Profiles of CFC-11 (CCl
The ground-based FTIR data have been compared with the collocated Michelson
Interferometer for Passive Atmospheric Sounding (MIPAS/ENVISAT) data and
found to be in good agreement: the observed mean relative biases and standard
deviations of the differences between the smoothed MIPAS and FTIR partial
columns (6–30 km) are (
The trends derived from the combined St Denis and Maïdo FTIR time series
are
CFC-11 (CCl
Because of the vital importance of these gases, the Advanced Global Atmospheric Gases Experiment (AGAGE) in situ network has been measuring CFC-11 and CFC-12 continuously since 1978 and HCFC-22 since the 1990s (Cunnold et al., 1997; Dunse et al., 2005). NOAA's Halocarbons & other Atmospheric Trace Species Group (HATS) sampling network started monitoring chlorofluorocarbon (CFCs) from flask grab samples in 1977 and via online in situ techniques starting in 1977 (Elkins et al., 1993). HCFC-22 was added to the NOAA/HATS measurements in 1992. Because of the use of CFCs as propellant and refrigerant in the 1980s, the in situ measurements show the rapid rise of CFC-11 and CFC-12 at that time. To reduce substances that deplete the ozone layer, amongst others CFCs, 27 nations around the world signed a global environmental treaty, the Montreal Protocol, in September 1987 (Murdoch and Sandler, 1997). The hydrochlorofluorocarbons (HCFCs) were applied to replace the CFCs after the Montreal Protocol, since they react with tropospheric hydroxyl (OH), resulting in a shorter lifetime compared with CFCs. As a result, accelerated increases are observed for HCFCs since 2004 in the global atmosphere (Montzka et al., 2009). The tropospheric concentrations of CFC-11 and CFC-12 reached their maximums in 1992 and 2003, respectively, and a decline has been observed since then (Elkins et al., 1993; Montzka et al., 1996; Walker et al., 2000).
Apart from the in situ measurements, observations of CFCs and HCFCs abundances have also been made using remote sensing infrared spectroscopy techniques. Space-based observations provide the global distributions and trends of CFCs and HCFCs; examples are the measurements of CFC-11 and CFC-12 from ILAS (Improved Limb Atmospheric Spectrometer), CFC-11, CFC-12, CFC-113, HCFC-22, HCFC-142a and HCFC-142b from ACE-FTS (Atmospheric Chemistry Experiment Fourier transform spectrometer) and CFC-11, CFC-12 and HCFC-22 from MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) (Khosrawi et al., 2004; Hoffmann et al., 2008; Mahieu et al., 2008). Also ground-based FTIR measurements are able to monitor the CFCs and HCFCs (Notholt, 1994), especially at the Swiss Jungfraujoch station (Zander et al., 2005; Mahieu et al., 2010, 2013), where comparisons with the ACE-FTS measurements show a good agreement (Mahieu et al., 2015). Trend studies of HCFC-22 from FTIR measurements have been conducted both in the Northern Hemisphere (Zander et al., 1994) and in the Southern Hemisphere at the Lauder station (Sherlock et al., 1997), for the period from the mid-1980s to the mid-1990s. FTIR can provide long time series of CFC-11, CFC-12 and HCFC-22 total columns and are therefore very good candidates for supporting the evaluation of satellite and model data and for the evaluation of trends: the Jungfraujoch CFC-11, CFC-12 and HCFC-22 time series and trends have been included in the most recent Scientific Assessment of Ozone Depletion (Carpenter et al., 2014).
In this study, we provide the first ground-based FTIR time series of CFC-11,
CFC-12 and HCFC-22 in the Southern Tropics, namely at two stations
located at Réunion Island (21
Microwindows, interfering gases, spectroscopic database, a priori profile and background parameters (slope, curvature, zshift and beam as discussed in Sect. 2.2.1) used for the SFIT4 retrievals of CFC-11, CFC-12 and HCFC-22. The degree of freedom for signal (DOFS, mean and standard deviation) of retrievals at St Denis and Maïdo.
