ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-6825-2017Mean age of stratospheric air derived from AirCore observationsEngelAndreasan.engel@iau.uni-frankfurt.dehttps://orcid.org/0000-0003-0557-3935BönischHaraldhttps://orcid.org/0000-0002-1004-0861UllrichMarkusSitalsRobertMembriveOlivierDanisFrancoisCrevoisierCyrilInstitute for Atmospheric and Environmental Science, Goethe
University Frankfurt, Frankfurt, GermanyLaboratoire de Météorologie Dynamique (LMD/IPSL), CNRS, Ecole
polytechnique, Université Paris-Saclay, Palaiseau, Francenow at: Karlsruhe Institute of Technology, KIT, Karlsruhe, GermanyAndreas Engel (an.engel@iau.uni-frankfurt.de)12June20171711682568383February20178February201728April20175May2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/6825/2017/acp-17-6825-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/6825/2017/acp-17-6825-2017.pdf
Mean age of stratospheric air can be derived from observations of
sufficiently long-lived trace gases with approximately linear trends in the
troposphere. Mean age can serve as a tracer to investigate stratospheric
transport and long-term changes in the strength of the overturning
Brewer–Dobson circulation of the stratosphere. For this purpose, a low-cost
method is required in order to allow for regular observations up to altitudes
of about 30 km. Despite the desired low costs, high precision and accuracy
are required in order to determine mean age. We present balloon-borne AirCore observations from two midlatitude sites: Timmins in
Ontario/Canada and Lindenberg in Germany. During the Timmins campaign, five
AirCores sampled air in parallel with a large stratospheric balloon and were
analysed for CO2, CH4 and partly CO. We show that there is good
agreement between the different AirCores (better than 0.1 %), especially
when vertical gradients are small. The measurements from Lindenberg were
performed using small low-cost balloons and yielded very comparable results.
We have used the observations to extend our long-term data set of mean age
observations at Northern Hemisphere midlatitudes. The time series now covers
more than 40 years and shows a small, statistically non-significant positive
trend of 0.15 ± 0.18 years decade-1. This trend is slightly
smaller than the previous estimate of 0.24 ± 0.22 years decade-1
which was based on observations up to the year 2006. These observations are
still in contrast to strong negative trends of mean age as derived from some
model calculations.
Introduction
Mean age of stratospheric air is the average time it takes for
the atmosphere to transport air from the tropospheric source region to a
given place in the stratosphere. The concept of mean age was first developed
by Kida (1983) and has since been refined and discussed in several high-quality reviews (Hall and Plumb, 1994; Waugh and Hall, 2002). In brief, the
concept divides an air parcel into irreducible fluid elements which are
irreversibly mixed during transport in the stratosphere. Each such fluid
element has a separate transport time and transport path associated with it.
The distribution of the statistical probability associated to the different
transit times is called the age spectrum and represents a probability density
function (pdf) for individual transit times to this air parcel. The first
moment of the age spectrum is called the mean age of air. While the age
spectrum cannot be measured, mean age can be derived from observations of
inert trace gases under certain conditions. In the case of an inert tracer with a
perfectly linear trend in the atmosphere, the time lag between the occurrence
of a given mixing ratio of a tracer in the troposphere and the occurrence of
the same mixing ratio at some place in the stratosphere would be the mean age of air. The two tracers which have been used most widely for this purpose are
CO2 and SF6. Neither of these gases increases completely linearly
with time, so the shape of the age spectrum needs to be taken into account in
deriving mean age.
Mean age has been identified as a valuable tracer to investigate
stratospheric transport timescales, for example, by comparing model-derived mean
age with observations. Long-term trends in mean age have been used to
investigate long-term changes in the overall overturning circulation of the
stratosphere (Brewer–Dobson circulation, BDC). An increase in the strength
of the BDC is expected from model calculation with increasing greenhouse gas
concentrations (Austin and Li, 2006; Butchart et al., 2006; Butchart,
2014). This is reflected in overall shorter transit times, thus also lower
mean age values in the models.
The experimental database of mean age observations for the verification of
such changes is sparse. It relies mainly on very sporadic balloon-borne
observations of CO2 and SF6 dating back to 1975 (Engel et al.,
2009) and on satellite observations of SF6 (Stiller et al., 2012; Haenel
et al., 2015). The balloon-borne observations used in Engel et al. (2009)
were taken in a region between 24 and 35 km where the vertical gradient in
mean age at Northern Hemisphere midlatitudes was found to be very small,
leading to little variability in this region. The balloon data were limited to
a total of 28 flights and showed a positive trend of 0.24 years per decade
for this region, which was, however, estimated to be non-significant. Satellite
observations of SF6 used in Stiller et al. (2012) and Haenel et
al. (2015) were limited to the lifetime of the Envisat satellite of about
10 years. They show an uneven distribution of trends with positive trends in
the middle stratosphere of the Northern Hemisphere but negative trends in the
Southern Hemisphere. Modelling work by Garny et al. (2014) showed that mixing
has a strong influence on mean age and that enhanced mixing leads to higher
mean ages in large parts of the stratosphere (aging by mixing). Ploeger
et al. (2015) then showed that trends in mean age are to a large degree also
influenced by trends in mixing and not only in residual transport. Overall,
it has become clear that the interpretation of changes in mean age as changes
in residual circulation is inadequate, and that it rather represents a
combination of changes in mixing and in residual transport.
The experimental investigation of changes in mean age of stratospheric air
is to a large degree restricted by the availability of observations. The
balloon-borne data set presented in Engel et al. (2009) relies to a large
part on samples collected in the stratosphere using large and heavy
cryogenic whole air samplers. These instruments require large and expensive
balloons to carry them to altitudes above 25 km. The use of these large
balloons involves a large operational team and is very expensive. The
uncontrolled parachute descent of such large payloads after the flights
further presents a large operational constraint due to safety regulations.
These safety regulations make it virtually impossible to fly such large
payloads in densely populated areas as central Europe. Due to these
operational constraints and in order to create a larger and more
representative database, an easy to launch and cheap technique to allow for
the measurement of age tracers would be required. AirCore, a new technique
for sampling air, which has been suggested by Karion et al. (2010), may
provide such an opportunity. In brief, this technique relies on collecting
air in a previously evacuated, long stainless steel tube. When deploying
AirCore on a balloon, the tube, which is open on one side and closed on the
other side, is filled with a fill gas (FG) which has different chemical
characteristics from the ambient air that is being sampled. The tube is emptied
during the ascent of the balloon due to the decreasing pressure with altitude.
