AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-5811-2016Adapted ECC ozonesonde for long-duration flights aboard boundary-layer pressurised balloonsGheusiFrançoisfrancois.gheusi@aero.obs-mip.frDurandPierreVerdierNicolasDulacFrançoisAttiéJean-LucCommunPhilippeBarretBriceBasdevantClaudeClenetAntoineDerrienSolèneDoerenbecherAlexisEl AmraouiLaazizFontaineAlainHacheEmericJambertCorinneJaumouilléElodieMeyerfeldYvesRoblouLaurenthttps://orcid.org/0000-0001-5370-0164TocquerFloreLaboratoire d'Aérologie, University of Toulouse, CNRS, UPS, Toulouse, FranceCentre National d'Études Spatiales, Toulouse, FranceLaboratoire des Sciences du Climat et de l'Environnement, IPSL-LSCE, CEA/CNRS/UVSQ, Gif-sur-Yvette, FranceCNRM-GAME, Météo-France/CNRS UMR 3589, Toulouse, FranceALTEN, Toulouse, FranceLaboratoire de Météorologie Dynamique, University Pierre et Marie Curie/Ecole Poytechnique/ Ecole Normale Supérieure de Paris/CNRS, Paris, FranceObservatoire Midi-Pyrénées, University of Toulouse/CNRS, Toulouse, FranceFrançois Gheusi (francois.gheusi@aero.obs-mip.fr)5December20169125811583216November20152February201621October201628October2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/9/5811/2016/amt-9-5811-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/5811/2016/amt-9-5811-2016.pdf
Since the 1970s, the French space agency CNES has developed
boundary-layer pressurised balloons (BLPBs) with the capability to transport
lightweight scientific payloads at isopycnic level and offer a
quasi-Lagrangian sampling of the lower atmosphere over very long distances
and durations (up to several weeks).
Electrochemical concentration cell (ECC) ozonesondes are widely used under
small sounding balloons. However, their autonomy is limited to a few hours
owing to power consumption and electrolyte evaporation. An adaptation of the
ECC sonde has been developed specifically for long-duration BLPB flights.
Compared to conventional ECC sondes, the main feature is the possibility of
programming periodic measurement sequences (with possible remote control
during the flight). To increase the ozonesonde autonomy, the strategy has
been adopted of short measurement sequences (2–3 min) regularly spaced in
time (e.g. every 15 min). The rest of the time, the sonde pump is turned
off.
Results of preliminary ground-based tests are first presented. In particular,
the sonde was able to provide correct ozone concentrations against a
reference UV-absorption ozone analyser every 15 min for 4 days.
Then we illustrate results from 16 BLBP flights launched over the western
Mediterranean during three summer field campaigns of the ChArMEx project
(http://charmex.lsce.ipsl.fr): TRAQA in 2012, and ADRIMED and SAFMED in
2013. BLPB drifting altitudes were in the range 0.25–3.2 km. The longest
flight lasted more than 32 h and covered more than 1000 km. Satisfactory
data were obtained when compared to independent ozone measurements close in
space and time. The quasi-Lagrangian measurements allowed a first look at
ozone diurnal evolution in the marine boundary layer as well as in the lower
free troposphere. During some flight segments, there was indication of
photochemical ozone production in the marine boundary layer or even in the
free troposphere, at rates ranging from 1 to 2 ppbv h -1, which is
slower than previously found in the boundary layer over land in the same
region.
Introduction
The Chemistry-Aerosol Mediterranean Experiment (ChArMEx;
http://charmex.lsce.ipsl.fr) project aims at an updated assessment of
the Mediterranean atmospheric environment. The Mediterranean troposphere is
indeed particularly rich in aerosol and ozone, especially during the long
Mediterranean dry summer season when concentrations are higher over the basin
than over most of continental Europe e.g.. In
this context, experimental campaigns including airborne observations were
performed in summer 2012 and 2013 in order to document the export of
continental air masses over the basin and their chemical evolution. The
present article focuses more specifically on the set-up of, and first results
from, drifting balloons carrying ozonesondes that were deployed during those
campaigns to perform Lagrangian observations of ozone concentration in the
low troposphere over the basin, following the former experience of
with shorter duration balloons.
The Lagrangian approach in fluid mechanics considers variables in a frame of
reference that moves with the fluid. This is a natural approach for dealing
with gas phase chemistry in the atmosphere .
A Lagrangian volume – hereafter a parcel – is a volume of air
sufficiently small to be coherently transported by the local wind and be
considered (in first order approximation) as isolated from its environment
(that is, no or reduced mass exchange occurs through its boundaries). Thus, a
Lagrangian air parcel can be viewed as a “smog chamber without walls”
.
A constant-volume balloon (hereafter CVB) is generally made of a rigid
pressurised envelope inflated with a mixture of helium and air, so that the
lift balances the balloon weight at a given air density level. A CVB is thus
drifting at nearly zero horizontal velocity relative to ambient air. Under
well-chosen conditions with negligible vertical air motion across density
levels, constant-volume balloons offer a method of performing
quasi-Lagrangian
measurements in the atmosphere. CVBs have been used as Lagrangian tracers as
early as in the 1950s. A first use of Lagrangian balloons for comparison of
turbulence with Eulerian tower-based observations was reported by
. have reviewed
the use of CVB in atmospheric research since that time and have also
discussed their limitations as Lagrangian markers. We only briefly recall
different types of use here.
CVBs have been intensively used as simple trajectory markers to document airflows. For instance, positions from five balloons released together at the
same density level can be used to derive the full kinematics of the flow:
divergence, vorticity and shear and stretching deformations
. During the AUTAN 84 field campaign, CVB trajectories
were used to build an interpolated wind field, the horizontal divergence and
vorticity of which were derived and analysed in relation to orographic
forcing . CVB trajectories launched
during the PYREX campaign (held in 1990) evidenced trapped lee waves downwind
of the Pyrenees . CVB trajectories were simulated in a
mesoscale model through the implementation of an equation describing the
balloon response to the vertical wind. This allowed a direct assessment of
the model performance by comparing the simulated and observed CVB
trajectories, thus coping with the non-Lagrangian character of the balloon
along the vertical .
With the view to measure the chemical evolution of a Lagrangian air parcel,
two strategies are possible: (i) using a CVB as a Lagrangian marker, thus
as a target for a research aircraft operating measurements close to the
balloon at repeated instants; (ii) directly using the CVB as conveyor for
on-board sensors.
The first strategy was used for instance in several Lagrangian experiments
during the ASTEX/MAGE (1992) ACE-1 (1995), ACE-2 (1997) and ICARTT (2004)
airborne campaigns, enabling the calculation of chemical budgets and aerosol
studies in the marine boundary layer and references
therein.
The second strategy (on-board sensors) might be difficult to carry out for
atmospheric gaseous chemistry and aerosol studies, because sensors with
sufficient accuracies (e.g. those used aboard research aircraft) are
generally either too heavy to be transported by small balloons or too
expensive to be lost. Only a few types of lightweight and reasonably
inexpensive sensors exist that are suitable for balloon-borne measurements.
Apart from water vapour, ozone is probably the gas which is most frequently
observed with balloons. It has been measured worldwide on a regular basis
since the 1970s with small sounding balloons and electrochemical sensors,
e.g. in the frame of the GAW, SHADOZ and NDACC networks
. A more experimental alternative was a light
UV-absorption sensor specifically designed for balloon flights, which was
carried out during ICARTT aboard CVB .
The most frequently used ozone sensors for balloon flights are based on the
principle of fast reaction of ozone with iodide ions within an
electrochemical cell. Three types of electrochemical ozonesondes exist: the
electrochemical concentration cell (ECC), the carbon iodine cell, and the
Brewer-Mast sonde and references therein. In this study,
we focus on the ECC type, which is in use in about 80 % of the stations
of the worldwide WMO/GAW ozone sounding network. The total weight of the
flight package is about 1 kg. Therefore, ECC ozonesondes are suitable for
tropospheric flights aboard small CVBs. The lifetime of standard ECC sondes
is, however, limited by power consumption but also by electrolyte evaporation.