As explained in Baray et al. (2013), the atmospheric observations at
Réunion Island are carried out at two sites, namely St Denis
(20.90
The Royal Belgian Institute for Space Aeronomy (BIRA–IASB) started the FTIR
solar absorption experiments at La Réunion in St Denis in 2002, with a
Bruker 120M FTIR spectrometer, first on a campaign basis with campaigns in
2002 (October), 2004 (August to October) and 2007 (May to November), and then
in continuous mode since June 2009 (Senten et al., 2008; Vigouroux et al.,
2009, 2012; Duflot et al., 2010; Baray et al., 2013). In September 2011,
BIRA-IASB started the replacement of the Bruker 120M by a Bruker 125HR: the
Bruker 125HR was installed next to the Bruker 120M and both instruments were
set up to make collocated measurements until November 2011, when BIRA-IASB
disassembled the Bruker 120M. Since then, at St Denis, BIRA-IASB operates
only the Bruker 125HR, and this instrument is primarily dedicated to TCCON
measurements Therefore, the KBr beam splitter and the MCT detector
(600–1400 cm
BIRA-IASB started operating a second Bruker 125HR FTIR spectrometer at the Maïdo observatory in March 2013 and dedicated it primarily to NDACC measurements with MCT and InSb detectors. As such, our CFCs and HCFC time series at Maïdo cover the March 2013–present time period.
The NDACC ground-based FTIR experiment observes the absorption of the direct
solar radiation with high spectral resolution (0.0035–0.0110 cm
The typical spectrum and averaging kernels of CFC-11 (upper), CFC-12
(middle) and HCFC-22 (bottom) at St Denis. The left panels show the
transmission and residual (observed
CFC-11, CFC-12 and HCFC-22 have weak absorptions in the infrared spectral
range, requiring careful selection of the retrieval spectral windows in order
to minimize the interfering absorptions from other species. The microwindows
(see Table 1) for CFC-11 are the same as in the work of Mahieu et al. (2010).
For CFC-12, the 922.50–923.60 cm
In each microwindow, the background transmittance
In Eq. (1),
Some low-frequency oscillations of the baseline can occur in the spectra,
resulting from the mirrors, filters or apertures. While this is not a problem
for small retrieval windows (a slope and a curvature are sufficient to fit
the baseline), it could be necessary in the case of wide window to include a
so-called “beam correction” to fit these oscillations. In SFIT4, this is
done by adding a zshift-like parameter
Table 1 lists the parameters used for fitting the background in the CFC-11,
CFC-12 and HCFC-22 retrievals. Since the retrieval windows of CFC-12 and
HCFC-22 are narrow, a linear fit is enough to characterize the spectral
background (
The mean residual transmittance (observed–calculated) of the CFC-11 retrievals with and without beam parameters at Maïdo. The IP beam fit line is used as a priori IP beam parameters.
We use the empirical pseudo-line-lists (PLL) created by G. Toon (details see
The a priori profiles of interfering gases, except H
The a priori covariance matrix (regularization matrix) is another important
input parameter in the optimal estimation method. Ideally, the diagonal
values of the covariance matrix represent the natural variability of the gas
concentration around the a priori profile. In our study, 2004–2016 monthly
data from WACCM is used to provide the variability for the FTIR retrieval,
which is the same data set used for creating the a priori profiles. The
variabilities of CFC-11, CFC-12 and HCFC-22 are then 5, 2 and 15 %,
respectively. The gas profile correlation width is set to 4 km from 0 to
100 km in the SFIT4 retrieval, and the retrieved profiles for CFC-11, CFC-12
and HCFC-22 are shown in Fig. 3. The retrieved HCFC-22 vertical profiles at
St Denis show a stronger variability in the troposphere/lower stratosphere
than at Maïdo. One reason is that St Denis data cover a much wider time
range (8 years) than Maïdo (4 years), so that part of the variability
simply comes from the trend in HCFC-22 amounts. Figure 3 shows the retrieved
profiles with different color for each year, indicating that part of the
variability comes from the trend. However the variability within 1 year is
still stronger at St Denis compared to Maïdo: this cannot come from
larger natural variability since this species is well mixed in the whole
troposphere. The larger scatter at St Denis within 1 year comes from the
larger random error budget at this station (see Tables 2 and 3) due to lower
signal to noise ratio (
The a priori (red line), retrieved profiles (each year with a different color) and the mean retrieved profile (blue line) of CFC-11, CFC-12 and HCFC-22 at St Denis (upper panels) and Maïdo (bottom panels).