Upon descent of the balloon, ambient air is pushed into the AirCore. Due to
the length of the tube and the laminar flow during the collection, the air
is only partially mixed and the information on the vertical distribution is
retained for a while before eventually being mixed due to molecular
diffusion. After collection, the sampled air can then be analysed by pushing
it out of the tube with a push gas (PG), which must again be well
distinguishable from ambient air.
A very lightweight AirCore developed at the University of Frankfurt for deployment
on small, cheap and easy to launch balloons used for launching, for
example,
ozone sondes is presented in Sect. 2 together with the analytical set-up for
measurements of the AirCore and the data retrieval. In Sect. 3 we present
observations from two midlatitude campaigns: the first is in Timmins,
Ontario in 2015 and the second is in Lindenberg, Germany in 2016. Results
from a first test campaign in Timmins, Ontario in 2014 have been published in
Membrive et al. (2016). Due to technical problems the results from the
campaign in 2014 cannot be used to derive mean age. The mean ages calculated
from the observations in 2015 and 2016 are presented in Sect. 4 together with
an updated long-term evolution of mean age. A summary and conclusions are given
in Sect. 5.
University of Frankfurt AirCore
The AirCore used by University of Frankfurt was developed under two main
aspects. The first aspect is that the instrument should be sufficiently
light to allow for flights under simple balloons at midlatitudes in Europe.
The second aspect is that the AirCore should be optimised to allow
measurements at high altitudes with an optimal resolution. The AirCore is
currently used for measurements of CO2, CH4 and CO.
Overall concept
As explained above, the University of Frankfurt AirCore is designed to provide
optimum resolution in the stratosphere while keeping the weight sufficiently
low for use under a small balloon. The vertical resolution, which can be
achieved by AirCore measurements, will generally depend on the geometry of
the AirCore itself, on the effective volume of the analyser deployed and on
the storage time between collection of the sample and the analysis. The
target of our AirCore is to derive the mean age from CO2. As the loss of
CH4 in the stratosphere results in the production of CO2, CH4
needs to be measured simultaneously. We therefore decided to use a Picarro
G2401 analyser for this, which is able to measure CO2, CH4, CO and
H2O with a temporal resolution of about 2–3 s and very high precision,
which is better than 0.01 % for CO2 and 0.05 % for CH4 over
a 5 s period under typical ambient conditions. Typical reproducibilities
observed during field operations showed precisions of 0.025 ppm of
CO2 and 0.2 ppb of CH4. For CO, which was mainly used to
distinguish between ambient air and PG, typical precision was 5 ppb.
Molecular diffusion is described by Fick's first law of diffusion, which
states that the diffusive flux J is proportional to the concentration
gradient ∂c∂x and the diffusion constant D.
J=-D×∂c∂x
A given amount of air stored in a short tube with large inner diameter will
be stretched out over a much shorter distance than the same amount in a narrow and longer
tube. The diffusive flux is thus lower when using a longer and thinner tube.
As the amount of sample collected by AirCore is proportional to the ambient
pressure, very little air is collected at high altitudes. In order to
minimise the loss of vertical resolution with altitude, it is desirable to
have long, thin tubes for the storage of stratospheric air. On the other
hand, the absolute amount of air collected by the AirCore is limited by the
total volume of the tube, which is low for tubing with a small inner diameter.
We have therefore decided to construct an AirCore from different diameter
tubes in such a way that the high-altitude air is stored in the thin-diameter
tubing part of the AirCore while the overall volume is provided by wider-diameter tubing in which the lower-altitude air will eventually be stored.
The AirCore operated at the University of Frankfurt is thus composed of 20 m of
8 mm O.D. tubing, and 40 m each of 4 and 2 mm outer diameter tubing. The
thinnest-walled tubing we could identify were 0.2 mm in thickness for 8
and 4 mm outer diameter tubes. A 2 mm outer diameter tube with a 0.12 mm
wall thickness is available. The volume per weight is highest for the large
outer diameter tubing. All tubes were custom produced for
our AirCores. The tubes are joined by solder and lightweight adaptors. As
suggested in Karion et al. (2010), all tubes were silanised prior to
soldering them together. The AirCore is closed during flight on the 2 mm
side, while the 8 mm O.D. tube is the open ended. The calculated weight of
the AirCore based on the specifications of the tubes is 1.4 kg for the
100 m-long tube. The final weight of the tube was, however, slightly higher
due to the wall thickness being on the high side of the specified tolerance.
An automated closure valve (Chen et al., 2017) is added onto the closed side
and a sample dryer is mounted on the open end. The drier is based on
Mg(ClO4)2 filled in a 1/2′′ O.D. tube of 50 mm in length, containing a
total of 4 cm3 of Mg(ClO4)2. The AirCore is mounted in a
Styrofoam box for thermal insulation and mechanical protection. As the
temperature of the AirCore during collection determines the amount of
air which can be sampled, we monitor this temperature with a minimum of
three temperature sensors. The automatic closure valve (Chen et al., 2017)
closes the AirCore after landing. This valve is controlled by a lightweight
electronics package which also includes the data logger for the temperature
sensors and was developed at the University of Groningen (Chen et al., 2017). The
overall weight of the AirCore in flight mode and using the University of
Groningen electronics package and closure valve is about 2.5 kg including
protective housing.
Estimated vertical resolution
The vertical resolution of the derived mixing ratio profiles is influenced by
sampling, storing and measurement procedures. The principal procedure to
estimate vertical resolution has been outlined by Karion et al. (2010).
Resolution is lost due to molecular diffusion during the storage of the
sampled air in the AirCore and due to mixing during the sampling and analysis
process. Molecular diffusion can be calculated using Fick's law. The mixing
process is essentially influenced by two parameters: (i) Taylor dispersion
during the collection and analysis of the samples and (ii) the effective cell
volume of the analyser, which has to be flushed. Mixing processes are species
independent, while the first effect (molecular diffusion during storage) is
species dependent, each species having a different molecular diffusion
coefficient. The cell volume of the analyser is not an intrinsic limitation
of the resolution of the AirCore itself, but will be included in the derived
resolution based on the Picarro G2401 analyser used for our analysis. As
molecular diffusion is a function of time and molecule, the resolution of our
AirCore is also a function of time and will deteriorate with time and differ
for each molecule. As explained above, molecular diffusion will lead to
a larger loss of resolution in a wider tube. Therefore, our AirCore loses resolution
much faster at lower altitudes during storage, where the sampled air is in
the wider tube. We have applied the same parameters as described in Karion et
al. (2010) to derive the vertical resolution for our AirCore. Figure 1
compares the vertical resolution of our AirCore to other AirCore systems
(Karion et al., 2010; Membrive et al., 2016). The calculation is based on the
assumption of sampling air down to 1000 hPa. At the upper altitudes, the
resolution is dominated in this calculation by the effective volume of the
analyser cell, while molecular diffusion is the dominant term at low
altitudes. The overall vertical resolution of the measurements is better than
1 km below 24 km in altitude and increases to about 2.5 km at 30 km
(Fig. 1). For comparison, the HR-AirCore described by Membrive et al. (2016)
achieves a vertical resolution which is better than 300 m below 15 km and
better than 500 m below 22 km. However, using much longer tubing results
in a higher weight.
