Introduction
The discovery of the Antarctic stratospheric “ozone hole” in the 1970s
(Farman et al., 1985) strongly intensified research to unravel the reason for
this ozone depletion. Chemically active chlorine (ClOx) species like Cl,
Cl2, ClO, ClOOCl, OClO, and HOCl are part of total inorganic chlorine
Cly (ClOx + HCl + ClONO2). They play a dominant role
in the catalytic destruction of stratospheric ozone during polar winter when
low temperatures and heterogeneous chemical reactions on polar stratospheric
cloud (PSC) particles have previously enabled active chlorine compounds
(mainly Cl2) to be produced from its reservoir species ClONO2, HCl,
and HOCl (e.g. Molina and Rowland, 1974; Solomon et al., 1986; Molina et
al., 1987; Solomon, 1999; Crutzen and Oppenheimer, 2008). Due to the Montreal
Protocol and successive agreements, emissions of dominant halocarbons were
reduced such that total tropospheric (organic) chlorine has been decreasing since
1994 after reaching a peak value of nearly 3.7 parts per billion by volume
(ppbv) (O'Doherty et al., 2004; WMO, 2011). The stratospheric total chlorine
peak occurred several years later, because it took this time for emitted air
masses to propagate into the stratosphere (Engel et al., 2002; WMO, 2011;
Kohlhepp et al., 2012). The amount of equivalent effective stratospheric
chlorine (chlorine and bromine halogens) is predicted to return to 1980
values around 2050 at mid-latitudes (Stolarski et al., 2010; WMO, 2011).
To assess and monitor the partitioning and budget of chlorine, a number of
measurements of its individual compounds have been carried out to calculate
the amount of inorganic (Cly), organic (CCly), and finally total
chlorine (Cltotal). An early observation based on data from the
Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument was published by
Zander et al. (1992). A mean stratospheric total chlorine volume mixing ratio
(VMR) of 2.58 ± 0.10 ppbv was observed at 30∘ N in 1985.
Significantly enhanced values between 3.4 and 3.5 (±0.4) ppbv in the
1992 Arctic lower stratosphere were estimated using retrieved data from the
balloon-borne Michelson Interferometer for Passive Atmospheric Sounding
(MIPAS-B) in combination with in situ measurements (von Clarmann et al.,
1995). A further slightly enhanced value of 3.53 ± 0.10 ppbv was
detected during the ATMOS/ATLAS-3 November 1994 mission at northern
mid-latitudes, also demonstrating the strong increase of stratospheric total
chlorine before regulating measures could alter this linear trend of
0.10 ppbv per year in the time period from northern spring 1985 to autumn
1994 (Zander et al., 1996). The same
trend has been deduced between 1991 and 1995 by estimating total chlorine
with the help of HCl observations from the HALogen Occultation Experiment
(HALOE; Russell III et al., 1996). A further increased Cltotal
value of 3.7 ± 0.2 ppbv was derived from MkIV balloon measurements
carried out in the 1997 Arctic summer (Sen et al., 1999). This measurement
took place close to the turnover of the total stratospheric chlorine amount.
Chlorine data obtained by the Atmospheric Chemistry Experiment Fourier
Transform Spectrometer (ACE-FTS) in combination with in situ measurements
from the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and
Validation Experiment (SOLVE) campaign (Schauffler et al., 2003; Nassar et
al., 2006) were used to estimate Cltotal between February 2004 and
January 2005 in five latitude zones. A mean stratospheric Cltotal
value of 3.65 ± 0.13 ppbv was determined for both the northern and
southern mid-latitudes. This start of a temporal decrease of stratospheric
chlorine was confirmed by observations from the Microwave Limb Sounder (MLS)
from August 2004 until January 2006 (Froidevaux et al., 2006). A
Cltotal value of 3.60 ± 0.13 ppbv at the end of this time
period was inferred from HCl measurements and a decrease of about 43 pptv in
the stratospheric chlorine loading within this 18-month period was detected.
The long term trend of stratospheric inorganic chlorine was investigated by
using data of multiple space-borne sensors like ACE-FTS, ATMOS, MLS, CLAES
(Cryogenic Limb Array Etalon Spectrometer), CRISTA (CRyogenic Infrared
Spectrometer and Telescope for the Atmosphere), and HALOE (Lary et al.,
2007). This time series confirms that stratospheric Cly peaked in the
late 1990s and started to decrease as expected due to the changing concentrations of tropospheric source gases and
related transport times from the troposphere to the stratosphere. More
recently published observations of Cltotal were performed by
ACE-FTS covering the years 2004 until 2009. Nine chlorine containing species
have been directly measured by the satellite instrument (Brown et al., 2011,
2013). These data were supplemented by a number of further trace gases
calculated using the SLIMCAT 3-dimensional chemical transport model
(Chipperfield, 2006). Global mean stratospheric chlorine was found to
decrease by 0.46 % per year in the time period under investigation.
