Atmos. Chem. Phys. Discuss., 8, 13355–13373, 2008
www.atmos-chem-phys-discuss.net/8/13355/2008/
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Atmospheric
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ACPD
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Nadir measurements
with ACE FTS
W. F. J. Evans et al.
Nadir measurements of the Earth’s
atmosphere with the ACE FTS: first
results
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W. F. J. Evans , E. Puckrin , K. Walker , D. Wunch , J. Drummond , S. McLeod ,
C. Boone4 , and P. Bernath4
Abstract
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1
NorthWest Research Associates, Inc., Bellevue, WA, USA
Defence R and D Canada (DRDC) – Valcartier, Quebec, Canada
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University of Toronto, Dept. of Physics, Toronto, Ontario, Canada
4
University of Waterloo, Dept. of Chemistry, Waterloo, Ontario, Canada
2
Received: 7 May 2008 – Accepted: 13 June 2008 – Published: 15 July 2008
Correspondence to: E. Puckrin ([email protected])
Published by Copernicus Publications on behalf of the European Geosciences Union.
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The primary objective of the Canadian SCISAT mission is to investigate the processes
that control the distribution of ozone in the stratosphere. The SCISAT satellite consists
of two major science instruments; an Atmospheric Chemistry Experiment (ACE) highresolution Fourier-transform spectrometer (FTS) and an ultraviolet/visible/near-infrared
spectrograph. These instruments primarily function in occultation mode; however, during the dark portion of the orbit the Earth passes between the sun and the satellite.
This configuration provides the opportunity to acquire some nadir-view FTIR spectra
of the Earth. Nadir spectra obtained with the ACE FTS are presented and analyzed
for methane, ozone and nitrous oxide. The measurements show that the instrument
should have sufficient signal-to-noise ratio to determine column gas amounts of the
major trace constituents in the atmosphere. Possible applications of these measurements to the study of global warming and air pollution monitoring are discussed.
1 Introduction
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The SciSat satellite was launched successfully on 12 August 2003 carrying the Atmospheric Chemistry Experiment (ACE) (Bernath, 2006). The satellite consists primarily
of two science instruments, the ACE Fourier-transform spectrometer (FTS) and an optical spectrograph for the Measurement of Aerosol Extinction in the Stratosphere and
Troposphere Retrieved by Occultation (MAESTRO). SCISAT uses the occultation of
the Sun by the Earth to make detailed determinations of the structure and chemistry
of the atmosphere in heights ranging from 4 to 100 km above the Earth’s surface. The
satellite orbits the Earth 15 times each day providing an opportunity to observe sunlight which has passed through the Earth’s atmosphere during 15 brief “sunrises” and
“sunsets”, which results in 30 sets of observations each day.
However, in between the sunset and sunrise occultation the ACE FTS is pointed
at the dark-side of the Earth, and therefore, is capable of acquiring and storing nadir
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Nadir measurements
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W. F. J. Evans et al.
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spectra of the Earth’s atmosphere. This could potentially result in the acquisition of another set of atmospheric gas measurements, i.e., column gas amounts, which can be
made for each satellite orbit (Kobayashi et al., 1999). Thus, the nadir-view data could
potentially contribute extra science to the ACE mission and provide complementary
data to the IASI (Phulpin et al., 2007) and TES (Beer, 2006) missions. For example,
radiative trapping or the absorption of upwelling thermal radiation by the atmosphere
could be measured for several greenhouse gases. This type of information is important
for verifying that climate models are correctly predicting the forcing function aspect of
global warming (Evans and Puckrin, 2001; IPCC, 2001; Harries et al., 2001). In addition, column amounts are useful for determining the presence of localized sources
of pollution (Reichle et al., 1990). The nadir data measured with the ACE instrument
could be very useful for filling in spatial gaps between occultations in the measurement
of several stratospheric gases, particularly ozone. The potential science for future nadir
missions could be evaluated as well. In the past we have measured the radiative trapping from CFC11 and CFC12 from IMG spectra (Evans and Puckrin, 2002; Kobayashi
et al., 1999). The radiative trapping and column amounts for ozone, methane, nitrous
oxide and carbon dioxide also can be easily measured with good signal-to-noise (SNR)
from the IMG spectra.