Calculated vertical resolution for CO2 with the Goethe
University Frankfurt (GUF) AirCore (AirCore-GUF) in comparison to other
AirCores (see text for description), assuming a delay of 3 h between
collection and sampling, a measurement flow of 40 mL min-1 and an effective
cell volume of 6 mL. The AirCore-HR and LMD-Light AirCores are operated by
LMD, while the original NOAA AirCore refers to the AirCore described by
Karion et al. (2010).
Operation and analytical set-up
Before the flight the AirCore is checked for leak tightness and cleanliness.
As a first test, a gas of known concentration is measured either directly or
by passing it through the AirCore and the measurements are compared. As a
further test, the AirCore is filled with a gas of known concentration and
analysed again after a storage time of 24 h. Only if the CO2 and
CH4 readings from both values agree within the uncertainties, the
AirCore is considered as clean and leak tight. It is then filled with a fill
gas (FG) of known CO2, CH4 and CO concentration up to
24 h before flight. Before flight the automatic valve mounted on the inlet
side of the AirCore is opened.
The AirCore should be analysed as quickly after the flight as possible as
molecular diffusion decreases the achievable vertical resolution. During the
flight in Timmins, Ontario, this occurred about 4 h after the
∼ 300 km flight, while the analysis started within an hour after
landing during the flights launched from Lindenberg. In order to achieve this
fast analysis, the analytical set-up consisting of a Picarro analyser and a
gas control system must be deployed in the field. For this purpose, we
mounted the analytical system in a car when operating from Timmins and inside
our laboratory bus during flights from Lindenberg. The set-up also included a
battery-operated inverter allowing us to run the Picarro for up to 6 h, which
also allowed the instrument to remain heated and under constant flow while
driving to the predicted landing area. We use the same gas as fill gas (FG)
and push gas (PG). This gas mixture contains typical atmospheric CO2
values, typical CH4 values expected around 30 km in altitude and
significantly higher CO values than observed either in the troposphere or in
the stratosphere. Based on the CO values, it is thus possible to distinguish
between the sampled atmospheric air, the PG and the FG, which is left in the
tube.
The gas flow system used for the analysis of AirCore is shown in Fig. 2. This
system allows the two lines, which are needed to connect the AirCore for the
analysis, to be flushed with a standard gas (Cal Gas) or the push gas used
for the analysis (connection and flushing, upper panel in Fig. 2). During the
connection all dead volumes of the connectors can be flushed, minimising the
contamination from ambient air and the Picarro is flushed
with push gas. The pressure of the PG is regulated to slightly above ambient
pressure (typically 1030 hPa) and the flow through the Picarro is regulated
to 40 mL min-1. Once all lines are flushed and connected and the
Picarro gives a stable reading of the expected value for the PG, the
two-position valve can be switched. PG regulated at 1030 hPa is
then flushed through the AirCore with a flow of 40 mL min-1. The
stratospheric (upper altitude) air is flushed out first. Before stratospheric
air arrives at the Picarro analyser, the standard gas that was used for
flushing the connection lines will arrive first, followed by the remaining FG
from the AirCore. The amount of FG left will depend on the lowest pressure
reached during flight and on the temperature of the AirCore during that phase
(see section data retrieval). Fig. 3 shows a typical example of the raw analytical results
from the measurements of two AirCores that embarked simultaneously on board the
flight from Timmins in 2015 (see Sect. 3.1). Figure 4
shows a zoomed-in image on the CO measurements shortly after switching the rotary valve.
The PG with its high CO values close to 1.4 ppm is measured. Then the CO
values drop due to the lower values of the calibration gas with which the
connection line was flushed. This is then followed by the FG remaining in the
tube. Note that during this flight the amount of FG left was not sufficient
for the Picarro to arrive at its expected value of close to 1.4 ppm. After
passing the peak with high CO from the FG, the values drop sharply,
showing much lower CO values. These lower CO values are expected in the
middle stratosphere (Toon et al., 1999; Engel et al., 2006b) due to the
photochemical balance between the production of CO from oxidation of CH4 and
the breakdown of CO due to oxidation with the OH radical. The transition from
high CO (remaining FG) to low CO thus marks the beginning of the sampled air
at the upper part of the profile. In a similar way, the transition from
rather low tropospheric CO to high CO marks the time at which all the sampled air
has been pushed out of the AirCore and the PG used to push the air out of the
AirCore is seen by the analyser.
Analytical set-up for the measurement of AirCore. The AirCore
including the valves which are flown is shown in red. In the bypass/flushing
position (a) the push gas (PG) is measured bypassing the AirCore.
The transfer lines to the AirCore can be flushed with PG or with a
calibration standard (Cal Gas), allowing us to connect the AirCore to the
analytical system without contamination. For the analysis the transfer lines
to the AirCore are closed, the AirCore valves are opened and the two
position valve is switched to the measurements mode (b). The PG is
passed through the AirCore and pushes the air to the Picarro. Pressure and
flow are controlled allowing for a very constant air flow.
Raw analytical results for CO2 from the measurements of the
two AirCores flown from Timmins in 2015. Note that AC2, which was measured
first, contained a closure valve, while AC3 was left open until the
recovery team was able to reach the instrument. The lowest part of the AC3
profile was therefore lost due to the warming of the AirCore on the ground. The
grey shaded areas denote the measurements of air from the AirCore. The
stratospheric part of the profiles is always measured first. Before the
measurements of AC2, between the measurements of the AirCores and after the
measurement of AC3, PG is measured by the Picarro analyser.
Data retrieval
The Picarro analyser will deliver a time series of mixing ratios as a
function of measurement time. The absolute values of the Picarro analyser are
transferred to the WMO scale (X2007 scale for CO2 and X2004a scale for
CH4) based on a calibration function derived from absolute values of
four gas bottles with a range of CO2 values between 390 and 416 ppm of
CO2 and 1.07 to 1.91 ppm of CH4 (only three gas bottles). The mixing
ratios determined by the Picarro analyser have to be matched to the altitude
at which the air was sampled by the AirCore during the flight. The basis of
this altitude attribution is the ideal gas law and the molar amount sampled
at each altitude during the flight. This matching is achieved in a four-stage
process. (i) The amount of remaining fill gas is determined,
(ii) the sampling of air based on the ideal gas law is calculated. (iii) The start and end times of AirCore in the analyser time
series are determined and finally (iv) the sampling and the analysis can be
matched based on the molar amount.