For a pump flow rate of 200 mL min-1 (usual value),
indicated a loss rate of about 0.2 mL h -1 (at 25 ∘C,
50 % humidity and sea level pressure). At this rate, the 3 mL cathode
chamber of an ECC sonde would be emptied within 15 h, but in reality, the
sonde performance lowers well before this time because the ozone measurement
is to some extent sensitive to the electrolyte concentration see
e.g.and also the discussion in Sect. .
carried out standard ECC ozonesondes aboard CVB for up to
6 h flights in the boundary layer and the lower free troposphere during two
ESCOMPTE field campaigns in 2000 and 2001, taking place on the French
Mediterranean coast
Note that in 2000–2001, radio
transponders were used for data transmission. Thus, the balloon range was
also limited by radio transmission, despite the deployment of a regional
network of radio receivers during the campaigns.
. From flight segments
during which the balloons remained in the same homogeneous air mass, the
authors were able to quantify quasi-Lagrangian ozone growth rates due to
photoproduction in the polluted summer boundary layer, ranging from 0 to
13 ppbv h -1 around a mean value of 6 ppbv h -1.
As early as in the 1970s, the French space agency CNES (Centre National
d'Études Spatiales) developed constant-volume balloons for long-range
scientific flights in the boundary layer or the low troposphere called
“boundary-layer pressurised balloons”, hereafter BLPBs details on
the recent generation of BLPB are also given
below. The use of satellite
data transmission allows for flights over several days or even weeks.
Clearly, standard ECC ozonesondes are not suited for such long-duration
flights. Concerning power consumption, the ozonesonde lifetime can be
considerably increased by use of high-performance lithium batteries. In spite
of this, the issue of electrolyte evaporation remains. A continuously working
ECC ozonesonde would not be able to cover a complete ozone diurnal cycle.
For this reason, we present a specific adaptation of ECC ozonesondes in this
article, whereby the sonde alternates between short working periods and
longer quiescence periods in order to save electrolyte and increase the
sonde lifetime up to several days. All technical details are given in
Sect. . Laboratory tests presented in
Sect. were preliminary to flights aboard BLPB during three
field campaigns in the western Mediterranean during summer 2012 and 2013.
The flights are detailed in Sect. . The main results are summarised in
the concluding Sect. .
ECC ozonesonde and specific adaptations
In all our experiments, we used commercial En-Sci Z ECC
ozonesondes
Now manufactured by Droplet Measurements Technologies,
Colorado, USA.
, either in their original form for conventional balloon
soundings or in a specific implementation for flights aboard CNES
constant-volume balloons. In the latter case, only a few elements of
commercial En-Sci Z sondes were kept (Sect. ).
ECC ozonesonde general features
ECC ozonesondes developed by are among the most commonly
used worldwide for tropospheric and stratospheric ozone soundings
. Ozone mole fractions
xO3=PO3/P (P being ambient pressure and
PO3 ozone partial pressure) are obtained from the sonde data as
follows:
xO3=R2FTPI-I0Qv,
where R=8.31 J mol-1 K-1 is the universal gas constant,
F=96485 C mol-1 is the Faraday constant, T is the pump
temperature, I is the current measured in the ozonesonde, I0 is the
sonde background current (residual current in absence of ozone) and Qv is
the pump volumetric flow rate
In Eq. (), a conversion
efficiency η=1 is taken, which is the usual assumption with the used
solutions .
. T, I and P are directly measured
on board during the flight, while I0 and Qv are derived from pre-flight
laboratory measurements. Qv is measured with a soap-film flowmeter, from
the time tp needed to fill a control volume V0=100 mL. tp is
usually referred to as the pump time.
The indicates that standard ECC ozonesondes, when operated
carefully, have a precision below 5 % and an absolute accuracy below
10 % in the troposphere. They also review the contributions from each
instrumental variables in Eq. () to the overall uncertainty in
great detail (their Fig. 3-1). In the troposphere, it is clearly dominated by
the uncertainty on the background current I0. The uncertainties of the
other variables together contribute less than 1 % of the ozone mole
fraction value.
To operate the ECC sondes, we mostly followed the GAW standard procedure
detailed in . In particular, we applied no altitude correction
on the background current value determined from ground measurements. As
recommended for En-Sci ECC sondes, we used the “0.5 % half-buffer”
cathode solution . For all flights, we charged
the ECC sonde chambers with 3 mL of cathode solution and 1,5 mL of anode
solution.
A few specific adaptations of the GAW standard operation procedure and
additional measurements were needed, which are listed below.
The procedure recommends to proceed with the advanced preparation 3–7 days
before the flight, which is suited for a single weekly sounding. During the
2012 and 2013 campaigns, intensive observation periods (IOPs) were triggered
upon meteorological alert, during which up to six ozonesondes were launched
within 24 h. A great number of sondes thus had to be prepared without
visibility on the flight date. Consequently, the advanced preparations were
proceeded 2 to 14 days before the flights.
Specific calibration data are available for the 2012 campaign. Measurements
were made during the pre-flight preparation phase, whereby the sonde pumped
air from an ozone calibrator (Ansyco KT-O3M) with scale points at 1.4, 56 and
106 ppbv. In this case, the values of the background current and pump flow
rate used for data processing were adjusted so that the derived ozone mole
fractions fit at best the calibrator scale points.
Unfortunately, no such calibration data are available for the 2013 campaigns.
Nevertheless, surface ozone mole fraction was continuously measured on the
launch site (by means of a TEI 49i UV-absorption ozone analyser). Unlike in
2012, the BLPB ozonesondes worked in continuous mode during the launch and
balloon ascent phases, measuring vertical profiles from the ground up to the
BLPB ceiling level. In such case, I0 was adjusted such that the lowest
data point from the ascent profile matches the analyser ozone reading at
launch time (Fig. ). The same was done to processed data
from the conventional radiosoundings with ECC ozonesondes also performed
during the 2012 and 2013 campaigns.
The only exception was the BLPB flight B55 (in 2013). The ozone data during
the launch and ascent phases were obviously not valid (possibly perturbed by
very high humidity in the lowest troposphere). Thus, the vertical profile was
not used to estimate I0, but instead the ground-based measurement of I0
during the final preparation phase was retained (as recommended in the GAW
procedure).
Specific implementation aboard CNES boundary-layer pressurised balloonsBLPB overview
The CNES BLPB consists of a spherical, non-dilatable and pressurised envelope
filled with a mixture of helium and air Fig. ;
. As the balloon volume and mass are constant,
the BLPB flies at constant-density (isopycnic) level in the atmosphere. The
desired flight level can be adjusted through the total mass of the balloon,
by varying the quantity of gas in the balloon (related to internal pressure)
or the proportions of air and helium. Two possible diameters exist: 2.5 and
2.6 m. The bigger version is used to reach higher altitudes (2000–3300 m
above the launch base).
Schematics of a CNES boundary-layer pressurised balloon and its
various payloads (2013 version).
The data exchange between the balloon and the operation centre is enabled
from anywhere on Earth through the Iridium satellite phone connection. This
allows for long-range flights (possibly several weeks). In the absence of
navigation constraint, the only limitation is the battery autonomy. However,
during the 2012 and 2013 campaigns over the western Mediterranean, flights
were restricted to a delimited zone over sea and over a number of islands for
short transits for security reasons (Fig. ). The flights
automatically aborted when the balloons exited from the authorised flight
zone (or were aborted upon request from the operation centre). To abort the
flight, a heated wire device perforates the envelope. The balloon slowly
loses its gas and softly touches down after a few minutes.
Authorised BLPB flight zone over the western Mediterranean (purple
shading) during the campaigns of summer 2013. During the 2012 campaign,
flights over the Corsica and Sardinia islands were not yet authorised, the
rest of the flight zone being the same. The three launch sites used in 2012
and 2013 (namely Martigues and the Minorca and Levant islands, see
Sect. ) are also indicated in the map.