Systematic and random uncertainties (in %) for CFC-11, CFC-12 and HCFC-22 at St Denis. Sb represents the relative uncertainties (absolute value) of the non-retrieved parameters (also in %), and the detail information about the sb value of each parameter is described in the text. For SZA, ILS and zshift, both the systematic uncertainty and the random uncertainty (in the bracket) are listed here. When a relative uncertainty is smaller than 0.01 %, it is considered negligible and represented as “–”.
Same as Table 2, but for Maïdo.
Table 1 lists the degree of freedom for signal (DOFS) of the total columns of
CFC-11, CFC-12 and HCFC-22, along with the standard deviation (
According to the optimal estimation method (Rodgers, 2000), the final state
We can rewrite Eq. (8) as
The first term in the right side of Eq. (8) is the smoothing error; the
second term contains three parts: the forward model error
Tables 2 and 3 list the different contributions to the total average
retrieval uncertainty at St Denis and Maïdo, respectively, including
smoothing, measurement noise, retrieval parameters (slope, curvature,
wavenumber shift, zero-level offset (zshift), beam parameters, solar line
shift, simple phase correction), interfering species, temperature profile,
solar zenith angle (SZA) and spectroscopic parameters (line intensity,
air-broadened half-width, temperature dependence of the air-broadened
half-width). We assume that the measurement and retrieval parameters have
very small systematic uncertainties (set to zero in our case) and that the
spectroscopic parameters have negligible random errors. Because of the strong
H
Sb in Tables 2 and 3 represents the relative uncertainties of the
non-retrieved parameters. For temperature, the systematic/random Sb matrix
was created by the mean/standard deviation of the differences between NCEP
and the balloon observations at St Denis. The random component is from 2K to
4K in the vertical range from 0 to 30 km and about 5K above 30 km, and the
systematic component is about 2K for the whole vertical range. For the target
spectroscopic parameters, 7, 1 and 5 % are the relative uncertainties of
CFC-11, CFC-12 and HCFC-22, respectively, according to the PLL database. For
the H
The time series of the total columns and total uncertainties of
CFC-11, CFC-12 and HCFC-22 at St Denis (black) and Maïdo (grey),
together with the a priori total columns at both sites (green dash lines).
The error bar contains both systematic and random uncertainties from SFIT4
retrieval
The total average systematic/random uncertainties associated with the retrieved columns for CFC-11, CFC-12 and HCFC-22 are 7.0/2.0 %, 1.8/1.1 % and 4.7/4.4 %, respectively, at St Denis and 6.7/1.6 %, 1.8/1.1 % and 4.4/3.6 %, respectively, at Maïdo. The systematic uncertainties originate mainly in the uncertainties on the spectroscopic parameters, as well as in the temperature uncertainty. The random uncertainty is dominated by the smoothing error, the uncertainty on the SZA and the measurement noise; especially for HCFC-22, the measurement noise error is very significant due to the narrow and weak absorption of HCFC-22 (see the left bottom panel in Fig. 1).
Figure 4 shows the time series of retrieved total columns of CFC-11, CFC-12
and HCFC-22 at St Denis and Maïdo, together with their uncertainties (in
unit of molecules cm
In situ daily mean (CFC-11 and CFC-12) and flask pair measurements (HCFC-22) at SMO site (blue) and individual FTIR column-averaged dry-air mole fractions at St Denis (light coral) and Maïdo (grey). Upper: CFC-11; middle: CFC-12; bottom: HCFC-22.