Determination of remaining fill gas
In the case of slow vertical displacement of the balloon, pressure
equilibrium between the AirCore and the surrounding air can be assumed.
Under this assumption of an instantaneous pressure equilibrium, the molar
amount n of an ideal gas stored in volume V at pressure p and temperature T is
according to the ideal gas law:
n=p×VR×T,
where R is the general gas constant. The temperature in this aspect is not
the ambient temperature but the temperature of the coil, as we assume an
instantaneous equilibrium between temperature of the air inside the AirCore
with the coil temperature. In the first step the amount of FG remaining
in the AirCore at the top of the profile is then calculated by searching for
the minimum in pT.
As noted above, the assumptions about pressure equilibrium between air inside
the AirCore and outside air needs to be made in this calculation. While this
is certainly a valid assumption for slowly descending balloon, it will be
less valid with a faster descending balloon. In the case of a rubber
balloon, which will burst while still ascending and then immediately start to
descend, the situation is even more difficult. The pressure inside the
AirCore will actually be higher than the outside pressure during the
beginning of the descent, because a non-equilibrium will exist both for the
emptying of the tube during ascent and the refilling of the tube during the
descent. The size of this non-equilibrium effect will depend on the geometry
of the AirCore but also on the filling of the sample dryer. In particular,
the latter may provide a significant flow restriction if the
Mg(ClO4)2 is packed very densely. The amount of FG left in the
AirCore is thus expected to differ significantly from the equilibrium amount
calculated before based on the minimum in pT. We have taken
particular care to have a short and loosely packed dryer providing minimal
flow restriction. As the FG used in our case differs significantly in CO
values from ambient air, and from the calibration gas used, it is possible to
determine the amount of remaining FG by integrating the CO peak observed
during the measurement of the AirCore (Chen et al., 2017), as illustrated in
Fig. 4. When switching from the bypass to the measurements mode, the gas
inside the Picarro measurement cell is first replaced by the calibration gas,
which was used to flush the transfer line to the AirCore during the
connection. The calibration gas (Cal Gas) is then replaced by the FG which
remained in the AirCore and then by ambient stratospheric air. All of these
gases are partially mixed and all of them contain some CO. In order to
separate the amount of CO (due to PG and Cal Gas) from the signal (due to
remaining FG) we performed a measurement in a similar set-up but our
AirCore was filled with pure nitrogen, which contained no detectable amounts of
CO. The CO in this set-up is thus not due to remaining FG and can be used to
correct the offset due to PG and Cal Gas when integrating the CO peak from
the remaining FG. Using the known mixing ratio of CO in the FG, the molar
amount of remaining FG can be determined. In the case of the two fast descent
profiles from Lindenberg (see Sect. 3), the pressure of remaining FG was
determined to be 17.2 and 7.3 hPa, respectively, while the corresponding
pressures derived from the minimum in pT was slightly lower at
15.2 and 7 hPa, respectively. The differences are rather small,
corresponding to altitude differences of a kilometre or less, as the flow
restriction of our AirCore is low due to the large inner diameter of the
8 mm tube (which carries the largest part of the volume) and the carefully
packed sample dryer. In the following we have therefore only corrected this
effect by adopting the upper sampling pressure during the Lindenberg flights
to the value calculated from the integration of the CO peak of the remaining
FG.
Zoomed in image of CO measurements from the flight on 25 May 2016. After
switching the two position valves to measurement mode. Before switching, the
analyser measures the high CO mixing ratios of the PG, then a decrease in CO
is observed, representing the low CO values of the Cal gas used for flushing
the transfer lines, and finally the CO peak from the remaining FG is measured (note that
PG and FG are taken from the same gas cylinder and thus have identical
mixing ratios). After the remaining PG has passed the analyser,
the CO values drop to the expected low-stratospheric CO values. The red line
shows the CO values measured using the same set-up but analysing an AirCore
filled with CO-free nitrogen. The area between the black CO peak and the red
baseline represents the amount of FG left in the AirCore.
Sampling of ambient air with AirCore
In the second step, we determine the amount of moles sampled at each
time step of the balloon trajectory. Again in the case of slow descent the
assumption of pressure equilibrium between the tube and the sampled ambient
air is justified. Starting at the molar amount determined during step 1 and
adding up over the pressure and coil temperature measured during the descent
of the balloon results in a matrix linking ambient pressure and altitude to
the sampled molar amount. In the case of a faster descent, the assumption of
pressure equilibrium is not completely valid, but as shown in Sect. 2.4.1.
the effect is small. To a first order this is compensated by starting the
summing at the molar amount determined from integration of the CO peak as
described in Sect. 2.4.1.
The approach of summing up the amount of moles in the AirCore during the
flight will also take into account that air from the AirCore can be lost
again if the pressure of ambient air is below that of the AirCore, e.g. in
case the balloon ascends (which can occur for large stratospheric balloons)
or if the AirCore heats up after landing without being closed. The procedure
is thus integrated in time until the moment that the AirCore is closed
either manually or via an automatic closure valve.
Matching AirCore and Picarro data
In the third step we determine the start and ends points in the
measurements of the air mass sampled and stored in the AirCore with the
Picarro (see Fig. 2). This is achieved by fitting a Gaussian curve to the CO
peak from the remaining FG (see Fig. 4). In the case that there is so much FG
left that the CO peak reaches a plateau, the left and right side of the
remaining CO from FG are fitted separately using only one side of the
Gaussian for fitting. The time when the peak reaches half its height is
chosen as the start time. The time derived for the rising CO peak is then
taken as the start time of the FG. The time derived for the descending part
of the peak is taken as the start of the AirCore, i.e. ambient stratospheric
air. We chose to use this second point as the start point and associate it
with the amount of remaining FG determined in Sect. 2.4.1. The determination
of start and end points of the AirCore analysis in the Picarro time
series is critical in correctly assigning the measured mixing ratio to
the sampling location and altitude. The start point of the
AirCore is an especially critical parameter as an offset of 1 hPa will result in a
significant shift in altitude in the stratosphere. The fourth step is then to
calculate the molar amount of air passing through the Picarro and link
this to the molar amount sampled with the AirCore. The link between the molar
amount and the time of measurement is straightforward, as the flow through
the Picarro is regulated to be constant (in our case 40 mL min-1) and
temperature and pressure of the measurement cell of the Picarro analyser
are also controlled.
