Calibration of the total carbon column observing network using aircraft profile data
ABSTRACT The Total Carbon Column Observing Network (TCCON) produces precise measurements of the column average dry-air mole fractions of CO2, CO, CH4, N2O and H2O at a variety of sites worldwide. These observations rely on spectroscopic parameters that are not known with sufficient accuracy to compute total columns that can be used in combination with in situ measurements. The TCCON must therefore be calibrated to World Meteorological Organization (WMO) in situ trace gas measurement scales. We present a calibration of TCCON data using WMO-scale instrumentation aboard aircraft that measured profiles over four TCCON stations during 2008 and 2009. The aircraft campaigns are the Stratosphere-Troposphere Analyses of Regional Transport 2008 (START-08), which included a profile over the Park Falls site, the HIAPER Pole-to-Pole Observations (HIPPO-1) campaign, which included profiles over the Lamont and Lauder sites, a series of Learjet profiles over the Lamont site, and a Beechcraft King Air profile over the Tsukuba site. These calibrations are compared with similar observations made during the INTEX-NA (2004), COBRA-ME (2004) and TWP-ICE (2006) campaigns. A single, global calibration factor for each gas accurately captures the TCCON total column data within error.
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Lawrence Berkeley National Laboratory
Calibration of the Total Carbon Column Observing Network using Aircraft Profile Data
Lawrence Berkeley National Laboratory
LBNL Paper LBNL-3578E
Atmospheric Measurement Techniques
Manuscript prepared for Atmos. Meas. Tech.
with version 3.0 of the LATEX class copernicus.cls.
Date: 26 May 2010
Calibration of the Total Carbon Column Observing
Network using Aircraft Profile Data
Debra Wunch1, Geoffrey C. Toon2,1, Paul O. Wennberg2,1, Steven C. Wofsy3,
Britton B. Stephens4, Marc L. Fischer10, Osamu Uchino14, James B. Abshire12,
Peter Bernath8,9, Sebastien C. Biraud10, Jean-Franc ¸ois L. Blavier2,1, Chris Boone8,
Kenneth P. Bowman13, Edward V. Browell11, Teresa Campos4, Brian J. Connor7,
Bruce C. Daube3, Nicholas M. Deutscher5, Minghui Diao15, James W. Elkins17,
Christoph Gerbig16, Elaine Gottlieb3, David W. T. Griffith5, Dale F. Hurst18,17,
Rodrigo Jim´ enez3, Gretchen Keppel-Aleks1, Eric Kort3, Ronald Macatangay5,
Toshinobu Machida14, Hidekazu Matsueda19, Fred Moore18, Isamu Morino14,
Sunyoung Park3, John Robinson20, Coleen M. Roehl1, Yusuke Sawa19, Vanessa
Sherlock6, Colm Sweeney18, Tomoaki Tanaka14, and Mark A. Zondlo15
1California Institute of Technology, Pasadena, CA, USA.
2Jet Propulsion Laboratory, Pasadena, CA, USA.
3Harvard University, Cambridge, MA, USA.
4National Center for Atmospheric Research, Boulder, CO, USA.
5Center for Atmospheric Chemistry, University of Wollongong, Wollongong, NSW, Australia.
6National Institute of Water & Atmospheric Research, Wellington, New Zealand.
7BC Consulting Limited, Alexandra, New Zealand.
8University of Waterloo, Waterloo, ON, Canada.
9York University, York, UK.
10Lawrence Berkeley National Laboratories, Berkeley, CA, USA.
11NASA Langley Research Center, Hampton, VA, USA.
12NASA Goddard Space Flight Center, Greenbelt, MD, USA.
13Texas A&M University, College Station, TX, USA.
14National Insitute for Environmental Studies, Tsukuba, Japan.
15Princeton University, Princeton, NJ, USA.
16Max-Planck-Institut f¨ ur Biogeochemie, Jena, Germany.
17National Oceanic and Atmospheric Administration, Boulder, CO, USA.
18Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder,
19Meteorological Research Institute, Tsukuba, Japan.
20National Institute of Water & Atmospheric Research, Lauder, New Zealand.