Previously, tests were performed on the ACE FTS instrument at the Space Instrument Calibration Facility at the University of Toronto in March 2003 (Puckrin et al.,
2003), using gases in a cell and a radiative contrast of about 100 K to approximately
simulate a nadir sensing scenario to determine if the ACE FTS instrument was, in principle, capable of measuring gases in a nadir configuration. Methane, ozone and carbon
monoxide gases were used in the cell for the purpose of simulating the measurement
characteristics of the ACE FTS instrument with respect to the nadir radiation emanating from the planet’s surface and atmosphere over most of the thermal infrared region.
These measurements indicated that the ACE FTS had a sufficient SNR to measure the
column amounts of several trace gases in the atmosphere.
In this paper, we present initial results from a collection of nearly 1000 nadir spectra
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Nadir measurements
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that were measured by the ACE FTS from orbit for a period of 2 min over South America on 6 May 2004 (Fig. 1). These spectra represent the first nadir measurements ever
−1
acquired with the ACE FTS. The ACE FTS operates at a resolution of 0.02 cm for
the occultation measurements; however, to reduce the noise in the nadir observations
of the Earth’s atmosphere the resolution was reduced to 0.4 cm−1 and measurements
taken over approximately 16 s were coadded (100 spectra). The measurement modes
of the ACE FTS are summarized in Table 1. This configuration results in the acquisition of column gas amounts that represent an average measurement over a horizontal
spatial scale of about 100 km at the Earth’s surface.
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2 Results and discussion
Title Page
An example of the raw nadir spectra collected over South America is presented in
Fig. 2a. In each figure the average uncalibrated radiance of approximately 100 individual scans collected over a period of 16 s are presented, along with the maximum
and minimum spectra in each collection of 100 scans. In Fig. 2b an indication of the
uniformity of each set of 100 spectra is presented in which the spectral radiance of
each scan has been integrated over wavenumber. It is apparent that typically there is
a variation of about 2% in the integrated radiance which is likely attributed to changes
in surface scene and possibly the instrument self emission.
In order to calibrate the radiance spectra shown in Fig. 2a, exoatmospheric measurements of the Sun and deep space were also obtained with the ACE FTS, as shown in
Figs. 3 and 4, respectively. The Sun spectra were used to represent a hot blackbody
calibration source, and they were obtained in occultation at very high altitude above the
Earth’s surface to avoid atmospheric absorption. The time series integrated spectra
show that these measurements are very uniform and have an rms noise value of 0.2%.
The deep space spectra were measured approximately 1.5 degrees off the Sun, and
these spectra represent a cold source for characterizing the instrument self-emission.
The cooling effect on the optics after pointing away from the Sun is evident in the time
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series integrated spectra.
The FTS spectra of the hot source (Sun) and cold source (deep space) were used
to calibrate the average nadir spectrum, as shown in Fig. 5a. The deep space and
nadir spectra have been expanded by a factor of 20 in order to compare more readily
the differences between them. The two measurements are very close in intensity. In
fact the deep space measurement (cold source) appears more intense than the nadir
measurement. This effect can be attributed to the fact that the temperature of the ACEFTS field-stop was decreasing between the time of the nadir measurements and the
−1
deep space measurements. The calibration result for a resolution of 0.4 cm is shown
by the blue curve in Fig. 5b. As expected, the calibrated spectrum is negative due to
the temperature impact on the deep space and nadir spectra. However, atmospheric
features can still be easily identified in the spectrum, particularly the ozone band at
−1
1050 cm . An attempt was made to correct for the variable temperature effect of the
FTS field stop. The cold source measurement (i.e., deep space) was corrected (gray
curve) by accounting for the maximum field-stop temperature difference of 3◦ C that
existed between the nadir and deep space measurements (Fig. 6). Implementing the
corrected deep space measurement in the calibration procedure yields a more realistic nadir radiance (red curve), which approximately corresponds to an Earth surface
temperature of 273 K. This is likely too low for this geographical region in May; however, it’s possible that some cloud cover may have been present which would have
resulted in a lower temperature than expected. It is probable that other optical components (e.g., beamsplitter) must also be considered to account correctly for the total
temperature change. In order to verify the temperature-related impact from other optical components it will be necessary to compare ACE nadir measurements with another
space-based sensor or some ground-truth measurements. This would help to confirm
the full thermal impact on the nadir measurements and lead to a possible method to
model the effect and account for it more accurately in the calibration procedure.