There are three different payloads aboard a BLPB, which are located either at
the “north” (upper) or “south” (lower) pole of the balloon
Fig. and:
the housekeeping gondola (south pole, inside the envelope) is devoted to
navigation control, communication with the other payloads, and remote data
transmission and control (plus a redundant GPS);
the north pole science gondola (outside the envelope) includes the main GPS,
and weather sensors of ambient temperature, pressure, humidity and global
radiation;
the south pole scientific payload (outside the envelope) is devoted to specific
sensors. During the 2012 and 2013 campaigns, this payload was either the ozone sensor
under consideration in this article or the LOAC
Light Optical Aerosol Counter.
sensor for measurement of aerosol properties not in the scope of the present article, see.
It is seen here that the south pole ozone sensor is located close to the
balloon envelope (air inlet approximately 20 cm below) and the question
arises whether ozone deposition on the envelope could perturb measurements.
To answer this question, a test was conducted in which the air inlet of a UV
ozone analyser was equipped with a sampling head made of the balloon envelope
material (an inextensible polymer). The sampling head consisted of a thin
cone (length of about 25 cm and maximum diameter of 2–3 cm) through which
ambient air flowed before entering the teflon air inlet of the analyser. No
detectable change in the analyser ozone reading was observed with or without
this sampling head. No significant perturbation is thus to be expected owing
to the balloon proximity.
Ozone payload
In the specific implementation of ECC ozonesonde for BLPB, the motor, the
pump, the electrochemical cell and the teflon tubing of original En-Sci Z
sondes were disassembled, then remounted onto an entirely new electronic
card. Compared to the standard electronic implementation of ECC sondes, the
major specific features are the following.
In standard Z sondes, the electronic card and the pump motor are powered
by separate batteries (at 9 and 12 V). In the BLPB implementation,
both motor and electronics are powered by a single lithium 3.6 V battery (Li-SOCl2).
The motor voltage is electronically multiplied up to about 10 V. Its rotation speed is a bit
lower than under nominal voltage (12 V), but this affects the pump flow rate by only a few percent.
The motor is switched on or off by electronic command following a programmable sequence described below.
The measurement cycle (Fig. ) is characterised by three
different time parameters. T0 is the overall period of the cycle, T1 is
the duration of a warm-up phase, and T2 is the duration of the measurement
phase. Two current values are recorded during the warm-up phase to check how
fast the current reaches its asymptotic value (Fig. , cyan
points). During the measurement phase, current intensities are regularly
recorded (Fig. , blue points). The Ti values mostly (but
not always) used during the 2012 and 2013 flights were T0=900 s
(15 min), T1=60 s and T2=120 s. This choice was inferred from
laboratory tests presented in Sect. .
Example (taken from the laboratory test detailed in
Sect. ) showing three measurement cycles of the BLPB
ozonesonde. In this illustration, the parameter values are T0=900 s,
T1=60 s and T2=60 s.
12 May 2011 ground-based experiment: (a) comparison between
the reference (SRef) and experimental (SExp) sondes,
with the colour indicating the time since the last motor restart. Data before
60 s were discarded in this analysis. The solid line is the least square
linear model and the dashed line the 1:1 fit. (b–d) Time series
of ozone mole fractions provided by the UV analyser (red) and retrieved from
currents in the reference (black) and the experimental (blue) ECC sondes. The
background shading indicates the time intervals when the sonde motor is on.
The bullets indicate the mean ozone values and the bars the standard
deviations over each working period (excluding the first 60 s). The
durations of the quiescence and working periods were equal, but changed at
different stages of the experiment: 10 min (b), 5 min (c)
and 3 min (d).
The sonde can also work in continuous mode as in a classical sounding to
profile the lower atmosphere during the BLPB ascent. In this case, current
intensity data are recorded every 10 s. The continuous mode was available
only during the 2013 campaigns (Sect. ). Once the balloons
had reached their ceiling altitude, cruise Ti values were set by remote
control from the operation centre.
Laboratory testsFirst tests in intermittent modeOzone current establishment
A first experiment was conducted on 12 May 2011 at a fixed outdoor place to
investigate the behaviour of an ECC ozonesonde (the experimental sonde,
hereafter referred to as SExp) with alternating quiescence (sonde
motor off) and run (sonde motor on) phases, and to evaluate its ability to
reproduce correct ozone mole fractions against reference measurements. Each
run–quiescence sequence was composed of two 10 min, 5 min and 3 min
periods during three stages of the test.
The reference measurements were (i) a standard (En-Sci Z) ECC ozonesonde
working as usual in continuous mode (the reference sonde, hereafter
SRef) and (ii) a UV-absorption analyser (TEI 42i). The expected
absolute accuracy is below 10 % for the reference sonde in the
troposphere , while it is better than 3 ppbv for the UV
analyser
The latter value was obtained combining (as rooted sum of
squares) the uncertainties given in for the analyser
measurement itself (1.2 ppbv) and the calibration chain (2.3 ppbv).
. The
background current value of SRef had to be adjusted to compensate
for an obvious bias of -5 ppbbv with respect to the other data sets
(presumably caused by incorrect measurement of the background current for
this sonde.)
12 May 2011 ground-based experiment. Deviation of SExp
data from the values predicted by the linear model (in relative value with
respect to SRef data), as a function of time elapsed since the
last restart of the motor. The red dashed line marks 60 s (warm-up time
adopted thereafter). The horizontal dashed lines mark deviations of
-10 %, 0 and +10 %.
We first compared the SExp values taken at least 60 s after
motor start to the reference sonde (Fig. a). A good
agreement was found between both ECC sondes (r2=0.93; root mean square
of the difference: (xExp-xRef)2‾1/2=0.7 ppbv). The comparison of both sondes with the UV analyser along the
course of the test is shown in Fig. b–d. The ozone
mole fractions are in fair agreement with each other (within 10 %), even
for the shortest alternation period of 3 min. Between 15:30 and 16:00, the
UV analyser was up to 5 ppb lower than the ECC sondes (no obvious
explanation for this discrepancy); nevertheless the two ECC sondes remained
consistent with each other during that interval.
It was also seen in this experiment that every time the motor restarted, the
ozone current rapidly grew from around zero to values corresponding to mole
fractions comparable to those of the two other instruments. The current
establishment is investigated quantitatively from the data presented in
Fig. . The linear model presented in
Fig. a provided a predicted SExp ozone
value for each SRef value. Real data from SExp were
then compared to the prediction, as a function of time elapsed since last
SExp motor restart (Fig. b). After a
rapid growth phase, a ±10 % agreement is achieved within a few tens
of seconds. This is not surprising since ECC ozone sensors are known to have
a response time to a step change in ozone of 20–30 s . In
the following, we therefore adopt a warm-up time T1=60 s after every
motor restart before considering measurements as valid.
Pump flow in warm-up regime
Pump flow rates are usually measured (during the pre-flight preparation) with
the motor having been running for a few tens of minutes. In contrast, BLPB
ozonesondes are designed to work for a few minutes between longer periods of
quiescence. We carried out a laboratory experiment to investigate whether,
after some period of quiescence, the pump flow rate of an En-Sci Z ECC sonde
varies in the first minutes after restart. To this goal, we made measurements
with a soap-film flowmeter, but in a timed way with respect to the instant of
motor start. As a single measurement is not instantaneous and takes about
30 s, we consider the middle of the interval as the measurement date.
Pump flow rate evolution after sonde motor restart (t=60 s
corresponding to the beginning of the measurement phase). Each data series is
differentiated with colours. (a) Volumetric flow rate
(L min-1). (b) Relative deviations (in %) from the flow
rate interpolated at t=60 s after restart.
31 May–6 June 2012 ground-based experiment. (a) Time
series of ozone mole fraction from the UV analyser (grey curve: 10 s
averages (analyser raw data); black dots: 60 s averages synchronised with
the BLPB ozonesonde data) and the BLPB ozonesonde (blue dots; bars represent
the standard deviation of the data recorded during each measurement phase).