Correspondence to: Debra Wunch (email@example.com)
Abstract. The Total Carbon Column Observing Network (TCCON) produces precise measure-
ments of the column average dry-air mole fractions of CO2, CO, CH4, N2O and H2O at a variety
of sites worldwide. These observations rely on spectroscopic parameters that are not known with
sufficient accuracy to compute total columns that can be used in combination with in situ measure-
ments. The TCCON must therefore be calibrated to World Meteorological Organization (WMO)
in situ trace gas measurement scales. We present a calibration of TCCON data using WMO-scale
instrumentation aboard aircraft that measured profiles over four TCCON stations during 2008 and
2009. The aircraft campaigns are the Stratosphere-Troposphere Analyses of Regional Transport
2008 (START-08), which included a profile over the Park Falls site, the HIAPER Pole-to-Pole Ob-
servations (HIPPO-1) campaign, which included profiles over the Lamont and Lauder sites, a series
of Learjet profiles over the Lamont site, and a Beechcraft King Air profile over the Tsukuba site.
These calibrations are compared with similar observations made during the INTEX-NA (2004),
COBRA-ME (2004) and TWP-ICE (2006) campaigns. A single, global calibration factor for each
gas accurately captures the TCCON total column data within error.
The Total Carbon Column Observing Network (TCCON) is a ground-based network of Fourier
transform spectrometers that precisely measure total columns of CO2, CO, CH4, N2O, H2O, HF and
other gases (Wunch et al., 2010). The TCCON instruments measure the absorption of direct sunlight
by atmospheric gases in the near infrared (NIR) spectral region. To derive a total column measure-
ment of the gases from these spectra, external information about the atmosphere (e.g. temperature,
pressure, a priori mixing ratio) and NIR spectroscopy is required. A significant effort is put into
minimizing errors in this external information, and the resulting total columns are precise (< 0.25%
Due to systematic biases in the spectroscopy, the absolute accuracy of the column measurements
is ~1% which is inadequate for use in combination with in situ measurements for carbon cycle
science. In order to make TCCON column measurements useful for these combined analyses, they
must be calibrated to the World Meteorological Organization (WMO) in situ trace gas measurement
scales. To do this, we use profiles obtained with in situ instrumentation flown on aircraft over
TCCON sites. A set of profiles were measured over the Park Falls, Wisconsin TCCON site in
2004-2005 (Washenfelder et al., 2006) during the Intercontinental Chemical Transport Experiment–
North America campaign (INTEX-NA, Singh et al., 2006) and the CO2Budget and Rectification
Airborne - Maine experiment (COBRA-ME, Gerbig et al., 2003; Lin et al., 2006). A single profile
was measured coincidently with the Darwin, Australia site in 2006 as part of the Tropical Warm
Pool International Cloud Experiment (TWP-ICE, Deutscher et al., 2010; May et al., 2008). Since
then, other TCCON sites have begun operational measurements. In this paper, we describe the first
global calibration of five TCCON sites (Park Falls, Lamont, Darwin, Lauder and Tsukuba), using
instrumentation calibrated to WMO scales aboard the HIAPER aircraft, during the START-08 and
HIPPO overpasses in 2008 and 2009, Learjet overflights of Lamont during summer of 2009, and
a Beechcraft King Air 200T aircraft profile over Tsukuba, Japan in January, 2009 (Tanaka et al.,
2009). We present the calibration of CO2, CO, CH4, N2O and H2O.
The TCCON was developed to provide a long, nearly continuous, time series to serve as a transfer
standard between in situ networks and satellite measurements, and to provide insights into the carbon
cycle (e.g. Yang et al. (2007); Keppel-Aleks et al. (2008); Wunch et al. (2009); Deutscher et al.
(2010)). TCCON sites are located worldwide (Figure 1). The first TCCON site, located in Park
Falls, is described by Washenfelder et al. (2006).
Total column abundances are retrieved from spectra measured with the TCCON instruments using
a nonlinear least-squares spectral fitting algorithm (GFIT), which scales an a priori profile to produce
a synthetic spectrum that achieves the best fit to the measured spectrum. We use spectral windows
and spectroscopic data listed in Table 1.
Column-averaged dry-air mole fractions (DMF), denoted XGfor gas G, are computed using the
retrieved O2columns as a measure of the dry air column.
Dividing by O2improves the precision of the measurement by significantly reducing the effects of
instrumental or measurement errors that are common to both the gases (e.g. solar tracker pointing
errors, zero level offsets, instrument line shape errors, etc. (Wunch et al., 2010)). However, any
errors specific to either columnGor columnO2will create errors in the DMFs of each gas.