A nadir spectrum of the US standard (USS) atmosphere (Anderson et al., 1986)
was simulated using the line-by-line radiative transfer model (LBLRTM) (Clough et al.,
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2005) and incorporated line parameters from the HITRAN 2000 database (Rothman
et al., 2003), and this result is compared in Fig. 7 to the nadir radiance measured by
ACE over South America. The qualitative comparison shows that absorption features
relating to ozone, methane and water vapour are clearly evident, but less so for nitrous
oxide. The noise equivalent spectral radiance (NESR) of the ACE measurement was
−7
2
−1
estimated to be about 3.4×10 W/(cm sr cm ), resulting in a SNR of about 12:1 for
ozone. A similar comparison is shown in Fig. 8 between the ACE measurement at
−1
−1
0.4 cm and an IMG measurement at 0.1 cm obtained with a higher SNR.
An improvement in NESR can be realized by degrading the resolution of the measurement by truncating the ACE-FTS interferogram and re-calculating the spectrum,
as shown in Fig. 9. Degrading the resolution to 1 cm−1 decreases the NESR to about
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2.2×10 W/(cm sr cm ) and increases the SNR to 18:1 for ozone (Fig. 9a). Fur−1
ther degradation of the resolution to 2 cm , as shown in Fig. 9b, results in an NESR
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of 1.2×10 W/(cm sr cm ) and a SNR of 33:1 for ozone. It is interesting to note
that this resolution is approximately equivalent to the 2.8 cm−1 that was used by the
Infrared Interferometric Spectrometer (IRIS) in 1970–1971 to make global measurements of atmospheric gases from orbit. Recently, it has been demonstrated that IRIS
data at this resolution can be compared with current satellite measurements in order to study the change in the radiative forcing of greenhouse gases over a period of
more than 30 years (Harries et al., 2001). This could present another possible application of ACE nadir measurements. However, it should be noted that an occultation
measurement is lost every time a nadir measurement is acquired. Figure 9c displays
−1
the nadir spectrum at a resolution of 4 cm . The resulting NESR has improved to
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2
−1
6.8×10 W/(cm sr cm ) and the ozone SNR is now about 60:1. The final degradation
to a resolution of 8 cm−1 is shown in Fig. 9d. The NESR is 2.9×10−8 W/(cm2 sr cm−1 ),
which results in a corresponding SNR of better than 130:1 for ozone. These nadir
spectra, obtained by manually degrading the ACE-FTS spectra, exhibit ozone SNRs
of greater than 50:1 may provide an important method for monitoring air pollution on
a global basis. Nadir spectra, which represent the total atmospheric ozone column,
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may potentially be combined with occultation spectra, which represent primarily the
stratospheric ozone contribution. This may provide a possible method to recover the
tropospheric ozone component, which would be useful for the global mapping of urban
air pollution.
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ACPD
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3 Conclusions
Example results from the first nadir spectral measurements ever obtained with the ACE
FTS have been presented in this paper. The measurements were obtained at a resolu−1
tion of 0.4 cm over South America on 6 May 2004. Average nadir spectra consisting
of 100 co-additions have been analyzed to determine if atmospheric absorption features are present. It has been shown that the integrated radiance can change by as
much as 4% over a measurement period of 16 s; it is, therefore, necessary to choose
sequential spectra that are relatively uniform in order to achieve a higher SNR.
It has also been shown that calibration difficulties exist due to the thermal drift of the
FTS optics. However, a pseudo-calibration that takes into consideration the changing
temperature of the FTS field stop during the calibration and nadir measurements has
permitted an analysis of ACE nadir spectra to show sufficiently that ozone and methane
features are readily visible. Future work incorporating a comparison of measurements
with other sensors or ground truth may lead to a satisfactory method of modelling the
optics’ temperature that will address the temperature variation impact on the calibration
procedure.