Correction of mixing ratios for mixing between AirCore and
fill gas
In order to keep the effect of mixing between FG and ambient stratospheric
air small, the FG had mixing ratios close to those expected in the middle
stratosphere. The FG, which is also used as PG, has a mixing ratio of about
407.75 ppm of CO2 and 1228.6 ppb of CH4. On the other hand, the
PG has much higher CO (about 1400 ppb) in order to allow a clear distinction
from both tropospheric and stratospheric air. In particular, stratospheric air
has much lower CO mixing ratios, which are on the order of 20 ppb (Engel et
al., 2006b; Toon et al., 1999). Our measurements showed a gradual decrease of
CO values from the high FG values to the significantly lower stratospheric
mixing ratios. Values as low as 20 ppb were only observed sporadically, with
values around 100 hPa pressure altitude typically being in the 30 ppb
range. This enhancement could either be caused by CO production from the
reaction of ozone with the tubing or it could be a measurement artefact as
the Picarro is not well suited for such low CO mixing ratios. This gradual
decrease from the FG values to stratospheric values is due to a combination
of mixing and diffusion. While mixing is similar for all species, molecular
diffusion depends on the diffusion coefficient and is different for each gas.
The upper part of the profile is stored in the 2 mm O.D. tube. In this tube
molecular diffusion only leads to a very gradual mixing of the two gases (FG
and ambient air). Most of the gradient is due to mixing during the analysis
(the limiting part is the volume of the analyser cell). Therefore, this
gradient can be treated as a gradient caused by mixing and not diffusion, and
the mixing should be similar for all species. We will thus use the large
difference in CO to characterise the fraction of FG in the analysis and
correct the observed mixing ratios of CH4 and CO2 for the remaining
impact of FG. In order to determine the fraction of remaining FG an
assumption must be made on the expected stratospheric mixing ratio of CO. As
we expect that the correction should approach zero once the cell has been
flushed a few times, we have chosen to use the average CO value observed
between 80 and 100 hPa as the expected value. Using this target value the
fraction of FG is calculated from the difference between the measured and the
expected CO mixing ratios and the observed mixing ratio from the Picarro
measurements is corrected accordingly.
Atmospheric observations
The AirCore developed at the University of Frankfurt is sufficiently light to be
flown with a small balloon. However, we performed our first test flights
using large stratospheric balloons launched by CNES from Timmins in Ontario.
Two such test flights were launched in order to compare our results with
those of other groups. The first test flights were launched in 2014. The
results from the first flight in 2014 is reported by Membrive et al. (2016).
Due to a balloon trajectory which was not adapted to AirCore measurements
(long ceiling and long float of the balloon around 20 km in altitude), the
profiles obtained for CO2 from AirCore could not be used to derive mean
age, as the AirCore showed unrealistically low values of CO2 around the
float altitude, possibly due to an interference with the sample dryer
(Membrive et al., 2016). These data are therefore not
discussed in this paper. During a second flight of the same payload as
reported in Membrive et al. (2016), the vertical velocities were much better
adapted and we could derive profiles of CO2, CO and CH4. These are
presented in Sect. 3.1. In Sect. 3.2. we present the first results from our
AirCore measurements at midlatitudes using a small and easy to launch rubber
balloon similar to those used for ozone soundings.
Timmins 2015
The payload launched from Timmins in the year 2015 was very similar to the
one described in Membrive et al. (2016). It consisted of a combination of two
AirCores by the University of Frankfurt, one high-resolution AirCore (Membrive et
al., 2016) and two lightweight AirCores by the Laboratoire de Meteorologie
Dynamique (LMD). In contrast to the Picarro G2401 used by the University of Frankfurt, the LMD team used a G2301 analyser, which lacks the capacity to
measure CO. The payload also included two pico-SDLA spectrometers (Ghysels et
al., 2014; Durry and Hauchecorne, 2005) for measurements of CO2 and
CH4, which are based on in situ infrared absorption measurements.
Results from the latter measurements were perturbed due to thermal drifts
in laser emission wavelength and are not available at the time of writing.
The balloon was launched from Timmins, Ontario, on 22 August 2015 and reached
a minimum pressure of about 11 hPa. In order to reach a zone where a safe
landing was possible, the balloon was left to drift westwards and a slow
descent of the balloon was started in the early morning of 23 August
(08:30 UT). The payload was separated from the balloon just below 100 hPa
pressure and the payload landed at about 10:40 UT. The recovery team was
able to recover the payload such that the analysis could be started about 4 h after landing.
Figures 5–7 show the vertical profiles of CO, CH4 and CO2 as
measured with the Picarro analyser for both AirCores by GUF and in comparison
to the LMD AirCores (only for CH4 and CO2). The altitude
attribution is based on the CO peak as described in Sect. 2.4. One of the
AirCores (AC-2) was operated with the automatic closure valve and thus did
not lose any air while warming up on the ground after landing. The profile
from this AirCore extends to the ground, while other profile (AC-3) ends
higher up due to the loss of air. First of all, two peaks in CO are observed
in the troposphere, which are found at the same altitude for both AirCores.
They could have been caused by biomass burning from wildfires occurring over
western Canada during the observation period. Lowest values of CO in the
stratosphere are of the order of 10–20 ppb, in agreement with expected
steady-state values (Toon et al., 1999; Engel et al., 2006b). AC-3, which was
measured after AC-2 shows an increase in CO mixing ratios above 20 km, which
is most probably due to the longer storage time, resulting in more diffusive
mixing with remaining fill gas.
Vertical profiles of CO derived for the flight on 23 August 2015
from Timmins, Ontario, Canada. The two peaks with enhanced CO around 5 km
in altitude are observed in both AirCores and are probably caused by wildfires
occurring in Canada during August 2015. There are no CO measurements from
the LMD AirCores due to the Picarro analyser used by LMD. The dashed lines
represent the thermal tropopause according to the WMO definition.
Vertical profiles of CH4 derived for the flight on 23 August 2015 from Timmins, Ontario, Canada. Excellent agreement is observed between
all AirCores. There is a slight high bias of the light LMD AirCores above 20 km with respect to both the University of Frankfurt AirCores (red and blue
trace) and the high-resolution (HR) AirCore (green trace). The fine
structures observed by the HR AirCore are smeared out in the lightweight
AirCores by the University of Frankfurt and LMD. The dashed lines represent the
thermal tropopause according to the WMO definition.
