AirCore's
ability to capture both vertical and horizontal trace gas profiles. The
usefulness of a UAV platform to quantify instantaneous CH4 fluxes from a
landfill has been demonstrated by . Our UAV-based active
AirCore system opens up a wide variety of opportunities, including
measurements of GHG on a local scale with high resolution; quantifying CH4
emissions from wetlands, landfills, and other CH4 hot spots; and the
quantification of CO2 emissions from power plants.