Degrading the resolution of the nadir spectra (by truncating the ACE-FTS interferograms) also resulted in much higher SNR values for ozone which should be sufficient
to extract total column amounts of this gas. This resolution is approximately the same
that was used by the IRIS instrument in the early 1970s to measure atmospheric gas
columns. Hence, the ACE FTS could potentially provide a useful database that may
be compared to the earlier radiative forcing results obtained by the IRIS instrument.
This could provide one of the very few experimental determinations of the increase in
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global warming from greenhouse gases such as carbon dioxide and methane. In addition, a comparison of occultation measurements with nadir measurements obtained
with the ACE FTS may potentially be useful for monitoring air pollution sources on a
global basis.
5
Acknowledgements. Funding for ACE is provided by the Canadian Space Agency and the
Natural Sciences and Engineering Research (NSERC) of Canada.
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W. F. J. Evans et al.
References
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Anderson, G. P., Clough, S. A., Kneizys, F. X., Chetwynd, J. H., and Shettle, E. P.: AFGL
Atmospheric Constituent Profiles (0–120 km), AFGL-TR-86-0110, Optical Physics Div., Air
Force Geophysics Laboratory, Hanscom AFB, MA, USA, 1986.
Beer, R.: TES on the Aura Mission: scientific objectives, measurements, and analysis overview,
IEEE T. Geosci. Remote, 44, 1102–1105, 2006.
Bernath, P. F.: Atmospheric chemistry experiment (ACE): analytical chemistry from orbit, Trends
in Analytical Chemistry, 25, 647–654, 2006.
Clough, S. A., Shephard, M. W., Mlawer, E. J., Delamere, J. S., Iacono, M. J., Cady-Pereira, K.,
Boukabara, S., and Brown, P. D.: Atmospheric radiative transfer modeling: a summary of the
AER codes, J. Quant. Spectrosc. Ra., 91(2), 233–244, 2005.
Evans, W. F. J. and Puckrin, E.: The surface radiative forcing of nitric acid for northern midlatitudes, Atmos. Environ., 35, 71–77, 2001.
Evans, W. F. J. and Puckrin, E.: Nadir measurements from ACE of column amounts of atmospheric gases, in: Proceedings of the 2002 International Geoscience and Remote Sensing
Symposium and 24th Canadian Symposium on Remote Sensing VI, Toronto, Canada, 24–28
June 2002, 3192–3195, 2002.
Harries, J. E., Brindley, H. E., Sagoo, P. J., and Bantges, R. J.: Increases in greenhouse forcing
inferred from the outgoing longwave radiation spectra of the Earth in 1970 and 1997, Nature,
410, 355–357, 2001.
IPCC: Climate Change 2001: the scientific basis: contribution of working group I to the Third
Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, New York, USA, 2001.
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Kobayashi, H., Shimota, A., Kondo, K., Okumura, E., Kameda, Y., Shimoda, H., and Ogawa,
T.: Development and evaluation of the interferometric monitor for greenhouse gases: a highthroughput Fourier-transform infrared radiometer for nadir Earth observation, Appl. Opt., 38,
6801–6807, 1999.
Phulpin, T., Blumstein, D., Prel, F., Tournier, B., Prunet, P., and Schlüssel, P.: Applications of IASI on MetOp-A : first results and illustration of potential use for meteorology, climate monitoring and atmospheric chemistry, SPIE Proceedings, 6684, 66840F,
doi:10.1117/12.736816, 2007.
Puckrin, E., Evans, W. F. J., Ferguson, C., Walker, K. A., and Dufour, D.: Test measurements
with the ACE/MAESTRO instruments using gases in a cell, Proceedings of the SPIE Conference on Earth Observing Systems VIII, San Diego, CA, USA, 3–6 August 2003, 5151,
192–200, 2003.
Reichle, H. G., Connors, V. S., Holland, J. A., Sherrill, R. T., Wallio, H. A., Casas, J. C., Condon, E. P., Gormsen, B. B., and Seiler, W.: The distribution of middle tropospheric carbon
monoxide during early October 1984, J. Geophys. Res., 95, 9845, 1990.