Figure 6 shows the vertical profiles of CH4 derived from the five
independent AirCores all mounted on the same gondola. A remarkably good
agreement is observed, as already shown for observations in 2014
(Membrive et al., 2016). As also discussed in
(Membrive et al., 2016) it is obvious that the
AirCore-HR is able to capture fine-scale vertical structures which are not
present in the profiles derived from the lightweight AirCore of the University of Frankfurt nor from the lightweight AirCore of LMD, which have a similar
vertical resolution in the troposphere but a lower vertical resolution in the
stratosphere. The vertical profile of the lightweight LMD AirCore is only
derived up to about 23 km in altitude. Above 19 km the lightweight
LMD AirCore shows some deviations from the high-resolution AirCore and the
University of Frankfurt AirCores. The University of Frankfurt AirCore on the other
hand is capable of capturing some local structure around 21–22 km in altitude,
although the two local minima from the high-resolution AirCore are smeared
out to one broader minimum. Above 23 km there seems to be a small
altitude mismatch between the University of Frankfurt and the high-resolution
AirCore, which is, however, less than 1 km. This altitude discrepancy is
explained by the uncertainty in matching the Picarro measurements to the
AirCore sampling, which is also treated slightly differently in the LMD and
the University of Frankfurt retrieval. In order to compare the values of the
different AirCores, we binned the data into 1 km intervals and then
calculated averages for each AirCore in these bins. In the troposphere
(values between 3 and 13 km in altitude), the standard deviation between these
1 km bins is 1.4 ppb, or 0.08 %. In the stratosphere the deviations are
higher due to the large vertical gradient. Absolute deviations are on average
(between 15 and 24 km in altitude) 11 ppb or 0.75 %. The agreement of the
two University of Frankfurt AirCores is much better (0.17 ppb or 0.01 % in
the troposphere and 3.8 ppb or 0.25 % in the stratosphere), as is that
of the two lightweight LMD AirCores (0.28 ppb or 0.015 % in the
troposphere and 1.6 ppb or 0.1 % in the stratosphere).
Vertical profiles of CO2 derived for the flight on 23 August 2015 from Timmins, Ontario, Canada. The overall structure is captured very
well by all AirCores. Again, a finer structure is obvious in high-resolution AirCore. See text for discussion of the differences between the
different AirCores. The dashed lines represent the thermal tropopause
according to the WMO definition.
The CO2 measurements from the five AirCore are compared in Fig. 7. The
overall shapes of the profiles from the different AirCores show good
agreement. In particular, rather small-scale phenomena are also resolved and
observed in all AirCores. For instance the small-scale structure at around
13 km is observed in all AirCores, again showing that the sampling and
altitude attribution give consistent results. As already discussed by
Membrive et al. (2016), CO2 measurements seem to show more deviations.
However, it should be noticed that the range shown for CO2 is much
smaller than for CH4. In contrast to CH4 the deviations in the
troposphere and the stratosphere are very similar, as the vertical gradient
is similar. In absolute terms, the deviations are typically 0.35 ppm or
about 0.09 %. This deviation is thus on a very similar level to those observed
for CH4 in the troposphere. Overall, this agreement is very good, taking
into account that the different AirCores partly use different data retrieval
algorithms, have different geometries and thus also have different vertical
resolutions. As in the case of CH4, we note that the agreement between
the two University of Frankfurt AirCores is much better (0.04 ppm or 0.01 %
in the troposphere and 0.17 ppm or 0.04 % in the stratosphere), as is
the agreement between the two lightweight LMD AirCores (0.05 ppm or
0.015 % in the troposphere and 0.07 ppm or 0.02 % in the
stratosphere). This shows that the differences are systematic and must be
related to the geometries of the different AirCores and the related
uncertainties in the altitude attributions.
Lindenberg 2016
A first test campaign to study the use of our AirCore using small balloons
was conducted from the Lindenberg Meteorological Observatory, Germany.
AirCores were launched on 20 and 25 May 2016. The balloon used for
the first flight was a TA 1500 balloon. A larger balloon (TA 3000) was used
for the flight on 25 May, which reached a higher ceiling altitude. Ceiling
pressures were 15.2 and 7 hPa, respectively. Large parachutes were used in
order to slow down the descent speed and minimise the effects due to
non-equilibrium of pressure inside and outside the AirCore. For
both measurement flights we were able to recover the AirCore very fast and
start the analysis within an hour after landing. The retrieval procedure was
similar to the one for the flight from Timmins in 2015 with the exception
that we derived the pressure at which sampling began not from the
measurements of ambient pressure but from the integration of the CO peak as
described in Sect. 2.4.1. (diploma thesis Markus Ullrich, the University of Frankfurt, December 2016).
Vertical profiles of CO derived for the flights on 20 and 25 May 2016 from Lindenberg in Germany. The flight on May 25 reached higher
altitudes due to use of a larger balloon. The dashed lines represent the thermal
tropopause according to the WMO definition.
Vertical profiles of CH4 derived for the flights on 20 and 25 May 2016 from Lindenberg in Germany. The flight on 25 May reached
higher altitudes due to use of a larger balloon. The dashed lines represent the
thermal tropopause according to the WMO definition.
Figures 8 to 10 show the vertical profiles of CO, CH4 and CO2 from
the two flights conducted in May 2016. For CO the general agreement between
both flights is very good, even though they are 5 days apart. CO values
are higher than observed in Timmins in 2015. This could either be due to the
use of a different Picarro analyser (note that these values are close to the
detection limits of CO) or to enhanced CO in early spring, e.g. due to
descending mesospheric air during the polar winter. For all species there is
a distinct change at the tropopause, which is observed around 10.4 km in
altitude on 20 May and 10.9 km on 25 May. The decrease in tracer mixing
ratios, especially for CO, is observed at the same altitude as the thermal
tropopause, showing that the altitude attribution as explained in Sect. 2
yields realistic results.
For CH4 and CO this is due to the chemical loss in the stratosphere,
whereas CO2 is very long lived in the stratosphere. The decrease in
CO2 values above the tropopause is mainly caused by the high values of
CO2 in the Northern Hemisphere troposphere during spring, while the air
above the tropopause partly entered through the tropical tropopause and
partly during late summer of the preceding year when tropospheric CO2
values were lower due to the seasonal cycle (Bönisch et al., 2009).
CH4 and CO2 show some fine structures in the stratosphere during
both flights. There is a local maximum in CO2 and CH4 at around
21 km in altitude on 20 May and a similar local maximum is observed on 25 May
at about 20.5 km in altitude. The maxima and minima in CO2 and CH4
are collocated at the same altitude. Therefore, this is clearly a dynamical
feature where CO2-rich (younger) air (see Sect. 4) is advected and at
the same time has higher CH4 mixing ratios. Such air masses would be
expected to occur in the tropics or subtropics. As the dynamical
interpretation of the profiles is not the focus of this paper, this is not
investigated further, for example, by using meteorological data.
