An original and innovative sampling system called AirCore was
presented by NOAA in 2010
Understanding the global atmospheric budget of the two major greenhouse gases
(GHG) emitted by human activities, carbon dioxide (CO
One of the main challenges for any satellite-based measurements is data
evaluation and the comparability to WMO standards. To that end the Total
Column Observing Network (TCCON;
Precise and regular vertical profile measurements from the surface to above the tropopause are currently missing to evaluate total or partial columns of GHG retrieved either from the ground or from space and to tie them to the calibrated measurements of the WMO.
Several aircraft missions contribute vertical information with
regular measurements along commercial airlines such as the CONTRAIL project
To overcome this limitation, several instruments to measure CO
In this context, an original and innovative atmospheric sampling system
called AirCore has been developed at the National Oceanic and Atmospheric
Administration Earth System Research Laboratory (NOAA/ESRL;
Schematic description of the five steps of the AirCore sampling method.
Since the development of the first AirCore
The design of this new high-resolution AirCore, named AirCore-HR, is
presented in Sect.
The general principle of an AirCore is illustrated in
Fig.
AirCores can be designed in a variety of configurations that determine the
vertical resolution that can be achieved with the instrument. The resolution
directly depends on the molecular diffusion and shear flow diffusivity,
otherwise known as Taylor dispersion, inside the tube
As described in
Most of the AirCore configurations will have the following characteristics:
(i)
Such values yield a number of Reynolds between
In all circumstances the flow in an AirCore is thus laminar since
Diffusion and dispersion are considered neither during ascent while the tube empties nor during descent when the sampled pressure range varies continuously and repartition of the air along the AirCore thus evolves rapidly. It is only from the moment the total column is sampled and the final air repartition reached that the described model is used to calculate the vertical resolution (i.e., from the moment the tube is sealed with the captured sample until the end of the analysis).
At first, during a given storage time before the payload is recovered only
molecular diffusion will affect the sample. As described in
Then, during analysis, both molecular diffusion and the Taylor dispersion affecting the sample have to be accounted for. During this phase the root mean square of the distance of molecular travel is given by
In addition to the effects of diffusion and dispersion, which are the main
drivers of the resulting vertical resolution, the smearing effect of the cell
of the analyzer during analysis has to be taken into account. The analyzer
used in this study (Picarro cavity ring-down spectrometer (CRDS); G2310) pulls
the sample at 110 sccm and measuring at 0.5 Hz makes one measurement every
3.7 scc (standard cubic centimeters). The analyzer cell has a standard volume
of approximately 6 scc, since it is 35 cc in volume, but is maintained at 187 hPa (140 torr) and 45
To account for mixing in the volume of the cell of the analyzer, a Gaussian
function characterized by the following standard deviation
As all mixing effects can be considered Gaussian, the total distance of
diffusion
Using Eq. (
Using a standard atmosphere temperature profile it is then possible using the
hydrostatic law to associate the atmospheric pressure with a given altitude. In
order to best represent the latitudes at which the AirCores are to be
deployed, we used the average temperature profile of the representative TIGR
(Thermodynamic Initial Guess Retrieval) dataset (
To appreciate the value of the AirCore-HR it is important to understand the factors that determine the resolution of an AirCore. The first factor is the sample cell of the analyzer that will limit the number of independent measurements over the sampled volume. The second factor is the diffusion distance (explained above) which, depending on the diameter of the tube and the lag between when air was sampled and when it is analyzed, will eventually limit the sampling resolution of the AirCore.
Comparison of the vertical resolution that can be expected with different AirCores for CO
At midlatitudes, air sampled over a 10 hPa descent between 20 and 30 hPa represents about 3 km of vertical distance whereas air sampled over a 10 hPa descent between 450 and 460 hPa represents about 200 m of vertical distance. This has a direct consequence for the observation of the stratosphere, for which the sampled air needs to be preserved while sampling as much of it as possible. This can be achieved by combining sections of tubes of different diameters. A given volume of air is indeed affected differently when stored in a section with a smaller diameter: although diffusion remains the same, the distance of travel for a molecule to impact an equivalent volume increases. Therefore, using at least two tubes, one characterized by a small diameter at the end that remains closed and one characterized by a larger diameter at the end that remains open, allows us to keep a high resolution for the stratosphere (by storing the stratospheric part of the sampled profile in the tube with the smallest diameter) while still sampling a consequent volume of air thanks to the larger tube. To maximize the total volume of the AirCore-HR and limit the impact of the diffusion distance, the AirCore-HR was designed with tubes of two different diameters.