Rothman, L. S., Barbe, A., Benner, D. C., Brown, L. R., Camy-Peyret, C., Carleer, M. R.,
Chance, K., Clerbaux, C., Dana, V., Devi, V. M., Fayt, A., Flaud, J.-M., Gamache, R. R.,
Goldman, A., Jacquemart, D., Jucks, K. W., Lafferty, W. J., Mandin, J. Y., Massie, S. T.,
Nemtchinov, V., Newnham, D. A., Perrin, A., Rinsland, C. P., Schroeder, J., Smith, K. M.,
Smith, M. A. H., Tang, K., Toth, R. A., van der Auwera, J., Varanasi, P., and Yoshino, K.: The
HITRAN molecular spectroscopic database: edition of 2000 including updates through 2001,
J. Quant. Spectrosc. Ra., 82, 5–44, 2003.
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Nadir measurements
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W. F. J. Evans et al.
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Table 1. ACE FTS Instrument settings for Occultation and Nadir Measurements.
Mode
Resolution
(cm−1 )
Single Scan
Duration (s)
Total Measurement
Duration (s)
Spectral Range
(cm−1 )
Footprint
(km)
ACE Occultation
Measurement
(high resolution)
0.02
2
2
700–4100
n/a
ACE Nadir
Measurement
(low resolution)
0.4
IMG Nadir
Measurement
0.1
0.1
16 (100 scans)
10
10 (1 scan)
700–4100
714–3030
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8×8
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SCISAT-1 infobox
Time (UTCG):
6 May 2004 05:31:15.00
Precision Pass Number:
3936.964595
Beta Angle (deg):
-21.422
Lat (deg):
-12.319
Lon (deg):
-58.388
Alt (km):
647.015069
q1:
0.650586
q2:
0.143302
q3:
0.363258
q4 :
0.651342
Local Time :
x (km) :
y (km):
z (km):
W. F. J. Evans et al.
Title Page
01:37:41.8114
-2484.571352
-6399.201539
-1488.728206
Boresight Grazing Point LLA
Time (UTCG):
6 May 2004 05:31:15.00
Latitude (deg):
-12.319
Longitude (deg):
-58.388
Altitude (km):
647.015069
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Fig. 1. Schematic diagram showing the path of SCISAT-1 over South America during the acquisition of 1000 nadir spectra on 6 May 2004.
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0.35
(a)
(b)
Percentage Change in Power Density
0.30
Radiance (arb. units)
0.25
0.20
0.15
0.10
0.05
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Integrated nadir radiance
measurement
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1400
Wavenumber (cm-1)
0
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100
Spectrum Number
Figure
(A) Raw
nadir spectra
radiance of
spectra
of the showing
Earth showing
the average
of 100
scans,
Fig. 2. (a)
Raw2:nadir
radiance
the Earth
the average
of 100
scans,
acquired
acquired over a time of 16 s, and the upper and lower limits. (B) Time sequence showing the
over a time
of
16
s,
and
the
upper
and
lower
limits.
(b)
Time
sequence
showing
the
variation
variation in the radiance of the 100 spectra. These variations may be associated with the
in the radiance
the 100
variations
may be associated with the presence of
presence of
of cloud
coverspectra.
or changesThese
in surface
reflectivity.
cloud cover or changes in surface reflectivity.
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Maximum nadir radiance
Minimum nadir radiance
Average nadir radiance
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Maximum high-sun radiance
Minimum high-sun radiance
Average high-sun radiance
14
Radiance (arb. units)
12
10
8
6
4
Integrated high-sun radiance
measurement
Percentage Change in Power Density
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99.5
2
(a)
(b)
99.0
0
800
1000
1200
0
1400
Wavenumber (cm-1)
100
200
300
400
500
Spectrum Number
Fig. 3. (a) Radiance measurements of the Sun at high altitude above Earths surface. (b) Time
Figure
Radiance
measurements
of the Sun
high altitude above Earth’s surface. (B)
sequence
of 3:
500(A)
Sun
spectra
showing stability
of at
measurements.