Vertical profiles of CO2 derived for the flights on 20 and 25 May 2016 from Lindenberg in Germany. The flight on 25 May reached
higher altitudes due to a larger balloon. The dashed lines represents the
thermal tropopause according to the WMO definition.
Age of air from AirCore
The main aim of our AirCore activities is to determine mean age of air and
use this to extend our long time series of mean age from balloon observations
(Engel et al., 2009). The two tracers most commonly used to derive mean age
are CO2 and SF6. As shown in Engel et al. (2009), the vertical
gradient of mean age becomes rather small at pressures below 30 hPa
(approximately altitudes above 24 km). The mean value of mean age above this
altitude has been used to investigate long-term changes in mean age and in
the stratospheric circulation (Engel et al., 2009). An ideal tracer for the
derivations of mean age should have neither sinks nor sources in the middle
atmosphere and show a monotonous, linear trend in the lower atmosphere (Hall
and Plumb, 1994; Waugh and Hall, 2002). Neither CO2 nor SF6
completely fulfill these requirements (Engel et al., 2009), leading to
uncertainties in the mean age values derived from observations. In the case
of CO2 there are three specific issues which need to be considered:
(i) the source of CO2 in the middle atmosphere due to the oxidation of
CH4, (ii) the seasonal cycle of CO2 and (iii) the deviation of the
deseasonalised long-term trend of CO2 in the troposphere from linearity.
The procedure of calculating mean age and the question of how to take these issues into account
are the same as in Engel et al. (2009) and only briefly summarised here. As
CH4 is oxidised in the stratosphere and thus provides a source for
CO2 in the stratosphere, the amount of CO2 produced from the
oxidation has to be subtracted from the observed CO2 mixing ratio. The
CO2 produced in the stratosphere is derived from the observed CH4
by subtracting the observed CH4 from the deseasonalised tropospheric CH4 at the
time of measurement. In this procedure the fact that CH4 has a
tropospheric trend and takes some time to propagate to the stratosphere is
ignored. The error in mean age due to this simplification is less than half a
month. CO2 has a seasonal cycle in the troposphere which can propagate
into the lower stratosphere (Andrews et al., 2001a, b; Hintsa et al., 1998;
Bönisch et al., 2009; Engel et al., 2006a). Rosenlof et al. (1997) found
that the seasonal cycle in water vapour is observable up to potential
temperatures of about 450 K and termed this region the tropically controlled
transition layer. In the stratospheric overworld (above 450 K potential
temperature) short-term influences, e.g. due to seasonal cycles in the
troposphere or tropopause region are much smaller. CO2 can thus only be
used as an age tracer for air at potential temperatures above 450 K where
the mixing ratios are no longer influenced by seasonality in the troposphere. Our analysis of mean age is thus restricted to potential
temperatures above 450 K. Thirdly, the deseasonalised tropospheric trend of
CO2 in the troposphere deviates from a perfect linear increase. The mean
age derived from CO2 observations will thus depend on the shape of the
age spectrum. To compensate for the effects of this deviation on the mean age
values derived, we again followed the same approach as in Engel et
al. (2009). We use a parameterisation of the width of the age spectrum
Δ as function of mean age Γ as suggested by Hall and
Plumb (1994), i.e. Δ2Γ=0.7 years with
the general shape of the age spectrum being an inverse Gaussian function. We
have further adapted the fitting period for the tropospheric trend so as to
represent 98 % of the air input for each individual data point (i.e.
shorter time periods for the fit are applied for younger air) in order to
find the best possible description of the tropospheric input time series.
The influence of all three effects on the mean age values has been included
in the error analysis, again following Engel et al. (2009).
Vertical profiles of CO2 derived mean age for the AirCore
observations by the University of Frankfurt in 2015 in Timmins, Canada and 2016 in
Lindenberg, Germany.
Time series of mean age derived from balloon observations. The data
prior to 2010 are those presented in Engel et al. (2009). The data from 2015
and 2016 are derived from the AirCore measurements presented here. Each data
point represents the average value of mean age derived above 30 and up to
5 hPa. The inner error bars represent the variability (error of the mean),
and
the larger outer error bars include the uncertainty as discussed in Engel et
al. (2009). A non-significant trend of 0.15 (±0.18) years per decade is
derived from these observations.
Vertical profile observations
Figure 11 shows the mean age profiles for the two flights from Lindenberg in
May 2016 and the two AirCores flown simultaneously in August 2015 from
Timmins. The data have been filtered to exclude air masses with potential
temperature below 450 K where the CO2 seasonal cycle is still expected
to have a significant impact. As for many other profile observations of mean
age (Andrews et al., 2001a; Engel et al., 2009; Schmidt and Khedim, 1991) an
increase of mean age with altitude is observed up to about 23–24 km, above which the vertical gradient becomes very small. Mean age
values above this layer are on the order of 5 years, in very good agreement
with other long-term data sets. The observations from Timmins in August 2015
show slightly higher mean age values than the observations in May 2016 from
Lindenberg in Germany. This could be explained by the seasonal cycle in mean
age derived from MIPAS Envisat observations of SF6, showing youngest air
in the Northern Hemisphere midlatitudes above 25 km during winter and
oldest air during summer (Stiller et al., 2012). The younger spring
measurements from Lindenberg could thus still be influenced from the lower
mean age values during winter, while the older observations from Timmins in
August 2015 should be during the maximum of the seasonal cycle. Note that the
accuracy of the mean age values determined here is not limited by the
analytical precision of the Picarro analyser, which is typically 0.025 ppm – less than a week when translated into mean age.
Extension of long-term time series
As explained above, we want to use AirCore observations to extend our long
time series of stratospheric mean age observations (Engel et al., 2009). The
calculation of a temporal trend in mean age is complicated by the sparsity of
the data set in combination with a vertical gradient in mean age. As the
AirCore observations, in agreement with other balloon data, only show a very
small vertical gradient above an altitude of 23–24 km, corresponding to
about 30 hPa, we have adopted the same procedure as used in
Engel et al. (2009); i.e. we average all data between 30 and 5 hPa and calculate an average value of mean age for this region of the
stratosphere.