(b) Comparison of the ozone mole fractions from the BLPB ozonesonde
against the data from the UV analyser (synchronised 60 s averages). The
numerical results of a linear regression (solid line) are given in the figure
panel. The dashed line represents the 1:1 correspondence.
Seven measurement sequences were conducted with the same motor and pump.
Between each sequence, the motor was quiescent for at least 5 min. The
obtained flow rates range between 0.206 and 0.212 L min-1
(corresponding to an overall dispersion of less than 3 %).
Figure shows the evolution (relative variation) of the
pump flow rate during a few tens of seconds after motor start. Globally,
there is a decay of the pump flow rate within the first 2 min. The decay is
in the range 0–1.7 % between 60 s (the beginning of the measurement
phase) and 120 s.
This might induce comparable variation of the ozone current in the sonde cell
for a given ozone concentration in air. If the ozone mole fraction is
retrieved from ozone current measurement using a constant value for Qv
(Eq. ), the result might be affected in the same way (about
3 %) due to unmeasured flow rate variation during each measurement phase
or between separate cycles. This is, therefore, a source of uncertainty that
adds to those already reported in the literature concerning the flow rate
determination. For instance, reports ±1 % of
uncertainty in the flow rate measurement by soap-film displacement technique.
For a future version of the BLPB ozonesonde, it would be interesting to
develop an on-board measurement of the flow rate – providing a sufficient
accuracy (less than 1 %) could be achieved with a light sensor.
In the present study, we will continue to use constant flow rate values
determined as usual. However, a 3 % uncertainty on ozone mole fraction
attached to flow rate variation during the BLPB sonde work phase should be
kept in mind.
Long-duration test in realistic conditions
We present here a ground-based experiment conducted from 31 May to
6 June 2012 to test the ability of the BLPB ozonesonde to monitor the
evolution of ozone in the boundary layer over several days. At this stage of
development, the sonde version was the same as those that flew a few weeks
later over the Mediterranean during the 2012 campaign (see
Sect. ). This experiment also provided an opportunity to
test the Iridium satellite connection. A TEI 49i UV-absorption ozone analyser
was again deployed in the vicinity of the BLPB ozonesonde.
The values for the sonde measurement cycles were T0=900 s (overall
period), T1=60 s (warm-up phase) and T2=60 s (measurement phase).
During each measurement phase, 12 values of ozone current were recorded, then
converted in ozone mole fractions using values measured during the sonde
preparation for the background current (I0=0.13µA) and the a
pump time (tp=32 s per 100 mL). The ambient pressure and
pump temperature were measured live by the working sonde. The 12 mole fractions were
finally aggregated into a single mean value (and the corresponding standard
deviation) available every 15 min.
The time series of ozone mole fraction from both instruments are shown in
Fig. a. The BLPB ozonesonde was able to provide realistic
measurements with respect to the UV analyser (±10 %) along almost
its entire lifetime, i.e. 5 days. In particular, the ozone diurnal cycles
occurring during these sunny days were well captured, as well as variations
on shorter timescales (e.g. on 3–4 June 2012). The linear correlation
between these measurements is fair (Fig. b: r2=0.88;
bias: -3.3 ppbv; standard deviation of sonde minus analyser: 5.6 ppbv).
The bias is certainly due in most part to the uncertainty on I0.
The sonde lifetime was limited by cathode solution evaporation. In our
experiment (T0=900 s; T1=T2=60 s ), the sonde worked for 3.2 h per
day. At the evaporation rate reported by (about 0.2 mL
per work hour for a pump flow rate of 200 mL min-1), the solution
would have completely evaporated in 4.7 days. This is consistent with the
duration of our experiment.
31 May–6 June 2012 ground-based experiment. Evolution of the sonde
behaviour over its lifetime. (a) Spin-up phase: for each measurement
cycle, the represented values are percentages of the eventually established
ozone value (see text for detailed definition). Grey dots represent the value
measured 20 s after motor start; black dots represent the value measured
40 s after motor start. Lines represent linear regressions over each data
set. (b) Deviation of the established ozone value from the UV
analyser as a function of time. The line again represents a linear
regression.
It is interesting to focus on the warm-up phase of each measurement cycle,
and its evolution through the sonde lifetime. For each sonde measurement
cycle, we compare the values measured by the sonde 20 and 40 s after motor
start, with the mean of the 12 mole fractions recorded during the measurement
phase (established current) between 60 and 120 s after motor start
(Fig. a). During the first day, the ozone current reaches
60 % (or 90 %) of the established value 20 s (or 40 s) after the
motor start. This is consistent with the result presented in
Fig. b. Over several days, both percentages are seen
to grow with time. By the end of the experiment, the 20 s values are near
90 % and the 40 s values above 95 %. This means that the response
time is shortening or, in other words, that the ozone sensor tends to be
faster. This is due to progressive evaporation of the electrolyte in the
cathode chamber. A current is induced in the electrochemical cell when some
imbalance is created due to I- oxidation by ozone, enhancing iodine
concentration in the cathode solution . It takes more time
to reach a given I2 excess concentration if the solution volume (and hence
the total amount of ions to oxidise) is large. As a result, the ozonesonde
response time is an indicator of the cathode solution level, and to some
extent could be used as electrolyte gauge for long-duration flights.
Figure b also shows an evolution through the sonde lifetime
of the absolute deviation from the UV analyser reference. This deviation
shows a positive trend at a rate of +1.63 ppbv day-1 (≈+0.07 ppbv h -1). This drift could be linked to cathode electrolyte
evaporation, which tends to increase the ion concentrations in the solution.
In our experiment, in which the sonde ran until almost complete evaporation,
the cathode concentrations might have doubled at half time, i.e. after about
2.5 days. From the JOSIE 2000 experiment, reported for ECC
sondes 5 % higher ozone values when using cathode solution concentrations
twice those of the 0.5 % half-buffer solution.
Another possible cause of measurement drift is the long-term drift of the
sonde background current. provided evidence that after a
brief period of fast decay (time constant of about 20 s), the background
current in an ECC sonde goes on decreasing slowly as the sonde runs (slow
decay with a time constant of 20–30 min). Using constant I0 instead of
actually decreasing background current should lead to underestimated ozone
mole fractions, hence to a negative trend. For instance, a decrease of I0
by 0.05 µA day-1 would lead to
an ozone trend by -0.08 ppbv h -1.
A greater number of similar long-duration tests against a reference
measurement would be needed to characterise the drift and to demonstrate the
links with either evaporation or background current drift – or a combination
of both. The present experiment at least suggests that ozone trends at rates
lower than a few ±0.1 ppbv h -1 should be considered cautiously,
but trends at rates well above this value should not be measurement artefacts
but real tendencies.
BLPB ozonesonde flights over the MediterraneanOperational overview
Sixteen BLBP flights equipped with an ozonesonde were launched in the low
troposphere over the Mediterranean Sea during three field campaigns of the
coordinated project ChArMEx (http://charmex.lsce.ipsl.fr):
TRAQA
French acronym for TRAnsport and Air Quality.
in summer 2012;
ADRI-MED
Aerosol Direct Radiative Impact in the MEDiterranean.
and
SAFMED
Secondary Aerosol Formation in the MEDiterranean.
, in
summer 2013. Each campaign had its own launch site (Fig.
and Table ) selected for both scientific and practical
reasons. All launch sites were located either on a coast or an island to
avoid flight over inhabited areas just after launch. TRAQA and SAFMED
were devoted to anthropogenic pollution transport and
chemistry. The launch sites (Martigues and Levant) were located on the French
Mediterranean south-east coast, which is a densely inhabited and
industrialised area and, therefore, a major source of pollution in the
western Mediterranean basin. The BLPB density was tuned for low-altitude
flights in the marine boundary layer or the lower free troposphere
(300–900 m, Table ). ADRIMED focused
mainly on the aerosol optical properties, especially in the case of dust
transport from the Saharan desert in the free troposphere. A possible
influence on ozone was also investigated. The chosen launch base was on the
island of Minorca. BLPB flights were performed at higher altitude
(2000–3000 m, Table ).