The purpose of this paper is to assess the partitioning and budget of
inorganic and organic stratospheric chlorine inside the late-winter Arctic
vortex. Winter 2010/2011 was characterized by a cold vortex defining a
strong transport barrier until approximately mid-April (Manney et al., 2011;
Sinnhuber et al., 2011). Temperatures were below the threshold associated for
chlorine activation (∼ 196 K) for more than 100 days between about 15
and 23 km. Consequently, an unprecedented Arctic ozone loss was observed
which could be described for the first time as an Arctic ozone hole since
ozone profiles in late March resembled typical Antarctic late-winter profiles
(Manney et al., 2011; Sinnhuber et al., 2011). Trace gas profiles of
individual chlorine compounds were retrieved from limb-emission spectra
recorded during a balloon flight of MIPAS-B and the TErahertz and
submillimeter LImb Sounder (TELIS) on 31 March 2011 inside the polar vortex.
A description of the instruments, data analysis, and chemical modelling is
given in Sect. 2. A discussion of the observed chlorine partitioning and
budget follows in Sect. 3 and in the conclusions, together with a comparison
of the combined measured data to simulations of the chemistry climate model
EMAC (ECHAM5/MESSy Atmospheric Chemistry model).
Instruments, data analysis, and modelling
The MIPAS-B/TELIS flight took place on 31 March 2011 over northern
Scandinavia inside the Arctic vortex at the end of the chlorine deactivation
period that started slowly in early March and accelerated towards the end of
this month (Manney et al., 2011; Sinnhuber et al., 2011). The balloon gondola
was launched from Esrange near Kiruna (Sweden, 67.9∘ N,
21.1∘ E) and reached its float level at about 35 km. Recorded limb
sequences of MIPAS-B and TELIS are depicted in Fig. 1.
Potential vorticity (PV) field (in
10-6 K m2 s-1 kg-1) from European Centre for
Medium-Range Weather Forecasts (ECMWF) analysis on 31 March 2011, 00:00 UTC.
MIPAS-B tangent points are plotted as black solid circles and TELIS tangent
points as yellow solid circles (altitude range: 9.1 to 35.4 km). Both
instruments look in the same direction. The vortex boundary which represents
the strongest PV gradient (Nash et al., 1996) is shown as black dashed line.
The insert (top left) shows the approximate measurement region (yellow
marker) in relation to the position of the whole polar vortex.
MIPAS-B instrument and data analysis
The balloon-borne limb-emission sounder MIPAS-B is a cryogenic Fourier
transform spectrometer which operates in the mid-infrared spectral range
between about 4 and 14 µm. The maximum optical path difference of
14.5 cm of the beam in the interferometer allows a high unapodized spectral
resolution of 0.0345 cm-1 (about 0.07 cm-1 after apodization with
the Norton and Beer (1976) “strong” function), which allows the separation
of individual spectral lines from continuum-like emissions in combination
with a high radiometric accuracy of typically 1 %. Values of the noise
equivalent spectral radiance (NESR) are typically within
1 × 10-9 and
7 × 10-9 W(cm2 sr cm-1)-1 for a single
calibrated spectrum. Averaging over n spectra (n≤5) per single
elevation scan reduces the spectral noise by a factor of 1/n. The
instrument is characterized by a high performance and flexibility of the
pointing system with a knowledge of the tangent altitude of better than 50 m
at the 1σ confidence limit. A comprehensive overview and description
of the instrument together with processing of recorded interferograms to
calibrated spectra including phase correction, Fourier Transformation to the
spectral domain, and two-point calibration of the spectra from arbitrary to
radiance units is given by Friedl-Vallon et al. (2004) and references
therein. This includes instrument characterization in terms of the
instrumental line shape, field of view, NESR, line of sight of the
instrument, detector non-linearity (Kleinert, 2006), and the error budget of
the calibrated spectra.
Temporal evolution of chlorine monoxide (ClO) mixing ratios (ppbv)
as observed by MIPAS-B (top) and TELIS (bottom) on 31 March 2011 above
northern Scandinavia between 02:00 and 04:38 UTC inside the Arctic vortex
within the latitude/longitude sector shown in Fig. 1. The black solid line
marks the sunrise terminator. A residual activation of chlorine is visible
between 16 and 22 km with slightly enhanced ClO values up to 0.4 ppbv.