We plot in Fig. 5 the column-averaged dry-air mole fractions at St Denis and
Maïdo. The column-averaged dry-air mole fraction is the ratio of the
column of the target gas with the dry-air column derived from the pressure.
Also shown for comparison in Fig. 5 are the in situ and flask daily mean
measurements at SMO. We use the Chromatograph for Atmospheric Trace Species
(CATS) in situ daily mean data for CFC-11 and CFC-12
(
Note that for CFC-12, it seems that there is an offset between the column-averaged dry-air mole fractions at St Denis and Maïdo, compared with SMO in situ measurements. Since CFC-12 mixes well within 0–20 km (see Fig. 2), the offset cannot be explained by the different pressures at two sites. The offset maybe caused by the systematic retrieval errors and should be taken into account when doing the trend analysis with the combined St Denis and Maïdo data.
ENVISAT was successfully launched into space on 1 March 2002 carrying several
sensors, including MIPAS, a cryogenic limb emission FTS which observes many trace gases from a wide spectrum
covering 865–2410 cm
There is no temporal overlap between MIPAS data and Maïdo measurements,
so the MIPAS footprints within
The number of collocated MIPAS-FTIR pairs, bias and standard
deviation (SD) of the relative differences
((MIPAS
Figure 6 shows the comparison of averaged profiles between FTIR measurements
and MIPAS data. The individual FTIR–MIPAS data pair was selected when the
FTIR measurement and the MIPAS observation were collocated within
Left panel, for each target species (from top to bottom: CFC-11,
CFC-12 and HCFC-22): averaged target species mixing ratio profile, random
uncertainty (error bar) and the standard deviation of all the co-existing
data pairs (shade area) for FTIR (in black) and for MIPAS (in red: raw data;
in sky blue: after smoothing with the corresponding FTIR averaging kernel).
The profiles (from 0 to 100 km) are also manifested in the left panels.
Right panel, for each target species, averaged relative difference between
MIPAS and FTIR ((MIPAS
In this section, we compare the MIPAS and St Denis FTIR partial columns (PC)
from 6 to 30 km, for the same collocated pairs as in Sect. 3.2. The DOFS of
the partial columns of CFC-11, CFC-12 and HCFC-22, respectively, are
0.6
The time series of the individual partial columns (6–30 km) of CFC-11, CFC-12 and HCFC-22 from St Denis FTIR measurements (grey) and raw MIPAS data (red). Error bars represent the retrieval errors.
Figure 7 shows the time series of the individual partial columns of CFC-11, CFC-12 and HCFC-22 FTIR measurements at St Denis (grey) along with the raw MIPAS data (red). The smoothed MIPAS data are not shown here because more than half of MIPAS data do not correspond with an individual FTIR measurement within 1 day or even 1 week, and the differences between partial columns of smoothed and unsmoothed data are within 1.0 %. Note that the bias between the raw MIPAS and FTIR data also contains the smoothing error, but the bias already lies within the uncertainty budget even without smoothing error (see Table 4). The individual partial columns of MIPAS and FTIR data, based on the respective original time series, are in a good agreement.
To derive the secular trends from the FTIR and in situ measurements daily
means
Since the Maïdo measurements only cover about 3 years, we cannot perform trend analysis on only Maïdo data. Therefore, we use the total columns at Maïdo in combination with the St Denis partial columns calculated at the altitude of Maïdo (2.155–100 km) to derive the trends of CFC-11, CFC-12 and HCFC-22 at Réunion Island for the period 2004–2016.