Figure
In order to achieve a higher resolution along the whole atmospheric column, a
design of a 300 m tube consisting of a 200 m of 0.125 in. (3.175 mm) tube and a
100 m of 0.25 in. (6.35 mm) tube linked together as one tube was selected for
AirCore-HR. The increase in overall volume of the AirCore-HR allows a
significant increase in resolution throughout the whole sampled air column
(Fig.
The resolution achievable by the lightweight AirCore-GUF designed and
developed at Goethe University Frankfurt is also shown in Fig.
Characteristics of the AirCore-HR.
Impact of the time delay between landing and analysis on the expected vertical resolution of AirCore-HR, for a storage time of 3 h (black), 6 h (blue) and 12 h (green), 24 h (orange) and 1 week (red).
Overview of the AirCore-HR and analysis system.
The storage time between landing and analysis of the sample is a key factor
influencing the vertical resolution. The resolutions plotted in Fig.
In order to obtain the vertical resolution shown in Fig.
The overall design is plotted in Fig.
The AirCore-HR payload has been designed to fit into a polystyrene foam box. It is flown together with an electronic data package designed at LMD that collects meteorological data from a pressure sensor and three temperature probes and also controls the opening and closing of a solenoid valve at the open end of the AirCore. Temperature probes are placed along the AirCore in contact with various segments of the tube and allow monitoring the mean temperature along the coil during the flight. The pressure sensor is an absolute pressure sensor that measures the ambient air pressure during the flight.
Several tests were conducted in the laboratory under monitored conditions to
evaluate the overall consistency of the AirCore-HR. In particular, the
AirCore-HR has been tested for leaks at the junctions and at the valves used
as closing points on each side of the AirCores. To test the preservation of
the concentration of the sample, calibrated dry standard gases of two
different values for both CO
For testing and analysis of the AirCores, two calibrated gas standards are used. The cylinders are connected to a multiport valve, allowing selection of one of the gases.
The first standard is composed of high concentrations of CO
Values of the calibrated gas standards using NOAA's WMO scale reference. The air of the two reference tanks used in this study was measured at LSCE with a Picarro G2401 calibrated with a scale of six tanks from NOAA/ESRL. The table shows the reproducibility of the measurements and standard deviation over three measurements made during a 15-day period.
All gas analyses of LMD AirCores were performed using one trace gas analyzer
by Picarro, Inc., model G2310
Upon recovery, the AirCore-HR is plugged into the prepared analysis system. It is first kept closed on both ends, allowing us to pull calibrated standards through the bypass into the analyzer. Once the values measured with the continuous analyzer are stabilized to the expected values for the calibration standard used as “push gas”, the analysis of the air captured in the coil can start. This phase is very important to make sure that, after plugging the AirCore-HR in the system, the mixing ratio read by the Picarro is not contaminated by water vapor that could have entered the analysis chain. The collected sample is then analyzed by opening both ends simultaneously; the air is pulled from one end into the continuous analyzer and low-concentration calibration standard is pulled through the other end. The top of the profile with the remaining fill gas is pulled first into the analyzer.
Picarro analysis of the AirCore-HR sample from the EdS-Stratéole flight on 29 August 2014.
The calibrated gas standards given in Table 2 allow replacing the values read
by the Picarro onto the WMO scale. The high-concentration standard is used as
fill gas to have a noticeable difference between fill gas and stratospheric
air sample at the top of the profile. The low-concentration calibration
standard is chosen to be used as push gas to have a noticeable difference of
the mixing ratios compared with the expected values of CO
Several steps are required to accurately place the Picarro measurements on a
vertical scale in order to retrieve the vertical profiles. The dry mole
fraction of CO
Figure
As a first approach, it is assumed that the air entering the tube
equilibrates the sample with ambient pressure and adjusts very quickly with
the mean coil temperature. As the characteristics of the AirCore (length,
diameter) do not change, ambient pressure and mean coil temperature are the
two main factors that regulate the number of moles in the AirCore. Using the
ideal gas law (Eq.
With measured time series of pressure (
This number is maximum when the AirCore reaches the Earth's surface, i.e.,
The critical orifice setting the flow during analysis at 38.5 sccm min
Using Eqs. (
AirCore-HR was flown for the first time during the StratoScience campaign
operated by the French space agency (CNES) in collaboration with the Canadian
Space Agency (CSA) in Timmins (Ontario, Canada; 48.57 N,
The carrier consists of a gondola that could accommodate a total of eight instruments including the AirCore-HR. All these instruments (consisting of small packages of several kg) were brought together on the same structure with the aim of studying simultaneously several climate variables. In total, the gondola weighed 248 kg.