All TCCON XCO2data have an airmass-dependent artifact, which causes the retrievals to be ~1%
larger at noon than at sunrise or sunset (Wunch et al., 2010). This artifact is caused primarily by
spectroscopic inadequacies which are common to all TCCON instruments (e.g. line widths, neglect
of line-mixing, inconsistencies in the relative strengths of weak and strong lines). The airmass-
dependent artifact is removed from the TCCON data with a single empirical correction factor before
calibration. Airmass dependent artifacts have not been seen in XCH4, XCO, XN2Oor XH2O.
Three independent aircraft campaigns were held in 2008 and 2009 that included profiles over four
TCCON stations. The instrumentation on each aircraft used for the calibration are listed in Table 2.
The WMO calibration scales used for the aircraft instrumentation are described for CO2in Zhao and
Tans (2006) and Keeling et al. (2002), for N2O in Hall et al. (2007), for CH4in Dlugokencky et al.
(2005) and for CO in Novelli et al. (1994).
3.1START-08/pre-HIPPO and HIPPO-1
The NCAR/NSF High-performance Instrumented Airborne Platform for Environmental Research
(HIAPER), is a modified Gulfstream V (GV) jet which hosted the Stratosphere-Troposphere Anal-
yses of Regional Transport 2008 (START-08) campaign (Pan et al., 2010) and the preliminary HI-
APER Pole-to-Pole Observations (pre-HIPPO) campaign during 2008. The two experiments shared
flight time and instrumentation and made observations across North America, including a vertical
profile above the Park Falls site. The HIAPER Pole-to-Pole Observations (HIPPO-1) campaign
(Wofsy et al., 2010) covered a cross-section of the globe that spanned the Arctic to the Antarctic
(Figure 1) with profiles over Lamont and Lauder. The START-08/pre-HIPPO and HIPPO-1 mis-
sions used similar in situ instrumentation (Table 2). The water profiles are from the available H2O
measurements on board the aircraft, with additional stratospheric information supplied by the noon-
time NCEP/NCAR specific humidity profile for that day. The HIPPO-1 profiles used in this analysis
over Lamont are shown in Figure 2.
The NASA Glenn Lear-25 aircraft performed three profiles from 5-13 km altitude over the Southern
Great Plains (SGP) Atmospheric Radiation Measurement (ARM) Lamont site during a campaign
from July 31, 2009 to August 5, 2009 (Abshire et al., 2010). Lower altitude (0.3 km – 5 km) profiles
were measured with a Cessna 210 at essentially the same times and locations. On both aircraft, the
CO2, CH4, N2O and CO measurements were made by flask samplers, which were analysed at the
National Oceanic and Atmospheric Administration’s Earth System Research Laboratory (NOAA’s
ESRL). Water profiles were obtained from on-site sonde measurements taken at 11:30 am local time.
Many years of bi-weekly Cessna 0-5km flights are available over Park Falls and Lamont and will
be used in a future analysis to assess possible calibration drifts for those sites. The ceiling of these
flights is insufficiently high for use in this analysis.
3.3 Beechcraft King Air
The Beechcraft King Air 200T aircraft measures CO2continuously with a Li-COR (Li-840) non-
dispersive infrared analyzer. CH4and other gases are measured using hand-operated flask samplers,
which are analysed at the National Institute for Environmental Studies (NIES). The aircraft over-
passes of the Tsukuba FTS instrument were carried out on 7 and 15 January, 2009 over Tsukuba
(36.1◦N, 140.1◦E) and Kumagaya (36.15◦N, 139.38◦E). Due to air traffic control restrictions, the
higher part of the profile (2 to 7 km) was observed over Kumagaya, and the lower altitude range
(0.5 to 2 km) was observed over Tsukuba. For the purposes of the FTS calibration, only data from
the January 15 overflight is used, because of heavy cloud cover on January 7. Water profiles were
obtained from nearby radiosonde measurements taken near the time of the overpass.