The last data point in Engel et al. (2009) was from the year 2005. There is
thus a gap of 10 years between these last measurements and the new AirCore
data. Overall the mean values derived from the AirCore data are in very good
agreement with the values published in Engel et al. (2009). The mean values
for mean age above 30 hPa from the Timmins flights are 4.9 ± 1 and
5.3 ± 1 years, while the Lindenberg flights yield slightly lower mean
age values at 4.7 ± 1 and 4.8 ± 1 years. Note that the
uncertainty ranges include uncertainties in the estimated representativeness,
the derivation of mean age and the observations themselves, based on the
error assessment given in Engel et al. (2009). Using all the available data,
we derive a new estimate of the long-term trend in mean age for the midlatitude stratosphere of the Northern Hemisphere between 30 and 5 hPa. The
time period covered is now more than 40 years (1975–2016), albeit with a
very restricted number of profile observations. The updated trend is now
calculated to be 0.15 ± 0.18 years decade-1. This trend is
smaller than the previous estimate (0.24 ± 0.22 years decade-1),
but agrees within the uncertainty range. The positive trend is not
significant within the 1σ uncertainty range. Due to the reduced
uncertainty of the new estimate, the largest negative trend which would be
compatible with our data within the 2σ uncertainty range remains
nearly unchanged (-0.21 years decade-1 instead of
-0.2 years decade-1).
Summary and conclusion
Observations of stratospheric trace gases are well suited to investigating
chemical and physical processes in the stratosphere. They are also well
suited to investigate long-term changes in the stratosphere. In particular,
for the investigation of long-term changes, high precision and accuracy are
needed in combination with rather low costs (Müller et al., 2016; Moore
et al., 2014). Most in situ measurements in the stratosphere require the use
of large and expensive balloons to carry instruments above altitudes of
20 km. The new technique of AirCore (Karion et al., 2010; Membrive et al.,
2016) is ideally suited to provide such low-cost long-term observations. We
have thus investigated the usefulness of AirCore for investigations of long-term evolution of mean age in order to extend the existing balloon data set
used in Engel et al. (2009) and (Ray et al., 2014). The University of Frankfurt has
developed an AirCore system from three different inner diameter tubes,
targeted at an optimal vertical resolution in the stratosphere, while still
being sufficiently lightweight to be deployed on a small balloon. During an
intercomparison campaign in Timmins, Ontario in August 2015 we compared five
different AirCores. We have shown that the AirCore technique can be used to
derive high-precision vertical profiles of CO2 and CH4. Both LMD
and the University of Frankfurt measurements are referenced to the same scales, but
use independent calibrations. This shows that the results are also of high
accuracy. For both trace gases the comparison between the different
independent AirCores was better than 0.1 % when vertical gradients are
small, as is the case for CO2 in the midlatitude stratosphere above a
pressure altitude of 30 hPa (about 24 km) and for tropospheric
CH4. For CH4 in the stratosphere, where there is a large vertical
gradient the typical agreement was still better than 1 %. The agreement
between two similar AirCores which sampled in parallel was always better
than the agreement between different AirCores when sampling in parallel. This
shows that the geometry, the analysis system and the data retrieval of the
AirCore has a significant impact. We have further performed the first
observations from the midlatitude site of Lindenberg in Germany using small
rubber balloons. Due to careful planning, it was possible to analyse the
AirCores within 1 h after landing. We showed that the CO peak from the
remaining fill gas in the AirCore can be used to derive the maximum sampling
altitude. In the case of the observations from Lindenberg the maximum
sampling altitudes were only slightly lower than the maximum pressure
altitude of the balloon. This is a good indication that the flow restriction
was rather small and that the pressure equilibrium between the tube and
outside air is rather fast. This was achieved in particular due to the use of
a large inner diameter tube for the main volume of the AirCore and by using a
sample dryer which was optimised for minimum flow restriction. Nevertheless,
the altitude attribution of the sampled air remains a difficult issue, in
particular when descent rates are high. The use of balloon techniques
allowing for rather slow descents should thus be considered when setting up
AirCore measurement sites.
We have used the new observations to calculate mean age of stratospheric air.
The results from our AirCore observations are in very good agreement with
previous observations using whole air sampling techniques (Engel et al.,
2009) with values ranging from 4.7 to 5.3 years of mean age above a pressure
altitude of 30 hPa. We have used these data to extend our long-term time
series of balloon-borne mean age observations. This time series now dates
from 1975 to 2016, thus spanning a total of more than 40 years. The long-term
trend of mean age in the Northern Hemisphere midlatitude stratosphere
deduced from this data set is 0.15 ± 0.18 years decade-1. This
trend is smaller than the previous estimate
(0.24 ± 0.22) years decade-1 but remains well within the
uncertainty limit. Based on this analysis, we thus sustain our result that no
significant change in mean age of air for the midlatitude stratosphere of
the Northern Hemisphere can be derived from our data set. Despite the smaller
positive trend derived from this extended data set, large negative changes in
mean age in this region can still be excluded, as the uncertainty on the
derived trend has been reduced. A negative trend in mean age of more than
-0.2 years decade-1 for the middle stratosphere of the Northern
Hemisphere midlatitudes can still be excluded with 95 % confidence.
We conclude that we have shown that AirCore measurements can be used to derive vertical profiles of CO2 and
CH4, even when using
small balloons. These observations are of sufficient quality to derive the mean age of
air and use this to extend the currently available data set of stratospheric
mean age observations. We suggest that long-term observations using AirCore
from a few selected stations covering different latitude bands may provide a
useful tool for investigating long-term changes in mean age. An extension of the
AirCore technique to other tracers gases as suggested by Moore et al. (2014)
may provide a valuable addition as these gases can also be used to study long-term changes in the stratosphere. Müller et al. (2016) suggested that a
long-term network for water vapour measurements in the stratosphere should be
set up to monitor this radiatively important trace gas. We suggest that such
a network could be complemented by AirCore observations. Such additional
AirCore observations would put the observations of changing water vapour into
the general context of a changing stratospheric circulation. In addition, if
both measurements are performed simultaneously, the observations of methane
from the AirCore instruments and H2O from the water vapour network could be
used to derive the sum of 2 × CH4+ H2O, which has been
identified to show much less variability in the stratosphere than H2O.
Data are available from the corresponding author upon
request in NASA AMES format.
The authors declare that they have no conflict of interest.
Acknowledgements
The work of the University of Frankfurt on AirCore has been funded through the
ROMIC programme of the German Ministry of Science and Education (Grant no. 01LG1221) and the EU Infrastructure Project RINGO (Grant agreement no. 730944).
We would like to thank the French Space Agency CNES for balloon operations
in Timmins and the team of Ruud Dirksen from the German Weather Service
(DWD) in Lindenberg. The support of the workshops and technicians at
the University of Frankfurt is gratefully acknowledged. Special thanks go to Huilin Chen from the University of Groningen in the Netherlands for many valuable
discussions on AirCore techniques and an introduction to AirCore
measurements during measurements from Sodankylä. Olivier Membrive was
funded by EIT/Climate-KIC, a body of the European Union.
Edited by: M. von Hobe
Reviewed by: E. Ray and D. Waugh
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