Forward radiance calculations were performed with the Karlsruhe Optimized and
Precise Radiative transfer Algorithm (KOPRA; Stiller et al., 2002), which is
a line-by-line and layer-by-layer model to simulate the infrared radiative
transfer through the atmosphere. Molecular spectroscopic parameters for the
calculation of limb-emission spectra were taken from the high-resolution
transmission molecular absorption database (HITRAN; Rothman et al., 2009) and
a MIPAS dedicated spectroscopic database (Raspollini et al., 2013). KOPRA
also calculates derivatives of the radiance spectrum with respect to
atmospheric state and instrument parameters and thus provides the Jacobians
for the retrieval procedure KOPRAFIT (Höpfner et al., 2002). Since the
vertical scan distance of adjacent tangent altitudes ranges between 1 and
1.5 km, the retrieval grid was set to 1 km up to the balloon float
altitude. Above this level, the vertical spacing increases gradually to
10 km at the top altitude at 100 km. Considering the smoothing of the
vertical part of the instrumental field of view, the retrieval grid is finer
than the achievable vertical resolution of the measurement for a large part
of the altitude region covered (especially above the observer altitude). To
avoid retrieval instabilities due to this oversampling of the vertical
retrieval grid, a Tikhonov–Phillips regularization approach (Phillips, 1962;
Tikhonov, 1963) was applied with a constraint with respect to a first
derivative of the a priori profile xa of the target species:
xi+1=xi+[KiTSy-1Ki+R]-1[KiTSy-1(ymeas-y(xi))-R(xi-xa)],
where xi+1 is the vector of the desired state parameters for
iteration i+1; ymeas is the measured radiance vector and
y(xi) the calculation of the radiative transfer model using
state parameters of iteration number i; K is the Jacobian matrix
containing partial derivatives ∂y(xi)/∂xi, while Sy-1 is the inverse noise measurement
covariance matrix and R a regularization matrix with the first
derivative operator and a regularization strength parameter.
Set-up for MIPAS-B trace species retrievals and typical (1σ)
errors. Results are given for different state parameters in corresponding
spectral windows together with the retrieval altitude resolution (Alt.
reso.).
Species
Spectral range (cm-1)
Noise errora (%)
Total errora (%)
Alt. reso. (km)
ClONO2
779.7–780.7
2–3
5–6
4–5
ClO
821.0–841.5
10–25b
20–30b
5–8
CFC-11
840.0–860.0
2–3
5–6
3–4
CFC-12
918.0–924.0
2–3
5–6
3–4
HCFC-22
828.0–830.0
3–10
8–15
3–6
CFC-113
813.0–830.0
3–10
20–25
3–6
CCl4
792.0–806.0
1–5
10–20
4–6
CH3Cl
742.5–755.0
1–5
8–15
9–13
HOCl
1215.0–1265.0
10–15
35–50
6–8
N2O
1227.8–1303.1
2–3
5–6
2–4
a In the altitude region around the VMR maximum;
b daytime errors.
In a first step, a temperature retrieval was performed using appropriate
CO2 lines of two separate bands around 810 and 950 cm-1 and a
priori pressure–temperature information from European Centre for Medium-Range
Weather Forecasts (ECMWF) analyses together with a CO2 VMR profile
updated with data from NOAA ESRL GMD (National Oceanic and Atmospheric
Administration, Earth System Research Laboratory, Global Monitoring Division;
Montzka et al., 1999). The temperature retrieval 1σ accuracy is
estimated to be within about 0.7 K. Then, VMR profiles of the target species
are individually retrieved in selected spectral regions (see Table 1).
Profiles of species interfering with the target molecule were adjusted
simultaneously during the retrieval procedure. An overview of the analysis of
spectra with regard to chlorine- and nitrogen-containing molecules is given
in von Clarmann et al. (1995) and Wetzel et al. (2002, 2010). The error
estimation of the target parameter consists of random and systematic errors
that were added in quadrature to yield the total error, which refers to the
1σ confidence limit. Random errors include spectral noise as well as
covariance effects of the simultaneously fitted parameters. Systematic errors
mainly comprise spectroscopic data inaccuracies (band intensities),
uncertainties in the line of sight, and gain calibration errors. The altitude
resolution is calculated from the number of degrees of freedom of the
retrieval, which corresponds to the trace of the averaging kernel matrix.
Typical values for the retrieved parameters are given in Table 1.