Launch bases during the 2012 and 2013
campaigns.
CampaignPeriodLaunch baseGeo. coord.TRAQA25 Jun –Martiguesa43∘19.96′ N15 Jul 20125∘05.22′ EADRIMED10 Jun –Minorcab39∘51.98′ N6 Jul 20134∘15.30′ ESAFMED22 Jul –Levantc43∘01.31′ N7 Aug 20136∘27.61′ E
a South-eastern France, Mediterranean coast.
b Sant Lluís, Minorca, Spain. c Island off
Hyères, south-eastern France, Mediterranean coast.
Details on the 16 ozone BLPB flights launched during the 2012 and 2013
campaigns. Note that 13 other BLPBs with LOAC payload were also launched during
these campaigns . Those flights are not in the scope of the
present article.
a EXIT is the limit of authorised flight zone reached; BLPB is the flight
aborted owing to balloon failure; O3 is ozonesonde failure but the flight
went on. b Maximum trend established over at least 4 h between sunrise and sunset. c See text
for details. d These values correspond to the time of the ozone record end, but B61 went on
further for about 10 h and 78 km south-eastwards.
Trajectories of the 16 ozone BLPB flights launched in 2012 and 2013.
The colour code (left box) represents the ozone mole fraction (in ppbv)
measured along the trajectory. The dot size represents the solar local time
(right box).
Table summarises the overall performance achieved by the
ozone BLPBs during the campaigns. Except in three cases (namely B53, B62 and
B69), all flights were terminated when they reached the limit of the
authorised flight zone (Figs. and ).
B53 (from Minorca) was prematurely aborted because of remote connection
failures. B62 and B69 (from Levant) were aborted owing to uncontrolled fall
below a critical flight altitude (200 m, defined for safety reasons – risk
of a sea surface touchdown which could damage the navigation gondola and
render the balloon out of control). This occurred at night for both flights
and might be caused by condensation on the envelope which weighted the
balloon. For all flights (including B53, B62 and B69) except B61, the
ozonesonde worked well until the end of the flight. In the course of flight
B61, the ozone signal was suddenly lost after a
turbulence
GPS-derived balloon vertical velocity showed quick
variations with 30 s averaged values larger than 1 m s-1.
area over
the crests of Cap Corse (the elongated mountain chain forming Corsica's
“index finger”), but the ozonesonde gave no sign of anomaly before that
time. The other payloads on B61 went on working well for hours. The BLPB
flight durations and ranges are reported in Table . In most
cases, the BLPB ozonesondes provided data over the full flight durations,
which are well beyond the lifetime of classical ozonesondes report
no ozone records longer than 6 h.. The ability to cover a full
diurnal cycle was demonstrated on the occasion of several favourable
trajectories (B55, B57, B62, B64 and B69).
In-flight validationsBLPB ascent profiles compared to conventional ozone soundings
During the 2012 and 2013 campaigns, conventional radiosoundings including ECC
ozonesondes were operated in addition to BLPB launches. Some of them were
launched sufficiently close in time to BLPBs (namely, B53, B54, B61 and B69)
to allow for comparisons of the ascent profiles. Two other BLBPs (B64 and
B65) were launched simultaneously and compared to each other. Such profile
comparisons were only possible in 2013 because, before that time, the BLPB
ozonesondes did not allow for continuous working mode during the launch and
ascent phases.
Ozone vertical profiles from radiosoundings and BLPBs during the
2013 campaigns. Note that the altitude range is not the same in
panels (a–b) (3000 m a.s.l.) as in panels (c–e)
(800 m a.s.l.). In all panels, dots represent BLPB measurements
every 10 s (in continuous mode) while radiosounding data are represented as
solid lines. The durations (in min) indicated in each panel give the time
needed for each balloon to reach an estimated ceiling altitude indicated by
the horizontal dashed line. Triangles represent surface ozone readings (UV
analyser) at the times of BLPB launches. All useful times (UTC) are specified
in figure legends.
Those radiosoundings and BLPB ascent profiles are displayed in
Fig. . Globally, balloons launched sufficiently close in
time (typically an hour or less) reveal very similar ozone profiles, whatever
the type of balloon (BLPB or conventional sounding balloon) or ozonesonde
(adapted or conventional ECC). This illustrates the correct behaviour of the
adapted ECC sondes when used in continuous mode. It is also an indication
that the proximity of the BLPB envelope from the sonde air inlet does not
significantly perturb the ozone measurements even during the balloon ascent,
which would be the worst configuration since the sonde is in the wake of the
balloon. Note, however, that these comparisons cannot be considered
validation elements for the intermittent working mode used during the BLPB
cruise at ceiling levels.
BLPBs at ceiling
Once the BLPBs had reached their ceiling level, it was difficult to carry out
ozone measurements specifically to validate the BLPB ozone data. We
nevertheless tried to compare these data to other concurrent ozone data
whenever possible.
During TRAQA, it was attempted to arrange in-flight rendezvous between the
BLPB and the French research aircraft ATR42
SAFIRE research service:
http://www.safire.fr.
, which was equipped with a UV-absorption ozone
analyser among many other sensors –. This was
especially challenging owing to many constraints in the airspace over the
western Mediterranean and the inability to control the balloon trajectories.
Nevertheless, the aircraft managed to fly as close as possible to the
balloons on rare occasions. This was the case on 6 July 2012. Two BLPBs (B08
and B06) were launched from Martigues in the early morning (at 02:37 and
04:46 UTC respectively), and followed similar trajectories toward Corsica
(Fig. a). They eventually reached the island in the evening.
B06, in particular, flew very close to the Ersa research station, where a
UV-absorption ozone analyser (type Thermo 49i) was in continuous operation.
The station is situated on a mountain crest at an altitude (533 m a.s.l.)
close to the balloon flight level (500–550 m a.s.l. during the last flight
hour).
(a) Blue and cyan curves: trajectories of BLPBs B06 and B08
launched from Martigues on 6 July 2012 during TRAQA. Red and magenta curves:
sections of the ATR42 research aircraft trajectory during two selected time
intervals corresponding to rendezvous with the balloons. The balloon
trajectories are broadened during these time intervals in order to indicate
their location. (b) Ozone time series on 6 July 2012 from different
measurements: BLPBs B06 and B08 (blue and cyan thick curves); ATR42 aircraft
at the time of the rendezvous (red and magenta curves); Ersa station surface
measurements at 533 m a.s.l. (grey curve). The station location is
indicated in panel (a). The balloon, aircraft and station altitudes
are also represented as thin curves (same colour code and right-hand scale).
The different ozone time series are shown in Fig. b. Between
03:00 and 06:00 UTC, B08 recorded questionably low ozone mole fractions (data
filtered out in Fig. b). In the mean time, the balloon
altitude dropped by about 100 m (Fig. b). It appeared from
the balloon's humidity data (not shown) that B08 encountered wet conditions
(relative humidity above 80 %). Possibly, water condensation on the
balloon envelope and the sensors affected the balloon weight and the
ozonesonde (e.g. water droplets sucked in the pump) – but we have no
definitive evidence of this. After sunrise, however, B08 seemed to again
provide reliable data.
First, it is interesting to see that B06 and B08 stayed close to each other
all their way (horizontally – less than 30 km – as well as vertically –
Fig. b), and that their ozone time series are in fairly good
agreement. A second validation element is the consistency of the BLPB data
with the aircraft measurements, especially during the rendezvous of the
aircraft flight #27, when the aircraft flew very close to B06 (balloon in
eye contact, as reported by the aircraft passengers). Lastly, ozone data from
B06 and from the Ersa surface station agree fairly well by the end of B06
flight (around 20:00 UTC) when the balloon got close to the station.