4 Numerical Integration of Aircraft In Situ Profiles
To calibrate the total column measurements of the TCCON network, the aircraft in situ profiles
must be integrated with respect to altitude. In order to properly compare the ground-based FTS
measurements with the in situ aircraft measurement, which we consider the best measure of the true
state of the atmosphere, the averaging kernels of the FTS measurements (A) must be taken into
account. From the aircraft profiles (xh), an averaging kernel-smoothed profile (xs) can be computed
that, when integrated, can be directly compared with the FTS retrieved total columns. The smoothed
profile represents the profile the FTS would retrieve, if it were measuring perfectly (i.e. without
spectroscopic errors), given the GFIT a priori profile (xa) and retrieved profile scale factor (γ). We
use equation 4 of Rodgers and Connor (2003),
xs= γxa+ A(xh− γxa).
Note that for a GFIT scaling retrieval, the kernels are calculated for the solution mole fraction profile,
not the a priori profile, so the point of linearization of the Taylor expansion producing equation 2 is
γxaand not xa.
For column measurement calibration, equation 2 is integrated vertically:
ˆ cs= γca+ aT(xh− γxa)
where ˆ csis the smoothed column-averaged DMF, cais the column-averaged DMF from integrat-
ing the a priori profile and a is the FTS dry pressure-weighted column averaging kernel. The
aT(xh− γxa) term represents the column averaging kernel-weighted vertical integration of the
difference between the in situ profile and the scaled a priori profile. Column averaging kernels vary
as a function of pressure and solar zenith angle.
Integrating these profiles is done most accurately on a pressure grid, under the assumption that the
atmosphere is in hydrostatic balance. The total vertical column for gas G (VCG) is then defined in
the following manner:
where fG= fdry
· (1 − fH2O) is the mole fraction of gas G, Psis the surface pressure, and g is
the gravitational acceleration, which is a function of altitude (z) and latitude (φ). We distinguish
between the true mole fraction (fG), and the dry mole fraction (fdry
situ instrumentation measures. The mean molecular weight of air, m, can be expressed in terms of
its wet and dry components as well: m = mH2O·fH2O+mdry
equation 4 and rearranging yields a useful, numerically integrable relationship to compute V CG.
g · mdp
G), which is what the aircraft in
air·(1−fH2O). Substituting these into
1 + fdry
H2O(p) · (mH2O/mdry
g(z(p),φ) · mdry
sonde profile, mH2O= 18.02 × 10−3/NAkg/molecule, mdry
and NAis Avogadro’s constant. To compute the column averaging kernel-weighted vertical column
is the aircraft profile of gas G, fdry
1−fH2O, where fH2Ois the H2O aircraft or
air= 28.964 × 10−3/NAkg/molecule,
(to satisfy the right-hand term in equation 3), the column averaging kernel (a(p)) must be included
at every level in the integral.
G(p) · a(p)
1 + fdry
g(z(p),φ) · mdry
H2O(p) · (mH2O/mdry
The column of dry air (VCair) is computed by setting the numerator in equation 6 to 1. The column-
averaged DMF is computed by dividing the appropriate vertical columns by the column of dry air.
Hence, equation 3 becomes:
ˆ cs= γca+
Aircraft measurements have good accuracy, but are limited in altitude floor and ceiling, and so
we must use additional information for the surface and the stratosphere. When multiple instruments
aboard the aircraft measure the same species, a running mean is applied. There is one instance
where two CO measurements on HIPPO-1 disagree over Lauder in the upper troposphere (RAF and
QCLS): in this case, QCLS is used.
For surface measurements, most TCCON sites are co-located with tower or surface in situ mea-
surements. In the event that there were no surface or tower measurements available, and the aircraft
did not measure to the surface, the lowest measured value was assumed to be the surface value (e.g.
Park Falls on July 14, 2004).
For stratospheric CO2, the mole fractions are predictable at the 0.3% level, as described in more
detail below. For N2O and CH4, the stratospheric mole fractions are more difficult to estimate
because they decrease rapidly with altitude, causing transport-driven variations in the stratospheric
column. In general, the unknown state of the atmosphere above the aircraft ceiling is the largest
source of uncertainty in the total integrated column.
The CO2profiles in the stratosphere are empirically derived from in situ measurements on high-
altitude balloons and include realistic latitude and time-dependencies. The stratosphere is set by an
exponential decrease above the tropopause, based on the age of air measurements of Andrews et al.