Many trace gases measured by the MIPAS-B instrument have been involved in a
large number of validation activities and cross-comparisons of satellite
sensors like MIPAS, ILAS/ILAS-II (Improved Limb Atmospheric Spectrometer),
and SMILES (Superconducting Submillimeter-Wave Limb-Emission Sounder). For
species used in this work we explicitly mention for evaluation the following
studies: ClONO2 (Höpfner et al., 2007; Wetzel et al., 2008, 2013),
CFC-11 and CFC-12 (Wetzel et al., 2008, 2013), ClO (Sagawa et al., 2013),
and N2O (Wetzel et al., 2008; Payan et al., 2009).
TELIS instrument and data analysis
The cryogenic heterodyne balloon sounder TELIS was developed in a
collaboration of three partners: the German Aerospace Centre (DLR),
Rutherford Appleton Laboratory (RAL), United Kingdom, and the Netherlands
Institute for Space Research (SRON). Each institute generated one channel:
1.8 THz (DLR), 500 GHz (RAL), and 480–650 GHz (SRON). A comprehensive
description of the instrument is given by Birk et al. (2010) and de Lange et
al. (2012). The HCl and ClO results presented here were derived from spectra
in the 480–650 GHz channel with a tunable superconducting integrated
receiver developed and characterized by de Lange et al. (2010, 2012). A local oscillator (LO) reference signal is mixed with
the atmospheric signal in a non-linear mixer. The measured spectrum is the
superposition of two spectra covering the frequency ranges νLO+νIF and νLO-νIF, where νIF is
the measured difference (intermediate) frequency (IF).
The analysis of the TELIS spectra is carried out in a similar way as for the
MIPAS-B retrieval procedure. A forward line-by-line model is used to model
the radiative transfer along the line of sight of the instrument.
Spectroscopic parameters are also taken from the HITRAN database (Rothman et
al., 2009). An instrument model to account for the specifics of the TELIS
instrument is included in the forward algorithm. Further details on the
forward model are described by de Lange et al. (2009) and references therein.
The forward model is inverted with a Gauss–Newton iteration scheme in
combination with a Tikhonov–Phillips regularization approach (Phillips, 1962;
Tikhonov, 1963) as described in the previous section.
Set-up for TELIS HCl and ClO retrievals with typical (1σ)
errors and retrieval altitude resolution (Alt. reso.).
Species
Spectral line (GHz)
Noise error∗ (%)
Total error∗ (%)
Alt. reso. (km)
H35Cl
625.9
< 1
10–15
2–5
H37Cl
624.8
< 1
10–15
2–5
ClO
501.3
< 1
10–15
2–4
∗ In the altitude region around the VMR maximum.
The HCl retrievals are performed for both chlorine isotopes H35Cl and
H37Cl. The total amount of HCl can be determined by taking into account
the isotope abundance of H35Cl (75.76 %) and H37Cl
(24.23 %). While the random error of HCl is very small
(∼ 0.01 ppbv), the systematic error estimate yields between 0.05 and
0.4 ppbv, resulting in a total error of about 10 to 15 % in the region of
the VMR maximum. Systematic error sources are instrumental uncertainties such
as instrumental line shape and side band ratio inaccuracies, detector
non-linearity, and calibration and pointing errors. Furthermore, errors in
the atmospheric pressure–temperature profile as well as spectroscopic data
errors are taken into account. The largest uncertainty stems from the
non-linear behaviour of the detector. This holds also for the ClO retrievals.
The overall accuracy of the ClO measurement is almost entirely determined by
systematic error sources. Similar to HCl, the total error for the species ClO
typically remains within 10 and 15 % in the altitude region of its VMR
maximum. An overview of the characteristics of the retrieved species is given
in Table 2.
TELIS HCl and ClO observations have been evaluated using MLS measurements
(de Lange et al., 2012). ClO was additionally compared to SMILES
observations (Sagawa et al., 2013). The Sagawa study also includes a
cross-comparison with MIPAS-B ClO observations.