Another interesting case for validation purpose is BLPB flight B61 on
29–30 July 2013 during SAFMED (Fig. ). The balloon was launched
from the island of Levant in the evening of 29 July. It flew toward Corsica
at levels between 400 and 500 m a.s.l. and reached the island's west coast
after about 3 h. Then, obviously under the effect of a flow-around regime
near the island, the balloon flew north-eastward along the coast,
experiencing turbulence and strong altitude variations. B61 touched Cap Corse around 03:30 UTC. The balloon crossed the mountain chain 7 km south of
the Ersa station. Unfortunately, B61 stopped transmitting ozone data after
this time, but the balloon nevertheless went on further between Corsica and
Italy for 10 h more (not shown in Fig. ). The ozone surface
record at Ersa (Fig. b) shows a homogeneous air mass all evening
and night long, with mole fractions in the range 35–40 ppbv, in fair
agreement with B61's ozone record.
B61 flight on 29–30 July 2013. In both panels, the rainbow
colour scale (box in panel a) represents time (UTC) at different
points of the balloon trajectory. (a) B61 trajectory.
(b) Ozone time series from BLPB B61 (blue curve) and Ersa surface
measurements (grey curve). The cyan curve represents the balloon altitude in
m a.s.l. (cyan axis). The cyan horizontal dashed line marks the altitude of
the Ersa station (533 m a.s.l.). The station location is indicated in
panel (a).
In conclusion from these comparisons, the BLPB ozonesondes demonstrated an
ability to provide ambient ozone mole fractions over the Mediterranean with
an accuracy of about 10 %.
General findings on ozone
Ozone levels recorded over the sea during BLPB flights are globally in the
range 20–80 ppbv (Table and Fig. ).
This range hides a variety of situations with different ozone backgrounds,
but a general feature is that relatively weak diurnal variations were
observed compared to usual ground-based observations in the summer
continental boundary layer (as illustrated in Fig. a).
During a given BLPB flight, the amplitude of ozone change generally did not
exceed 20 ppbv
Greater amplitudes are reported in
Table for some flights, but the reported values include
outliers.
. This contrasts also with the results reported by ,
who investigated the pollution plume transported from New York City over the
Atlantic during the 2004 ICARTT campaign, by means of ozone sensors aboard
“smart balloons” (a type of constant-volume balloons used by this research
group). They found high spatial and temporal ozone variability in the
pollution plume at low level (∼ 500 m) over the ocean (e.g.
variations exceeding 80 ppbv within 10 km and 15 min). They attributed
this variability to the patchiness of the ozone field in the plume, where
small pockets of high concentrations could result from a combination of
factors including strong daytime ozone photoproduction and transport at
small-scale.
Our BPLB ozone measurements over the Mediterranean during the three ChArMEx
campaigns showed no such high variability. An explanation could be that no
major ozone pollution episode occurred over the pollution source area during
the ChArMEx campaigns, contrasting with the situations investigated during
ICARTT. In addition, the explored environments were quite different (in terms
of geographical area, local climate, sea surface temperature, weather
conditions, etc.). Among those differences, highly contrasting weather
conditions were encountered. While the ChArMEx flights occurred mostly in
calm, fair weather conditions, the weather during ICARTT was atypically
changing and complex for the summer season. For all those reasons, comparable
results were not necessarily expected in terms of ozone variability.
Nevertheless, evidence of ozone photochemistry over the Mediterranean could
be found during some BLPB flights. investigated whether
Lagrangian photochemical ozone production could be evidenced and quantified
from constant-volume balloon (CVB) measurements carried out in 2000 and 2001
during the ESCOMPTE project. The authors identified CVB trajectory sections
in which the considered balloon clearly drifted inside the same air mass.
Ozone change in this air mass might be due to ozone chemistry, but also to
vertical turbulent transport. The latter might be strong, especially near the
top of the boundary layer, where large vertical gradients of ozone and other
atmospheric species – especially water vapour – are often encountered
e.g.. An ozone
trend in this case is likely to coincide with a trend in specific humidity as
well. Conversely, constant specific humidity is an indication that the
balloon flew in a well-mixed air mass, and ozone change in this case is more
likely related to in situ chemistry.
Such an analysis has been conducted based on the ozone and specific humidity
time series from the 2012–2013 BLPB flights. Ozone trends during the daytime
over intervals of at least 4 h were observed for 12 flights out of 16
(Table ). Ozone mole fraction increased in a majority of
cases (9 out of 16). No obvious trend was found in four cases. Ozone decrease
was observed in three cases (B57, B65, B69).
For seven flights (namely B06, B08, B10, B55, B57, B59 and B62), specific
humidity (as well as potential and equivalent potential temperatures) was
found to be almost constant over the considered time intervals; therefore the
ozone trend can be likely attributed to ozone chemistry. Of these flights,
six showed ozone build-up at rates ranging from 1.2 to 2.2 ppbv h -1.
Such values are lower than those reported in , who found a
mean growth rate of +6 ppbv h -1 in the case of ozone production.
During ESCOMPTE, most CVB flights took place in the continental boundary
layer. This makes a major difference with over-sea flights, since the
continental boundary layer is constantly supplied with ozone precursors
(nitrogen oxides and volatile organic compounds) from the surface. The
Marseilles area is especially favourable to ozone production. The CVBs during
ESCOMPTE were launched from industrial or urban sites, and it is likely that
the air masses were initially rich in nitrogen oxides. Then the balloons were
transported over the rural hinterland, where emissions of biogenic volatile
organic compounds from the Mediterranean vegetation are strong in summer
. This forms the cocktail for explosive ozone production in
the boundary layer, as observed by . The 2012 TRAQA and 2013
SAFMED BLPBs were launched from the same area as during ESCOMPTE, but wind
conditions were chosen for flights over sea. In such conditions, the initial
precursor concentrations in the air mass are potentially similar but no
further supply of precursors is expected from the sea surface. This may be an
explanation for slower ozone growth in the air mass. Another point is that no
major pollution episode was encountered in the area during TRAQA and SAFMED,
unlike what was observed during ESCOMPTE, and this might bias the
comparison
Considering the likely cases of ozone photoproduction
(Table , flights B06, B08, B10, B55, B59 and B62), the mean
ozone mixing ratio recorded during these flights is 45 ppbv. A similar
estimation from their Table 3, cases 1, 3, 7, 10–12, 15, 18–20,
22, 26–28 yields 60 ppbv.
. B63 is the only flight that
exhibits rapid ozone increase (+6.5 ppbv h -1) between 12 and
16 h UTC, but this is associated to large specific humidity variations as
well as a balloon descent of about 100 m. Hence, it is not obvious to
conclude in situ ozone production. At least part of this growth might be
attributed to turbulent transport or to the fact that the balloon sampled
different layers in the meantime (the balloon not being Lagrangian along the
vertical).
A remarkable case of in situ ozone production was found during flight B55
from Minorca. Even though the ozone growth is relatively slow
(+1.2 ppbv h -1), it occurs at high altitude (2400–2500 m) in the
free troposphere. Prior to our study, reported ozone
production in a free tropospheric air mass downwind of a tropical convective
cell, also measured by an ozonesonde. In this case, however, the air mass
tracking was not intentional and resulted by chance from vertical
oscillations of a conventional sounding balloon in up- and downdraughts.
Flight B55, in contrast, was intentionally designed to follow a Lagrangian
trajectory in the free troposphere. The flight is presented in more detail in
Sect. .
B57 is the only case of ozone decrease likely related to in situ destruction
(in the free troposphere, again). The other two cases of decrease (B65, B69,
both at low altitude) are more ambiguous, owing to a larger variability in
specific humidity.
Remarkable flightsLow-altitude flight B62
B62 (Fig. ) is an especially interesting flight, which covered
almost a full diurnal cycle. Its remarkable trajectory passed between the
Corsica and Sardinia islands, and the flight revealed interesting features of
the Mediterranean lower troposphere. The specific humidity time series allows
us to clearly distinguish four flight sections during which it remained
roughly constant (Fig. b). It can be assumed that the balloon
sampled the same air mass inside each flight section; therefore the time
evolution of the measured variables can be considered quasi-Lagrangian.