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Time sequence of 500 Sun spectra showing stability of measurements.
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0.4
Radiance (arb. units)
0.3
0.2
0.1
Percentage Change in Power Density
100.0
Maximum deep-space radiance
Minimum deep-space radiance
Average deep-space radiance
Integrated deep-space radiance
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98.0
(a)
(b)
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1400
-1
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200
300
400
500
Spectrum Number
Wavenumber (cm )
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Figure 4: (A) Radiance measurements of deep space at high altitude above Earth’s surface. (B)
Fig. 4. (a)
Radiance
measurements
of deep
space
at high
altitude
above Earths
(b)
Time
sequence of
500 deep-space spectra
showing
relative
change
of measurements
due tosurface.
the
Time sequence
of
500
deep-space
spectra
showing
relative
change
of
measurements
due
to
decreasing temperature of the FTS optics.
the decreasing temperature of the FTS optics.
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Hot Source (Sun)
Cold Source (x20) (Deep space)
Average nadir measurement (x20)
Corrected deep space
measurement (x20)
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5e-6
2
Intensity (arb. units)
-1
Radiance (W/cm sr cm )
12
Uncorrected calibration
Corrected calibration
Blackbody @ 273K
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-5e-6
2
(a)
0
800
(b)
-1e-5
900 1000 1100 1200 1300 1400
800
-1
900
1000 1100 1200 1300 1400
Wavenumber (cm -1)
Wavenumber (cm )
Fig. 5. (a) Raw ACE spectra used to calibrate nadir measurements. The cold source measureFigure 5: (A) Raw ACE spectra used to calibrate nadir measurements. The cold source
ment (i.e.,
deep space)
wasspace)
corrected
(gray (gray
curve)
by by
accounting
for the
themaximum
maximum
measurement
(i.e., deep
was corrected
curve)
accounting for
field-field-stop
◦
oexisted between the nadir and deep space measurements
temperature
difference
of
3
C
that
stop temperature difference of 3 C that existed between the nadir and deep space measurements
6). (B)
The uncorrected
calibration
resultsinina anegative
negative nadir
nadir radiance
(blue
curve).
(Fig. 6). (Fig.
(b) The
uncorrected
calibration
results
radiance
(blue
curve). Acfor the inchange
in the field-stop
temperature
betweenthe
thenadir
nadir and
and deep
countingAccounting
for the change
the field-stop
temperature
between
deepspace
space meameasurements
yieldsrealistic
a morenadir
realistic
nadir radiance
(red which
curve), approximately
which approximately
surements
yields a more
radiance
(red curve),
corresponds
corresponds to an Earth surface temperature of 273 K.
to an Earth surface temperature of 273 K.
13369
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Interactive Discussion
ACPD
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FTS Aperture Stop - Field Stop Temperature - 6 May 2004
26
Nadir measurements
with ACE FTS
25
SS ~3:30 UTC
SS ~5:08 UTC
SS ~6:45 UTC
W. F. J. Evans et al.
24
Temperature (C)
23
Title Page
22
21
Deep space part of
nadir_calibration_1
T = 22.33 - 22.30 C
~ 4:40 UTC
20
19
SR ~4:07
18
6/5/2004 2:24
6/5/2004 3:36
nadir spectra
taken here ~20 C
~ 5:27 UTC
6/5/2004 4:48
Deep space part of
nadir_calibration_2
T = 23.32 - 22.23 C
~ 6:18 UTC
SR ~5:44
6/5/2004 6:00
6/5/2004 7:12
6/5/2004 8:24
Date (UTC)
Fig. 6. Temperature behaviour of the ACE FTS field stop. The plot shows that the temperature
of the stop changes by about 2◦ C between the time of the deep space cold measurements
and the nadir measurements. The temperatures of other optical components will be necessary
to consider in the calibration procedure of the nadir spectra. SR represents sunrise and SS
represents sunset.