Model calculations
Measured data are compared to simulations performed with the chemistry
climate model EMAC, which is a numerical chemistry and climate simulation
system that includes submodels describing tropospheric and middle atmosphere
processes (Jöckel et al., 2010). It uses the second version of the
Modular Earth Submodel System (MESSy2) to link multi-institutional computer
codes. The core atmospheric model is the fifth-generation European Centre
Hamburg general circulation model (ECHAM5, Roeckner et al., 2006). For the
present study we applied EMAC (ECHAM5 version 5.3.02, MESSy version 2.50) in
the T42L39MA-resolution, i.e. with a spherical truncation of T42
(corresponding to a quadratic Gaussian grid of approximately 2.8∘ by
2.8∘ in latitude and longitude) with 39 vertical hybrid pressure
levels from the ground up to 0.01 hPa. The applied model set-up comprised,
among others, the submodels MECCA (Sander et al., 2005) for the calculation
of gas-phase chemistry and the submodel MSBM (Kirner et al., 2011) for the
simulation of polar stratospheric clouds and the calculation of heterogeneous
reaction rates. The PSC scheme was validated with the help of HNO3, ClO,
and O3 data from the MLS instrument (Kirner et al., 2015).
A Newtonian relaxation technique of the prognostic variables temperature,
vorticity, divergence, and surface pressure above the boundary layer and
below 1 hPa towards the ECMWF reanalysis ERA-Interim (Dee et al., 2011) has
been applied to the model to simulate realistic synoptic conditions (van
Aalst, 2005). Boundary conditions for greenhouse gases, chlorofluorocarbons
(CFCs), and halons are adapted from observations (WMO, 2011; Meinshausen et
al., 2011). Halogenated hydrocarbons are included according to the WMO-A1
scenario (WMO, 2011). Chlorine-containing tropospheric source gases
considered in EMAC are CFC-11, CFC-12, HCFC-22, CFC-113, CCl4,
CH3Cl, and CH3CCl3. Photolysis rates of HCFC-22 and CFC-113
are the same as for CFC-12. The simulation includes a comprehensive chemistry
set-up from the troposphere to the lower mesosphere with 104 gas-phase
species, 234 gas-phase reactions, 67 photolysis reactions, and 11
heterogeneous reactions on liquid aerosols, nitric acid trihydrate, and
ice particles. Rate constants of gas-phase reactions are taken from Atkinson
et al. (2007) and Sander et al. (2011). The model output data were saved
every 10 min. The temporally closest model output to the MIPAS-B
measurements has been interpolated in space to the observed geolocations.
Chlorine partitioning and budget
The combination of two different sensors, MIPAS-B and TELIS, working in
different spectral regions (mid-infrared and microwave), enables the
simultaneous measurement of virtually all relevant inorganic and organic
chlorine molecules. The amount of inorganic chlorine [Cly] is defined
as
[Cly]=[ClOx]+[HCl]+[ClONO2],
where active chlorine [ClOx] is calculated via
[ClOx]=[ClO]+[HOCl]+2[ClOOCl].
The amount of organic chlorine [CCly] is composed of
[CCly]=2[CFC-12]+3[CFC-11]+[HCFC-22]+3[CFC-113]+4[CCl4]+[CH3Cl].
Total chlorine [Cltotal] is given as the sum of both budgets:
[Cltotal]=[Cly]+[CCly].
Constituents, which are of minor importance for the Arctic stratospheric
chlorine budget (like Cl2, Cl, OClO, CH3CCl3, CFC-114,
CFC-115, HCFC-141b, HCFC-142b, Halon-1211; see e.g. Prinn et al., 2000) are
neglected here. All the quantities defined in Eqs. (2) to (5) can be deduced
from TELIS (measuring ClO and HCl) and MIPAS-B (measuring all gases except
HCl) observations. However, the chlorine monoxide dimer ClOOCl is only
measurable by MIPAS-B under activated chlorine conditions
([ClOOCl] > 0.5 ppbv) without any PSC emissions in the recorded spectra
(Wetzel et al., 2010). On 31 March 2011, no PSC signatures are visible in the
MIPAS-B spectra but ClOOCl concentrations are below the detection limit.
However, [ClOOCl] can be estimated from [ClO] with the following relation
(Wetzel et al., 2012):
[ClOOCl]=([ClOnoon]+2[ClOOClnoon]-[ClO])/2,
while the amounts of [ClOnoon] and [ClOOClnoon] which
correspond to noon maximum and minimum values respectively can be both
taken from EMAC simulations if the modelled ClO is constrained to the
measured one.
MIPAS-B spectra have been recorded from night until day. The sunrise took
place between 02:38 UTC at 36 km and 03:10 UTC at 9 km altitude. Figure 2
shows the measured ClO cross section from 02:00 to 04:38 UTC, corresponding
to 64.0∘ N, 30.1∘ E and 63.5∘ N, 28.9∘ E.