B62 flight launched on 30 July 2013 at 02:59 UTC. In both panels,
the rainbow colour scale represents time (UTC) at different points of the
balloon trajectory. Black square marks delimit 4 flight sections numbered 1
to 4 (see text). (a) B62 trajectory. (b) Time series from
B62 measurements: ozone mole fraction (black dots; bars represent one
standard deviation around the mean value during the measurement phase – see
Sect. ); air specific humidity (blue line and related
scale in g kg-1); balloon flight altitude (cyan line and related scale
in m a.s.l.); incoming shortwave irradiance (red line and related scale in
W m-2).
Flight section 1 occurred in the late night and early morning (B62 launched
at 02:59 UTC) and ended around 06:30 UTC. Once the balloon had reached
its ceiling altitude, it oscillated between 400 and 500 m a.s.l. The
specific humidity also fluctuated between 6 and 8 g kg-1, and, to some
extent, mirrored the ozone variations. It may be concluded that the balloon
flew in a turbulent air mass where vertical gradients of both humidity and
ozone existed – probably near the top of the marine boundary layer. As
specific humidity is most likely to decrease with height, and humidity and
ozone variations are opposite in this case, higher ozone concentrations seem
to be present aloft. Ozone shows no overall trend over flight section 1, and
no chemical evolution is to be expected.
Flight section 2 occurred between 06:30 and 10:40 UTC. This new air mass was
significantly more moist (11–12 g kg-1) than the previous one. The
flight altitude again showed fluctuations but specific humidity remained
almost constant, indicating a turbulent well-mixed layer – the marine
boundary layer. In the meantime, ozone concentration showed a linear increase
of +1.4 ppbv h -1. This is a clear indication of ozone
photochemical production in the marine boundary layer.
By the end of flight section 2, B62 accelerated (from 8 up to 15 m s-1
– not shown) while passing between the two islands. This is the evidence of
a gap flow acceleration (Venturi effect). In addition, the flow tended to
further accelerate after the point of maximum constriction between the
islands – the signature of a supercritical hydraulic flow. This might occur
in particular when the lower troposphere acts as a stable two-layer flow, the
lower one being the marine boundary layer and the upper one the stable free
troposphere, separated by a temperature inversion e.g.and references
therein. A noticeable point is that the interface lowers as the
lower layer accelerates (owing to conversion of potential into kinetic
energy). This can explain the sudden change of air mass at 10:40 UTC shortly
after the gap: the quasi-horizontal isopycnic balloon trajectory probably
crossed the lowering interface. The sudden decrease of specific humidity
(down to 8 g kg-1, Fig. b), balloon deceleration
(Fig. a) and temperature jump by about +4 ∘C (not
shown) support the assumption that the balloon exited the boundary layer and
entered the free troposphere. The temperature jump also supports the
existence of a temperature inversion between the layers.
Flight section 3 occurred between 10:40 and 17:50 UTC. During this time the
balloon sampled the lower free troposphere, although it flew in the same
altitude range as in the previous flight sections. This implies that the
marine boundary layer was not as deep as on the other side of Corsica and
Sardinia. Ozone increased globally by 0.7 ppbv h -1, at a higher rate
during the first 2–3 h and more slowly after that. Here again, this
positive trend can be attributed to photochemical ozone production.
During the last flight, flight section 4 in the evening (17:50–20:20 UTC),
B62 experienced very moist conditions (specific humidity around
14 g kg-1, relative humidity above 80 %), again within the marine
boundary layer. The balloon progressively lost its altitude. Ozone decrease
was observed while specific humidity was relatively constant. However, the
balloon descent is significant and the ozone trend might be linked to either
a vertical gradient or ozone chemical destruction. Finally, the balloon went
below the critical altitude of 200 m and the flight was aborted.
(a) Ozone mole fraction (colour code in ppbv) and wind
field (vectors) at 950 hPa pressure level given by the chemistry-transport
model MOCAGE, at 06:00 UTC on 30 July 2013. (b) As in (a),
but at 12:00 UTC. In panels (a) and (b),
balloon B62 positions at the respective times are marked by stars.
(c) Observed (cross marks) and simulated (solid line) ozone mole
fraction time series (in ppbv) along the B62 trajectory (abscissa: time
in UTC on 30 July 2015).
A numerical simulation covering the B62 flight period was performed by means
of the chemistry-transport model MOCA-GE
MOdèle de Chimie
Atmosphérique à Grande Echelle (Large-Scale Chemistry Atmospheric
MOdel).
developed by Météo-France . The model covers
the planetary boundary layer, the free troposphere, and the stratosphere. It
provides a number of optional configurations with varying domain geometries
and resolutions, as well as chemical and physical parameterisation packages
see. It offers the flexibility to use
several chemical schemes for stratospheric and tropospheric studies. The
model uses a semi-Lagrangian transport scheme and includes 47 hybrid vertical
levels from the surface up to 5 hPa, giving the model a vertical resolution
between 40 and 400 m in the boundary layer and between 400 and 800 m in the
upper layers. In this study, MOCAGE is forced dynamically by wind and
temperature fields from the ARPEGE model analyses . It is
run over a regional nested domain (Mediterranean area, see Fig. 15) at a
horizontal resolution of 0.2∘× 0.2∘ forced by the
2∘× 2∘ global domain.
For the global domain, we used the GEIA and the IPCC
inventories for natural and anthropogenic emissions
respectively. For the regional domain, we used the MACC II inventory
for the anthropogenic emissions, the GFAS 1.1 product
for biomass burning emissions and the GEIA inventory for
the natural emissions.
Figure a and b show the ozone and wind vector fields from
MOCAGE approximately at the balloon altitude (950 hPa) at 06:00 and
12:00 UTC respectively. The balloon was transported offshore from the
continent along the north-eastern edge of a low-level wind jet locally called
mistral. This wind jet is caused by the Venturi effect between two mountain
areas in France (Alps and Massif Central), then further accelerates over the
sea due to the supercritical nature of the flow . On
this day, a branch of the mistral jet was channelled between Corsica and
Sardinia, and the balloon was obviously driven in this branch. The model wind
field is consistent with the real balloon trajectory (Fig. a).
In the model, ozone increase can be seen in this air mass, caused by in situ
photochemical production. The direct comparison of the observed and simulated
ozone time series along the (real) balloon trajectory reveals a parallel
evolution over the course of the day, with a daytime increase of about
10 ppbv. However, the model overestimates ozone mole fractions by about
15 ppbv compared to the observations (Fig. c).
Free troposphere flight B55
B55 flight was launched on 2 July 2013 at 18:00 UTC from Minorca. The
balloon flew east-south-eastward for 32 h in the free troposphere at
altitudes ranging from 2350 to 2480 m a.s.l. (Fig. ). It
exited the authorised flight zone while approaching Sicily.
B55 flight launched on 2 July 2013 at 18:00 UTC. Same legend as in
Fig. concerning the balloon. Panel (a) shows in
addition an ensemble of 10-day HYSPLIT backward trajectories (diamonds)
ending on 2 July 2013 21:00 UTC at the current balloon position (trajectory
step 1 h; for one illustrative trajectory, larger diamonds mark the parcel
position daily at 00:00 UTC; the parcel altitudes are represented as a
brown-to-blue colour scale, in m a.s.l.).
In order to characterise the origin of the air mass sampled by the balloon,
an ensemble of 27 ten-day backward trajectories was computed with the online
HYSPLIT model . The trajectory endpoints all
correspond to the balloon current position at 21:00 UTC on
2 July 2013
Control parameters used for the HYSPLIT simulation:
trajectory endpoint at 39.912260∘ N, 4.563480∘ E,
2400 m a.s.l. on 2 July 2013, 21:00 UTC; ensemble option activated; global
REANALYSIS archive used for meteorological fields (details available on
https://www.ready.noaa.gov/gbl_reanalysis.php); model vertical velocity
used for vertical motion calculation.