(2001). The tropopause pressure comes from the NCEP/NCAR four-times daily analysis, which is
interpolated to local noon at the latitude and longitude of the site. A generous error of ±1 ppm is
assumed for the GFIT stratospheric a priori. These stratospheric profiles are used as a prioris for all
TCCONretrievals. A priorisforthe troposphere are derived from GLOBALVIEW (GLOBALVIEW-
The GFIT CH4, N2O, CO and HF a prioris are generated from MkIV FTS balloon profiles (Toon,
1991). The profiles are shifted up or down in altitude depending on the tropopause pressure for
local noon on that day. The CH4-HF and N2O-HF correlations in the a priori profiles are consistent
with those observed by Luo et al. (1995) and Washenfelder et al. (2003) and are preserved under the
vertical shifting. Due to a complete absence of HF in the troposphere, HF is a sensitive indicator
of ascent and descent in the stratosphere. Indeed, a 1 km vertical shift in the HF profile produces
a ~15% change in the total column, which is easily measureable. Since HF is a long-lived, stable
stratospheric tracer, we assume that any difference in the retrieved HF column from the a priori value
is due to the stratospheric dynamics and will be anti-correlated with the stratospheric N2O and CH4.
The magnitude of the deviation of the HF column from the a priori HF column is used to adjust
the CH4and N2O stratospheric profiles to generate our best estimate of the “true” stratospheric
profile for a given overpass. An illustration of this is in Figure 3. Note that even small errors in the
stratospheric a priori profile of N2O will be very important in this analysis, because the N2O column
averaging kernels increase significantly in the stratosphere. The stratospheric error contribution
for both CH4and N2O is estimated by shifting the stratospheric profile up and down by 1 km and
Unlike CH4and N2O, stratospheric CO is highly variable and does not have a simple relationship
to HF. To estimate the CO stratospheric contributions, v2.2 profiles from the low-Earth orbiting
ACE-FTS instrument (Bernath et al., 2005) were averaged within one month of the overpass and ±5
degrees latitude of the site. The work by Clerbaux et al. (2008) has shown that the ACE-FTS CO
values are accurate to 30% in the upper troposphere/lower stratosphere, and 25% above. For our
stratospheric error budget, we have taken the larger of the standard deviation of the ACE profiles,
and the estimated error by Clerbaux et al. (2008).
If water vapor profiles are not available from the aircraft in situ data (Tsukuba, Darwin and dur-
ing the Learjet overpasses of Lamont), radiosonde measurements of H2O are used. Any additional
stratospheric information is provided from GFIT a prioris, which are derived from NCEP profiles,
and extended upwards using a model based on MkIV balloon profiles. Because most of the water
column is located at altitudes below ~5 km, errors in the upper altitude water profile do not signifi-
cantly affect the total column. The errors on the H2O columns are estimated to be 10% of the total
To estimate the H2O calibration curve for the TCCON, the radiosonde profiles over Tsukuba, Dar-
win, Lamont, Lauder and Park Falls are used, which tend to reach higher altitudes than the aircraft
(generally well above the tropopause). Water profiles are available from daily sonde measurements
at Lamont and Darwin.
Once full profiles of the gas of interest and H2O are generated on a fine altitude or pressure grid,
the profiles are integrated via equation 5 or 6, and the smoothed profile is computed via equation 7.
Aircraft overflights of the Park Falls, Darwin, Lamont, Lauder and Tsukuba TCCON stations are
listed in Table 3, including their dates and which molecules were measured on the aircraft. Sample
profiles from the HIPPO aircraft over Lamont are shown in Figure 2 and the derived column-average
calibration data are shown in Figures 4–8. Errors on the FTS measurements are quoted as the 1-
σ standard deviations during the duration of the overflight for CO2, CO, CH4and N2O, and 2-σ
standard deviations for H2O, because the atmospheric variability of H2O over the course of a day can
be much greater than for the other molecules. Errors computed for the smoothed, integrated aircraft
measurements are the sum in quadrature of estimated stratospheric uncertainties and the estimated
error on the aircraft or sonde profiles in the troposphere. In all cases (except H2O), the stratospheric
uncertainty is a significant component of the total error. The slopes of the calibration curves are
listed in Table 4. Errors on the slopes are quoted as standard errors on the best fit, calculated using
the errors in both the x and y axis (York et al., 2004) and as 2 standard deviations of the individual
Our retrieval method is predicted to be both linear and have no intercept. We thus fit the data with
a linear least-squares and force a zero intercept. When the least-squares fits are allowed a nonzero
y-intercept, all but H2O have a y-intercept that is zero within the uncertainty. To attempt to remove
any biases added from errors in the GFIT a priori, the aircraft profile with our best estimate of
the stratospheric profile was input as the a priori profile. The same spectra were processed using the
standard GFIT a priori as well. The calibration coefficient for both cases have identical slopes within
standard error, suggesting that the GFIT a priori does not add a significant bias to the retrievals.