A temporal variation of ClO is visible. The concentration of this species is
a measure of whether the air masses sounded are still chlorine-activated or
not. After sunrise the mixing ratio of ClO increases in a layer between 16
and 22 km from nighttime values below 0.05 ppbv to daytime mixing ratios up
to 0.4 ppbv. During periods of strong chlorine activation, significantly
higher values around 2 ppbv are observed (see e.g. Santee et al., 2003;
Wetzel et al., 2012). The ClO increase is shown by both instruments, MIPAS-B
and TELIS. The latter instrument measured not only with higher vertical
resolution but also with higher temporal resolution compared to MIPAS-B;
hence the TELIS data were transferred to the coarser temporal grid of MIPAS-B
for better comparability. At altitudes above 26 km, MIPAS-B ClO temporal
retrieval fluctuations are visible due to the large spectral noise error in
this altitude region. As a consequence, the TELIS ClO data were used for
calculating the chlorine partitioning and budget in the whole altitude range.
The decreasing ClOx at the end of the Arctic winter in the lower
stratosphere due to rising temperatures followed by shrinking ClOx
production from heterogeneous chemical reactions is in line with high amounts
of ClONO2 in this altitude region. The reaction of ClO with NO2
produces the reservoir species ClONO2. The measured time evolution of
this molecule is displayed in Fig. 3. Measured ClONO2 data exhibit high
values that are typical for observations in the late Arctic winter (see
e.g. Oelhaf et al., 1994; von Clarmann et al., 1997, 2009; Wetzel et al.,
2002). Only in an atmospheric layer around 19 km is the vertical mixing ratio
gradient small, since ClONO2 values (around 19 km) are slightly lower
than they would be if chlorine was completely deactivated. This observed
signature is in line with the enhanced ClO amounts around 19 km as seen in
Fig. 2. A significant diurnal temporal variation is not visible in the
ClONO2 data.
ClONO2 mixing ratios (ppbv) as seen by MIPAS-B above northern
Scandinavia on 31 March 2011 inside the late-winter Arctic vortex. The black
solid line marks the sunrise terminator.
The mean measured chlorine partitioning and budget for early morning is
displayed in Fig. 4. A spectral noise error weighted averaging was applied to
calculate the mean profiles, although statistical errors of the individual
species profiles are similar. The molecules ClO and ClOOCl exhibit a temporal
variation over the measured time period. However, since their mixing ratios
are very low at this time in the year, vertical profiles of these species
have also been averaged over the observed time period, with almost no
consequence. To obtain a proxy of total inorganic chlorine, a
N2O–Cly correlation was derived from air samples collected with the
balloon-borne cryogenic whole air sampler BONBON in the Arctic between 2009
and 2011 according to the method described in Engel et al. (2002) and Wetzel
et al. (2010). Cly from the cryosampler measurements is calculated as
the difference between total chlorine and observed organic chlorine from the
source gases CFC-11, CFC-12, CFC-113, CH3CCl3, CCl4, HCFC-22,
HCFC-141b, and HCFC-142b. In addition, an input of 50 pptv of chlorine from
short-lived source gases is taken into account, which is assumed to be
transformed immediately to inorganic chlorine. Total chlorine from the gases
is propagated into the stratosphere in the same way as an inert tracer, as
described in Engel et al. (2002), using global mean observation data from
NOAA ESRL. The proxy inorganic chlorine [Cly∗] is calculated with
the following dependence on the amount of [N2O], both given in ppbv:
Partitioning and budget of inorganic, organic, and total chlorine as
measured by TELIS (HCl and ClO) and MIPAS-B (all other species) in the Arctic
stratosphere on 31 March 2011 (see legend for line style and note non-linear
abscissa). The reservoir species HCl and ClONO2 dominate the
stratospheric inorganic chlorine budget. Cly∗ deduced from
observed N2O data with the help of a N2O–Cly correlation (see
Eq. 7) is shown for comparison. Note that for the calculation of the chlorine
budgets the atomic content for each species has to be considered (some error
bars have been omitted for clarity).
[Cly∗]=3.2008346+8.7786479×10-6[N2O]-2.9132361×10-5[N2O]2.