In addition to AirCore-HR, two AirCores-GUF from Goethe University Frankfurt were also flown during this flight.
Flight plan from the StratoScience 2014 EdS-Stratéole flight on 29 August 2014 with main operating
states of
To fulfill the requirements of the eight instruments, the EdS-Stratéole flight
had a very specific flight trajectory. The takeoff (release of the balloon)
took place on 28 August 2014 at 20:33 local time in Timmins (00:33 UTC,
29 August 2014). After the ascent phase, the flight consisted of a monitored
and controlled descent with two stops. Following a short stop at the ceiling
at a barometric altitude of 14 hPa (29 km), an evacuation trap allowed us to let
some gas out to engage in a descent phase down to a barometric altitude of 54 hPa (
Figure
Joint efforts of CNES and CSA teams allowed access of the AirCores and
analysis less than 3 h after landing. Unfortunately, at
the end of the flight, the electronic circuit keeping the solenoid valve
closed experienced a short power cut of about an hour, which resulted in
sampled air evacuating from the AirCore. The AirCore-HR coil heated up after
reaching the ground since it had been exposed to cold temperatures during
the flight. During this period, the heating that occurred resulted in the
loss of
a fraction of the profile equivalent to the air sampled from 900 to 980 hPa.
The loss of that fraction of the total sample had an impact on the retrieved
vertical profiles (see Sect.
Recorded temperature from the three probes on the AirCore-HR and ambient pressure during the EdS-Stratéole flight on 29 August 2014. The three temperature probes (red, pink and purple lines) are presented in degrees Celsius as a function of time; the temperature axis is located on the right side. Ambient pressure (black line) is presented in hPa as a function of UTC time, with the vertical scale on the left side.
Vertical profiles retrieved from the air sampled with AirCore-HR on the EdS-Stratéole
flight on 29 August 2014:
The specific periods of interaction with ambient air of the AirCores-GUF are
highlighted in Fig.
In order to determine the vertical profiles of CO
Comparison of AirCore-HR
Comparison between AirCore-HR and other pressure measurements highlighted a small drift in AirCore-HR data pressure recordings. Therefore the pressure profile recorded with the electronics of AirCore-HR has been corrected to fit the high precision of the records of a Paroscientific, Inc., absolute pressure gauge that is characterized by an accuracy of 10 Pa and a precision of 0.1 Pa.
Additionally, GPS coordinates and altitudes from CNES were used to complete the dataset.
Figure
In Fig.
As can be seen in Fig.
The CH
A comparison between Fig.
A comparison was performed with CO
The agreement between both CO
Although fewer vertical structures are seen in the forecast, the CH
Monte Carlo simulations were performed to assess the uncertainty associated
with the retrieved constituent profiles. The retrieval process of the
vertical profiles was iterated a 1000 times by randomly changing the original
datasets within the estimated uncertainty range of every identified
uncertainty source. This allowed us to produce a set of 1000 slightly different
outcomes for the vertical profiles in terms of both mixing ratios and
vertical position. A standard deviation of the mixing ratios at a given
position was then calculated based on this dataset. In these simulations we
took into account the following uncertainties:
The accuracy of the gas analyzer: Picarro measurement accuracy was
defined as a Gaussian standard deviation of the mixing ratios based on the
instrument specification (i.e., deviations of 0.5 ppb for CH The mean temperature profile: to account for the impact of temperature
correction, the temperature profile was randomly chosen among the three profiles
measured by the three probes. Indeed, the three temperature probes are placed at
different positions along the tube (near the entrance, in the middle of the
AirCore and near the closed end) and, depending on the distance to the inlet,
they have recorded different temperatures along the AirCore. Choosing
randomly between one of the three probes is thus the conservative way to account
for the uncertainty related to the mean temperature of the AirCore. The pressure profile: an uncertainty of 0.1 Pa corresponding to the
accuracy of the Paroscientific, Inc., absolute pressure gauge was used. The selection of the sample: the choice of the exact midpoint of
transition between either push gas and sample or remaining gas and sample
(see Sect. The potential loss of air sample resulting from the tube remaining open
after landing as occurred during this flight (see Sect.
The uncertainties discussed here are related to the analysis and processing
of the sampled air and are only valid for the AirCore-HR in the case of this
flight and may have different results in other situations. The CO
Comparing Fig.
Additionally, the impact of the variability in the measurements of the three
temperature probes has been studied. It was found that temperature
uncertainty has a very limited influence on the overall uncertainties, of the
order of 6 %, despite differences of several degrees Celsius (Fig.