Figures 4–8 show the calibration curves calculated using the aircraft profile as the a priori.
For all molecules, there is excellent consistency between the TCCON calibrations obtained from
different sites and seasons. Within measurement error, all stations can be described by a single re-
gression line and hence single calibration factor, with variations around the regression line being
explicable by instrumental and site differences. Hence, the reported TCCON columns are produced
by dividing the retrieved columns by the values listed in Table 4. The largest uncertainties in the
calibration coefficients are for H2O. The H2O calibration curve shows that over a large range of hu-
midities, the FTS instruments are capable of measuring water columns to a good degree of accuracy,
but due to the high variability of tropospheric H2O, we do not expect calibration errors as small as
for CO2, CH4, CO or N2O.
The uncertainties on the slopes, listed in Table 4, are used to compute the species uncertainty
of each molecule, and can be compared with the WMO-recommended intercomparability for the
molecules (WMO, 2007). The calibrated TCCON data, though less precise and accurate than the in
situ data, provide long time series of precise and accurate total column measurements of atmospheric
CO2, CH4, CO and N2O.
The TCCON column-averaged dry-air mole fractions of CO2, CO, CH4and N2O have been cali-
brated to the WMO scale using aircraft profiles measured between 2004 and 2009. The TCCON
H2O columns have been calibrated using radiosonde measurements. The calibration curves show
excellent consistency between the different TCCON sites and seasons, and can be described by a
single calibration factor for each molecule. Future plans include extending this calibration set us-
ing additional HIPPO campaigns and other aircraft programs. We expect that all TCCON sites will
eventually be calibrated using WMO-scale in situ measurements.
Acknowledgements. NCEP Reanalysis data is provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado,
USA, from their Web site at http://www.cdc.noaa.gov/. Data were obtained through the Atmospheric Radi-
ation Measurement (ARM) Program sponsored by the U.S. Department of Energy, Office of Science, Office
of Biological and Environmental Research under contract DE-AC02-05CH11231. Data were generated
by the National Oceanic and Atmospheric Administration(NOAA),EarthSystem
Research Laboratory (ESRL), Carbon Cycle Greenhouse Gases Group, including flask data from
Andrews et al. (2009). US funding for TCCON comes from NASA’s Terrestrial Ecology
Program, the Orbiting Carbon Observatory project and the DOE/ARM Program. ACE is funded primarily
by the Canadian Space Agency. Support for the Learjet-25 measurements was provided by the NASA AS-
CENDS development and ESTO IIP programs. We acknowledge funding for Darwin and Wollongong from
the Australian Research Council, Projects DP0879468 and LP0562346 with the Australian Greenhouse Office.
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HIPPO−1 Flight Path
START−08/pre−HIPPO Flight Path
King Air Flight Path
Lear Flight Path
Fig. 1. TCCON Site Locations. The HIPPO flight path is overlaid in solid black, START-08 in solid green, the
King Air in black stars (*) and the Lear in black plusses (+).
380 385390 395
GFIT A Priori
GFIT A Priori
GFIT A Priori
GFIT A Priori
Fig. 2. Lamont profiles from the January 30, 2009 HIPPO overpass. The colored dots show the aircraft data.
The thick grey line in the CO panel shows the mean ACE-FTS CO profile. The thin black lines show the GFIT
a priori profile for January 30, 2009 over Lamont. The thick black line is the profile that is integrated.
Fig. 3. An illustration of the HF correction used to determine the best stratospheric profiles. Panel (a) shows the
MkIV FTS balloon profiles before correcting with the NCEP tropopause pressure. The tropopause height for the
balloon profiles is indicated with the horizontal dotted line. Panel (b) shows the standard GFIT a priori profiles,
which uses the NCEP tropopause height (indicated by the horizontal dashed line), pressure, temperature and
altitude to scale the gas profiles. Panel (c) shows the adjusted GFIT a priori profiles, using the scale factors
from the retrieved HF columns. The thick black dash-dot line shows the altitude shift (0.8 km) from an HF
scale factor of 0.9. The correlation between HF and CH4is preserved for all panels.