This correlation has been applied to MIPAS-B measured N2O and yields up
to 3.20 ppbv Cly∗ in the stratosphere. The amount of inorganic
chlorine is dominated by the chlorine reservoir species ClONO2 and HCl,
the latter one especially above 24 km. Above this altitude, where the
Cly VMR is (vertically) approximately constant, the mean observed
Cly amounts to 3.25 ± 0.30 ppbv, which is in agreement with the
deduced Cly∗ within the error bars although there is a tendency
towards a small positive deviation in the observations compared to the
Cly∗ reference according to Eq. (7). The deviation between
Cly and Cly∗ below 21 km is caused by different degrees of
subsidence of the air masses in the case of the discussed balloon flight and
the Cly∗ reference, which results in different N2O mixing
ratios in a specific altitude. Cly species play by far the largest part
in the total chlorine budget from the lower to the upper Arctic winter
stratosphere. From about 17 km downwards, the amount of organic chlorine
gets increasingly dominant in the total chlorine budget. Source gases that
contribute to CCly are visible in Fig. 4: CFC-12 (CCl2F2),
CFC-11 (CCl3F), HCFC-22 (CHClF2), CFC-113 (C2Cl3F3),
CCl4, and CH3Cl. The mean amount of Cltotal is calculated
as 3.41 ± 0.30 ppbv above 24 km. From the ratio Cly to
Cltotal it follows that about 95 % of total chlorine is
inorganic in this altitude region.
The mean chlorine partitioning and budget as simulated by EMAC is shown in
Fig. 5. The principal vertical profile shape of the measured chlorine species
is well reproduced by the model. However, some differences in detail between
simulated and observed data are visible. The modelled HCl VMR maximum appears
slightly broader than the measured one. Below about 20 km, the simulation
shows significantly lower values compared to the observation by TELIS. A
difference is visible in the case of ClONO2. The model underestimates
this reservoir species and deviates by 0.8 ppbv (42 %) from the MIPAS-B
data in the region of the VMR maximum at 22 km, although simulated and
measured NOy and NO2 (a necessary reactant in the production of
ClONO2 via NO2 plus ClO) agree in this altitude region. Since
simulated HCl and ClOx (near 22 km) are in agreement with the observed
data, the simulated Cly deviation from the measurement can be attributed
to the ClONO2 deficit in EMAC. Around 19 km, the difference in
simulated and measured Cly is largest due to very low HCl values in EMAC
compared to the HCl seen by TELIS. The amount of available Cly below
about 24 km is dependent on the degree of downwelling of the air masses
inside the polar vortex. In EMAC, the subsidence of the air masses in the
course of the winter was underestimated such that we find higher values of
tracers like N2O and CFCs at a given altitude of the lower stratosphere
compared to the measurements. These higher N2O values are connected with
lower Cly values according to the compact N2O–Cly
relationship, resulting in an underestimation of the chlorine reservoir
species (especially ClONO2). So, at least part of the ClONO2
deficit in EMAC can be explained by the underestimation of the subsidence in
the model.
Partitioning and budget of inorganic, organic, and total chlorine as
simulated with the chemistry climate model EMAC on 31 March 2011 (see legend
for line style). Cly∗ has been calculated from the simulated
N2O data according to Eq. (7). The budgets ClOx, Cly,
CCly, and Cltotal are calculated as listed in Eqs. (2) to (5)
respectively. The shaded region of the budgets takes into account all minor
chlorine species contained in EMAC (Cl2, Cl, OClO, CH3CCl3)
that were not measured by MIPAS-B and TELIS.
The simulated Cly reaches its maximum VMR in the quasi-altitude-constant
region above 24 km with a mean value of 3.16 ppbv, which is slightly lower
than the measured one and close to the simulated value of Cly∗
(deduced from EMAC), which gives 3.19 ppbv. Below this altitude region, a
similar bias between Cly and Cly∗, as in the case of the
observations, is visible.
The mean amount of Cltotal in the model run is calculated as
3.21 ppbv above 24 km, which is 0.20 ppbv lower than the observed one.
About half of this simulated chlorine deficit can be explained by the fact
that some minor CFCs (e.g. CFC-114 and CFC-115) and HCFCs (e.g. HCFC-141b and
HCFC-142b) as well as halons are not included in the EMAC model. Their
contribution to Cltotal is not more than 1 % above 24 km
(Brown et al., 2013). The remaining deficit can be explained by very
short-lived chlorine species which altogether amount to about 0.1 ppbv
(Mébarki et al., 2010; WMO, 2011) and which are also not contained in the
model simulation. However, the chlorine amount of these missing species is
implicitly contained in the HCl measurement (since the short-lived chlorine
species are converted to HCl after being photolysed) and hence included in
the observed chlorine budget. In the altitude region above 24 km, about
98 % of total chlorine in EMAC is inorganic. The shaded region of the
budget profiles of ClOx, Cly, CCly, and Cltotal
shown in Fig. 5 takes into account all available chlorine species in EMAC
that were not measured by MIPAS-B and TELIS. These molecules comprise Cl,
Cl2, OClO (belonging to ClOx and Cly), and CH3CCl3
(belonging to CCly) and add up to 0.1 ppbv at 16 km to the total
chlorine budget (mainly due to Cl2 and OClO). However, at altitudes
between 22 and 36 km, contributions of these gases to the chlorine budget are
insignificant.