. The trajectory ensemble is shown in
Fig. a. Before passing over Minorca, the trajectories had
followed one of two main paths.
Some trajectories experienced a slow anticyclonic motion over northern Africa,
and before this in the western Mediterranean boundary layer. In this case, the
model suggests no major recent anthropogenic influence, but aged and diluted
residual pollution from the Mediterranean boundary layer might be present.
Other trajectories were more recently in the boundary layer over south-eastern Spain.
In this case, fresh and more concentrated pollution can be expected.
The day of 3 July 2013 was completely covered by the balloon measurements. A
remarkable ozone increase of more than 25 ppbv was observed during the
daytime along the flight track, while specific humidity remained relatively
constant around 4–5 g kg-1. Again, we defined four flight sections
for convenience (Fig. ). Section 1 was mostly during the night,
with a near-constant ozone mole fraction around 27–28 ppbv. Ozone started
then to increase in the early morning at around 03:00 UTC. During a 3 h
transition period, both the balloon altitude and the specific humidity
varied. Therefore, the cause of the ozone mole fraction increase is not clear
for that period.
During flight section 2, the specific humidity was remarkably constant
(4.7–4.9 g kg-1) while the ozone mole fraction was growing at a rate
of 1.0 ppbv h -1. The most likely explanation for this ozone increase
is in situ photochemistry. During section 3, ozone variations around the
overall trend mirror humidity variations but still the ozone baseline keeps
on growing at about the same average rate (1.0 ppbv h -1). During the
final night-time section 4, ozone shows no obvious trend after 21:00 UTC.
To our knowledge, such a continuous Lagrangian observation of ozone
photochemical production in the free troposphere has not been reported
previously. This case study deserves further work, especially with numerical
modelling, to give more support to this hypothesis and specify the chemical
mechanism in play.
Summary and future work
A specific adaptation of electrochemical concentration cell (ECC) ozonesonde
has been developed for long-duration isopycnic flights in the lower
atmosphere aboard the last generation of boundary-layer pressurised balloons
(BLPBs): small constant-volume balloons developed by CNES. The main challenge
was the relatively short lifetime of conventional ECC ozonesondes. Whereas
BLPBs can fly and transmit data for days or even weeks, the working time of
ECC ozonesondes is limited to a few hours, chiefly owing to electrolyte
evaporation in the cathode chamber. The adopted strategy was to save
electrolytes by alternating short working phases (pump motor on) and longer
quiescence periods (pump motor off).
The adaptation consists of an entirely new electronic implementation of
existing elements from commercial ECC En-Sci Z sondes, namely motor, pump and
electrochemical cell. The major specific feature of the new electronic card
is that the pump motor can be switched on or off following a programmable
sequence composed of three steps: (i) a warm-up period (motor on), (ii) a
measurement period (motor still on) and (iii) a quiescence period (motor
off). Laboratory tests presented in this article show that a 1 min warm-up
period is sufficient to reach a stabilised ozone measurement that is
consistent with the typical response time of ECC sondes to an ozone step,
which is a few tens of seconds. The durations of the measurement period
(typically 1–2 min) and of the quiescence period (such that the overall
3-step sequence is typically 15 to 30 min) can be adjusted to consume the
electrolyte more or less rapidly, depending on the expected flight duration
and desired sampling rate.
Among other laboratory tests, an outdoor ground-based experiment was
conducted over several days in order to evaluate the new ozonesonde
performance against the data from a UV absorption analyser, which was
considered to be a reference. With warm-up and measurement periods of 1 min
each and an overall sequence of 15 min, the ozonesonde provided data within
±10 % from the reference for more than 4 days, capturing several
pronounced ozone diurnal cycles (in the range ∼0–60 ppbv) as well as
features at shorter timescale. The obtained agreement is within the expected
absolute accuracy of ECC ozonesonde data in the troposphere 10 %,
according to the.
The new ozonesonde was then carried out over the western Mediterranean aboard
16 BLPB flights during three campaigns in summer 2012 and 2013. Two launch
bases were located on the French Mediterranean coast, in the Marseilles and
Toulon areas, and a third one was on Minorca. Drifting altitudes were in the
range 0.25–3.2 km. The longest flight lasted more than 32 h and covered
more than 1000 km from Minorca to the south of Malta.
The concurrent data sets available from the campaigns (aircraft or
ground-based UV analyser measurements) suitable for in-flight validation all
show reasonable agreement with the BLPB ozone data.
Prior to our study, considered ozonesonde measurements
from low-altitude isopycnic balloons launched in 2000 and 2001 from the
Marseilles area (at that time, standard ECC ozonesonde were used, over much
shorter flight durations). Following the method used by these authors, we
also identified flight segments for which specific humidity remained nearly
constant – an indication that the balloon flew for some time within the same
homogeneous air mass and, therefore, that the ozone measurement can be
considered quasi-Lagrangian with good confidence. In such cases, the observed
ozone trend can be attributed with good confidence to ozone chemistry. In a
majority of cases, the ozone mole fraction was found to increase during the
daytime, with growth rates in the range 1–2 ppbv h -1. This is
significatively less than the mean growth rate found by
(6 ppbv h -1), but in our case, all flights were over sea, whereas
their results were obtained mainly over land. Moreover, several major
pollution episodes were experienced during the 2000–2001 campaigns, but this
was not the case in 2012–2013.
Beyond the overview presented in this article, several interesting flights
were investigated in more detail, with more attention paid to other
experimental data from the 2012–2013 campaigns as well as
chemistry-transport numerical simulations. Of these flights, one (B55) flew
in the free troposphere (around 3000 m a.s.l.) and revealed ozone growth by
about 1 ppbv h -1 during the daytime, while specific humidity
remained nearly constant. This is potentially the first in situ observation
of ozone photoproduction in the free troposphere along a continuous
Lagrangian trajectory. However, further work is needed to confirm this result
and study the cause of the observed evolution.
From a technical point of view, an interesting evolution of the BLPB
ozonesonde (and potentially also of conventional ECC sondes) would be the
on-board measurement of the pump flow rate. Indeed, the ozone current
measured in the ECC is proportional to the pump flow rate. From our
laboratory tests, it was found to vary by 1–2 % during the first 3 min
after the pump motor has been turned on. This could help to reduce the
uncertainty associated with the ozone measurement. However, the major
uncertainty source for tropospheric ozone measurements is related to the
sonde background current (i.e. the current measured in absence of ozone).
This is a general concern for all types of ECC ozonesondes and reducing this
source of uncertainty remains an open research challenge
.
Data availability
The data sets used in this study are all available on http://mistrals.sedoo.fr/ChArMEx/ under the following DOIs:
Laboratory tests
10.6096/MISTRALS-ChArMEx.1456
10.6096/MISTRALS-ChArMEx.1458
10.6096/MISTRALS-ChArMEx.1459
Field campaign 2012
10.6096/MISTRALS-ChArMEx.998
10.6096/MISTRALS-ChArMEx.765
10.6096/MISTRALS-ChArMEx.764
Field campaign 2013
10.6096/MISTRALS-ChArMEx.1450
10.6096/MISTRALS-ChArMEx.1451
10.6096/MISTRALS-ChArMEx.1452
10.6096/MISTRALS-ChArMEx.1453
10.6096/MISTRALS-ChArMEx.1454
10.6096/MISTRALS-ChArMEx.1455
Acknowledgements
The ozone BLPB project was supported and funded by the French space agency
CNES, and also by the French national research institute CNRS/INSU through
its research programme MISTRALS/ChArMEx. The TRAQA campaign was partly
supported by ADEME (French environment and energy management agency). We
gratefully thank all members of the CNES technical staff, who brought
priceless support during and around the field campaigns, as well as the SEDOO
group of the Observatoire Midi-Pyrénées who made the forecast
trajectories used to plan the BLPB launches available. The authors also
acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the
HYSPLIT transport and dispersion model and READY website
(http://www.ready.noaa.gov) used in this publication. Edited by: J. Joiner Reviewed by: two
anonymous referees
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