Overall, the average uncertainty on the CO
The average uncertainty on the overall CH
Benefiting from the accommodation of several AirCores on board the CNES
gondola, the AirCore-HR profiles can be compared with those of the lighter
AirCores-GUF (see Sect.
The particular descent profile of this flight had several impacts on the
AirCore-GUF profiles:
As for AirCore-HR, unrealistic low values of CO Since the lower-resolution AirCore-GUF captured a smaller volume than
AirCore-HR, the stratospheric part of the profile was impacted by diffusion
during the 7 h plateau phase. Indeed, during the plateau phase at about
90 hPa, the air sampled from 20 to 90 hPa by AirCore-GUF remained in the first
tube of 20 m/8 mm diameter whereas it was stored over the 100 m/0.25 in. (6.35 mm) tube for AirCore-HR. This led to a more intense diffusion in
the AirCore-GUF sample.
Vertical profiles retrieved from the air sampled with the AirCore-HR and an AirCore-GUF on the EdS-Stratéole
flight on 29 August 2014.
Therefore, all the CO
The comparison between AirCore-HR (black) and AirCore-GUF (blue) highlights
that both CO
For CH
Overall, the comparison between both AirCores reveals that the high resolution captures more information on the vertical distribution along the atmospheric column.
To perform a fair comparison between the different AirCore profiles, the
degradation of the resolution of AirCore-HR profiles to that of lower-resolution AirCore-GUF has to be performed. This exercise aims also
to
evaluate the theoretical calculation of the expected resolution (Sect.
The vertical resolutions shown in Fig.
Degradation of the AirCore-HR profiles is performed through the convolution
with a Gaussian window with a standard deviation of the lower vertical
resolution at each given altitude:
It allows retrieving a degraded version of the profiles:
The effect of the degradation of the AirCore-HR profile to the lower
resolution of AirCore-GUF is presented in Figs.
The comparison of the CO
Concerning CH
In this paper, a new AirCore (AirCore-HR) allowing high-resolution
measurements of CO
The AirCore-HR was flown for the first time on a multi-instrument gondola,
which allowed us to perform comparisons of the vertical profiles retrieved with
AirCore-HR and lower-resolution AirCore-GUF. The degradation of the profile
given by AirCore-HR to the resolution of AirCore-GUF revealed an excellent
agreement between both profiles for CH
CO
By designing a method that takes into account all the sources of uncertainties in
the processing of the data, the overall uncertainty is estimated to be less
than 3 ppb on the CH
Comparison between AirCore data and forecasts from CAMS-ECMWF has yielded
satisfying agreements between AirCore-HR profiles and simulated profiles. In
particular, well-pronounced vertical transport signatures in the troposphere
in both CO
This comparison illustrates the potential of AirCores to evaluate atmospheric transport models, as well as GHG satellite retrievals from TIR and SWIR instruments. In particular, light AirCores flown from weather balloons could be deployed at various locations to complete an effective system together with ground stations and regular aircraft campaigns. Such lightweight systems could also contribute to specific campaigns for calibration and validation of future space missions. In order to fit these applications, the spatial and temporal resolution requirements necessary to evaluate the models or satellite retrievals efficiently need to be assessed.
Along with the development of robust lightweight systems, it is also important to continue development strategies of AirCores for large platforms carrying heavy payloads. Such platforms, flown during specific stratospheric balloon campaigns, allow unique multi-instrument measurements of the same or complementary atmospheric variables. The simultaneous use of laser-diode spectrometers, cryosamplers and AirCores, which can only be performed during these specific campaigns, is necessary to evaluate the retrievals performed with various AirCores and test improvements of the instruments.
AirCore data presented in this paper are available via the Ether database
through the following link:
The authors declare that they have no conflict of interest.
This work was supported by CNES and benefited during the StratoScience 2014 campaign from the infrastructure developed by CSA in Timmins. École Polytechnique provided additional funding for the instruments and funded the 6-month sabbatical of Colm Sweeney from NOAA to LMD. The first author is funded by EIT/Climate-KIC under contract with UPMC. Technical support to develop the instrument was provided by Olivier Bousquet and Olivier Godde from the technical team of LMD. Sebastien Massart and Anna Agusti-Panareda from ECMWF provided collocated CAMS-ECMWF data generated using Copernicus Atmosphere Monitoring Service Information (2016) and are to be thanked for their valuable input and scientific discussions. The development and deployment of AirCore at GUF was funded by the German Federal Ministry of Education and Research (BMBF) within the ROMIC program under project 01LG1221A. Edited by: M. Hamilton Reviewed by: two anonymous referees