Conclusions
Observations from MIPAS-B/TELIS were performed at the end of the cold
2010/2011 stratospheric winter that was characterized by a persistent polar
vortex enabling strong chlorine activation and ozone loss. The chlorine
partitioning measured on 31 March 2011 reveals that in the outer part of the
polar vortex (above Finland) the recovery of active chlorine (ClOx) into
the reservoir species (mainly ClONO2) is almost completed by the end of
March, only a few days before the cold period had finished (Manney et al.,
2011). This is verified by low amounts of daytime ClO of up to 0.4 ppbv
around 19 km. The observed total stratospheric chlorine amounts to
3.41 ± 0.30 ppbv above 24 km (see Table 3). This is in accordance
with the EMAC simulation (3.21 ppbv), taking into account the fact that some
chlorine source gases and very short-lived species are not included in the
model. The horizontal Cltotal distribution in EMAC (above 24 km)
exhibits virtually no variation inside the polar vortex. The variation
inside/outside vortex is no larger than 0.1 ppbv. That is smaller than the
estimated Cltotal measurement accuracy of 0.3 ppbv so that the
observations can be treated as representative at least for the geographical
region of the Arctic vortex. Mean Cltotal values deduced from
spectra recorded by the ACE-FTS instrument (Brown et al., 2013) give
3.44 ± 0.18 ppbv (morning occultations) and 3.50 ± 0.13 ppbv
(evening occultations) for northern mid-latitudes and the Arctic in 2009.
Extrapolating these data to 2011 with the chlorine trend (between 2004 and
2009) obtained from these ACE-FTS observations (about -0.4 % per year)
yields Cltotal values of 3.41 ppbv (morning occultations) and
3.47 ppbv (evening occultations) comparable to the MIPAS-B/TELIS data. The
accumulated amount of minor species (not measured by MIPAS-B/TELIS) like
CFC-114, CFC-115, HCFC-141b, HCFC-142b, and Halon-1211 was estimated to be
about 0.7 % (∼ 0.02 ppbv) of total chlorine at 30 km (Brown et
al., 2013). Hence, the MIPAS-B/TELIS Cltotal value is in line with
the data obtained from ACE-FTS solar occultations and is consistent with the
decreasing amount of stratospheric chlorine. Considering the 2005 mean global
tropospheric Cltotal from in situ data of AGAGE (Advanced Global
Atmospheric Gases Experiment) and NOAA ESRL databases, as compiled in
WMO (2011), and transferring this value to 30 km, taking into account a
typical time lag of 6 years of stratospheric mean age of air (Engel et al.,
2002, 2009; Stiller et al., 2008; WMO, 2011), we get an estimated
Cltotal value of 3.40 ppbv for the year 2011, which is very close
to the MIPAS-B/TELIS result (3.41 ppbv).
Mean stratospheric chlorine budgets (ppbv, including 1σ
total errors) as measured by MIPAS-B/TELIS and simulated by EMAC in
comparison to ACE-FTS observations (Brown et al., 2013) and in situ data from
AGAGE and NOAA ESRL databases (WMO, 2011).
Budget
MIPAS-B/TELIS
EMAC
ACE-FTS
In situ
Cltotal
3.41 ± 0.30a
3.21a
3.41/3.47b
3.40c
Cly
3.25 ± 0.30a
3.16a
–
–
Cly∗
3.19 ± 0.002a
3.19a
–
–
a Mean value between 25 and 36 km; b mean
value (for morning/evening occultations) between 30 and 70∘ N for
2011, extrapolated from 2009 with trend between 2004 and 2009;
c mean global tropospheric value from 2005 corresponding to a
stratospheric value of 2011 assuming a stratospheric mean age of air of 6 years.
We finally conclude that the stratospheric total chlorine as deduced from
Arctic MIPAS-B/TELIS observations on 31 March 2011 confirms previously
published total chlorine assessments and their related trends. A recently
published study by Mahieu et al. (2014) shows a HCl concentration increase
between 2005/2006 and 2010/2011 in large parts of the northern hemispheric
lower stratosphere in combination with an increase in the mean age of
stratospheric air of up to 0.4 years. However, in the Arctic above 24 km,
ascertained changes of mean age of stratospheric air are small (Mahieu et
al., 2014) and, therefore, do not alter the findings above.