The annual percent changes (in % year
Table 5 gives the annual percent changes and their uncertainties of CFC-11,
CFC-12 and HCF-22 (% year
For CFC-11, the annual change of the combined FTIR partial columns
(2.155–100 km) at St Denis and total columns at Maïdo (2004–2016) is
very close to the SMO measurements (
For CFC-12, Fig. 5 shows that its concentration has a significant trend
change around 2004 (increasing before 2004 and decreasing after); therefore,
it is relevant, in order to capture the most recent trend, to select the data
after 2009 to do the trend analysis. The annual change of the combined
partial columns (2.155–100 km) at St Denis and total columns at Maïdo
(2009–2016) is stronger than that derived from the SMO measurements
(
For HCFC-22 the trend of the combined partial columns (2.155–100 km) at St
Denis and total columns at Maïdo (2004–2016) is smaller than that of
the SMO measurements (2.84
Seasonal cycles of CFC-11, CFC-12 and HCFC-22 based on FTIR
measurements (grey) and MIPAS data (red). The mean of the measurements for
each month during the 2004–2016 period for FTIR measurements and 2004–2011
for MIPAS data, after subtraction of the trend, is shown as circle (FTIR) and
asterisk (MIPAS), together with the
Figure 8 shows the seasonal cycles of CFC-11, CFC-12 and HCFC-22 based on
FTIR (both St Denis and Maïdo) partial columns of 6–30 km (grey) and
MIPAS partial columns of 6–30 km (red). The mean of the measurements for
each month during the 2004–2016 period for FTIR measurements and 2004–2011
for MIPAS data, after subtraction of the trend, is shown, together with the
CFC-11, CFC-12 and HCFC-22 mixing ratio profiles were retrieved at
Réunion Island from St Denis and Maïdo ground-based solar absorption
FTIR measurements between 2004 and 2016. The retrieval microwindows are
carefully selected to minimize the interfering absorptions from other
species. The AKs of CFC-11, CFC-12 and HCFC-22 are very
similar, and the retrieved information comes mainly from the troposphere and
lower stratosphere with low vertical resolution. As expected as a response to
the Montreal Protocol, negative trends of total columns of CFC-11 and CFC-12
and a positive trend of HCFC-22 were observed at St Denis and Maïdo,
which is in good agreement with the in situ surface data and other remote
sensing results (e.g., SMO in situ and flask measurements and Jungfraujoch
FTIR data, respectively). The observed FTIR total column trends above St
Denis between 2004 and 2011 are
The FTIR measurements were also compared with collocated MIPAS/ENVISAT data
around St Denis. There are 60, 86 and 50 FTIR–MIPAS collocated data pairs for
CFC-11, CFC-12 and HCFC-22 within
The FTIR CFC-11, CFC-12 and HCFC-22 retrievals at St Denis and Maïdo are not
publicly available yet because they are not standard NDACC species. To obtain access
to site data, please contact the author or BIRA-IASB FTIR group. MIPAS/ENVISAT versions
V5H (FR) and V5R (RR) Level 2 CFC-11, CFC-12 and HCFC-22 data are publicly available
from KIT/IMK (
This work is supported by the National Natural Science Foundation of China (41575034). The authors wish to thank Steve Montzka from NOAA for providing the HCFC-22 flask measurements, the Université de la Réunion, as well as the Belgian Science Policy and AGACC-II project for supporting the NDACC operations in Réunion Island, the MIPAS satellite group at KIT/IMK for the provision of CFC-11, CFC-12 and HCFC-22 MIPAS data. The authors the European Communities, the Région Réunion, CNRS, and Université de la Réunion for their support and contribution in the construction phase of the research infrastructure OPAR (Observatoire de Physique de l'Atmosphère à La Réunion). OPAR is presently funded by CNRS (INSU) and Université de La Réunion, and managed by OSU-R (Observatoire des Sciences de l'Univers à La Réunion, UMS 3365). The authors also want to thank Emmanuel Mahieu, Philippe Demoulin and Stephanie Conway for helpful discussions. Part of this work was performed at Jet Propulsion Laboratory, California Institute of Technology, under contact with NASA. Edited by: H. Maring Reviewed by: O. Garciar and one anonymous referee