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1.4e-5
NESR = 3.4x10
Nadir measurements
with ACE FTS
-1
LBLRTM simulation for 1976 USS conditions @ 0.4 cm
-1
ACE nadir measurement over South America @ 0.4 cm
1.2e-5
O3
1.0e-5
W. F. J. Evans et al.
-1
Radiance (W/cm sr cm )
-7
N 2O
2
8.0e-6
Title Page
CH 4
6.0e-6
Abstract
Introduction
4.0e-6
Conclusions
References
2.0e-6
Tables
Figures
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0.0
-2.0e-6
900
1000
1100
1200
1300
1400
Wavenumber (cm -1)
Figure 7: Nadir radiance spectra of the Earth measured by the ACE FTS at a
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-1
0.4 cm ofand
compared
to the radiance
USS atmosphere
Fig. 7. Nadir resolution
radianceofspectra
the
Earth measured
by for
thea ACE
FTS at a resolution of
−1
simulated
by
the
LBLRTM
model.
Important
absorption
features
of significance
0.4 cm and compared to the radiance for a USS atmosphere simulated
by the LBLRTM
are shown in red.
model. Important absorption features of significance are shown in red.
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ACPD
8, 13355–13373, 2008
1e-5
Nadir measurements
with ACE FTS
Typical IMG nadir measurement @ 0.1 cm -1
ACE nadir measurement over South America @ 0.4 cm -1
8e-6
6e-6
Title Page
2
-1
Radiance (W/cm sr cm )
W. F. J. Evans et al.
4e-6
2e-6
Abstract
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Conclusions
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0
-2e-6
900
1000
1100
1200
1300
1400
Wavenumber (cm -1)
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Figure 8: A qualitative comparison between the nadir spectra measured by the ACE and IMG FTS systems. The
A qualitative
between
the
spectra ismeasured
the nadir
IMG measurement
at 0.1 cm-1. by the ACE and
ACE
measurement comparison
is at a resolution of
0.4 cm-1, and
Fig. 8.
IMG FTS
systems. The ACE measurement is at a resolution of 0.4 cm−1 , and the IMG measurement is
at 0.1 cm−1 .
13372
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ACPD
8, 13355–13373, 2008
L B L R T M s im u la tio n fo r 1 9 7 6 U S S c o n d itio n s
A C E n a d ir m e a s u r e m e n t o v e r S o u th A m e r ic a
2
-1
Radiance (W/cm sr cm )
1.4e-5
1.2e-5
1.0e-5
1.0e-5
8.0e-6
8.0e-6
6.0e-6
6.0e-6
4.0e-6
4.0e-6
2.0e-6
2.0e-6
0.0
0.0
(a)
900
1000
1100
1200
1300
-1
W avenum ber (cm )
1400
-1
2
N ESR = 6.8x10 -8
1.2e-5
900
1.2e-5
1.0e-5
8.0e-6
8.0e-6
6.0e-6
6.0e-6
4.0e-6
4.0e-6
2.0e-6
2.0e-6
0.0
(c)
900
W. F. J. Evans et al.
Title Page
(b)
1.0e-5
0.0
N E SR = 1.2x10 -7
1000
1100
1200
1300
-1
W avenum ber (cm )
1400
1.4e-5
1.4e-5
Radiance (W/cm sr cm )
Nadir measurements
with ACE FTS
1.4e-5
N ES R = 2.2x10 -7
1.2e-5
1000
1100
1200
1300
-1
W avenum ber (cm )
1400
N ES R = 2.9x10 -8
Abstract
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(d)
900
1000
1100
1200
1300
-1
W avenum ber (cm )
1400
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Figureradiance
9: Nadir radiance
spectra
of the
Earthmeasured
measured byby
the the
ACEACE
FTS atFTS
a resolution
of (A) 1 of (a)
Fig. 9. Nadir
spectra
of the
Earth
at a resolution
-1
cm-1, (B)−12 cm-1, (C) 4−1cm-1 and (D) 8 cm
−1
−1 . The spectra of three trace gases (in red), ozone,
1 cm , (b)
2 cmoxide
, (c)
cm and
8 cm
. The
spectra
of three
tracetransfer
gasesmodel
(in red),
nitrous
and4methane,
have(d)
been
simulated
with
the LBLRTM
radiative
for ozone,
nitrous oxide
and methane, have been simulated with the LBLRTM radiative transfer model for
comparison.
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comparison.
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13373