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Gas Correlation Spectroscopy and the MOPITT Instrument

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University of Toronto. York University. University of Saskatchewan. McGill University ... G. Mand, Toronto. J.C. McConnell, York. G.D. Peskett, Oxford, UK. H.G. ... – PowerPoint PPT presentation

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Title: Gas Correlation Spectroscopy and the MOPITT Instrument


1
Gas Correlation Spectroscopy and the MOPITT
Instrument
August 30, 2002, LAquila, Italy
  • James R. Drummond
  • Department of Physics,
  • University of Toronto
  • james.drummond_at_utoronto.ca Tel 416-978-4723

2
MOPITT
  • Introduction
  • What does MOPITT do?
  • How does it do it?
  • How is the instrument designed?
  • Conclusions

3
MOPITT Team
  • Principal Investigator
  • James R. Drummond
  • Co-Investigators
  • G.P. Brasseur, Max-Plank
  • G.R. Davis, Saskatchewan
  • D. Edwards, NCAR
  • J.C. Gille, NCAR
  • G. Mand, Toronto
  • J.C. McConnell, York
  • G.D. Peskett, Oxford, UK
  • H.G. Reichle, Jr., NCSU
  • N. Roulet, McGill
  • J. Wang, SPIE
  • AGENCIES
  • University of Toronto
  • York University
  • University of Saskatchewan
  • McGill University
  • National Center for Atmospheric Research (NCAR)
  • Oxford University
  • Canadian Space Agency (CSA)
  • Meteorological Service of Canada (MSC)
  • North Carolina State University (NCSU)
  • COM DEV International
  • Hughes-Leitz
  • BOMEM
  • SED
  • NSERC
  • NASA

http//www.atmosp.physics.utoronto.ca/mopitt/home.
html http//www.eos.ucar.edu/mopitt/home.html
4
Why The Troposphere?
  • Lowest region of the atmosphere
  • Contains 80 of atmospheric mass
  • Continual vertical and horizontal mixing
  • Anything that reaches the higher atmosphere goes
    through the troposphere first
  • Exchanges with all surface biological systems
  • Its the bit we breathe!

5
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6
Why The Troposphere?
  • Contains a variety of gases
  • Major constituents
  • Oxygen, nitrogen, argon,……
  • Water vapour
  • Minor constituents
  • Carbon dioxide, methane, carbon monoxide, ozone
  • For many purposes the minor constituents are the
    most important
  • Determine interaction with sun and surface
  • Greenhouse effect,…..
  • Determine air quality and pollution

7
Tropospheric Chemistry
  • Tropospheric chemistry can be regarded as
    chemistry of OH/HO2
  • OH acts as a detergent which oxidises many
    pollutants
  • These compounds are very difficult to measure
  • Look at measuring allied compounds which can be
    measured
  • E.g. carbon monoxide (CO)

8
And Dynamics and Radiation..
  • The troposphere is in a constant state of motion
    and transformation
  • Surface sources can be highly variable and very
    strong
  • Gaseous chemistry transforms things over a period
    of time
  • Compounds can be transported over great distances
    in the middle troposphere
  • Effects can happen far from sources

9
Sorting out the Signal
  • The CO spectrum (bottom panel) is weak compared
    to the more plentiful H2O (top panel) and other
    compounds.
  • Need to be extremely selective or the signal will
    get lost in the atmospheric noise of the other
    gases

10
Problems
  • CLOUDS!
  • Penetration zero until the microwave region
  • Measure over the clouds only
  • Surface
  • Large signal
  • Irrelevant
  • Heterogeneous

11
Space Measurement Techniques
  • Active vs passive
  • Virtually all passive
  • Nadir viewing vs limb viewing/occultation
  • Solar reflection vs thermal emission
  • Spectroscopy
  • UV-Visible, infrared
  • Grating, FTS, correlation
  • Limb vs nadir
  • Limb gives a larger signal and good vertical
    resolution, but cannot penetrate far into the
    troposphere because of clouds, aerosols and
    refraction. Also has poor horizontal resolution
  • Nadir has a lower signal and a surface
    background, but has better horizontal resolution
    and can penetrate to the ground

12
MAPS
  • The MAPS instrument on the shuttle showed some of
    the features of the global CO field.
  • Outflow from biomass burning events can be seen
    in this map. Other features can be seen in other
    seasons.
  • But… incomplete...

13
MOPITT
  • MOPITT measures gas concentrations in the
    troposphere (0-15km)
  • A very challenging region because of
  • Underlying surface
  • Heterogeneous structure
  • Clouds
  • Stratosphere has very little of any of the above
    (by comparison)

14
Measurement Scenario
  • MOPITT operates by sensing infra-red radiation
    from either
  • The surface thermal emission at 4.7µm for CO
    profiles
  • Reflected sunlight at about 2.2-2.4µm for CO and
    CH4 column measurements in daylight
  • The radiation is modified by absorption/emission
    processes in the atmosphere and these changes are
    detected in the MOPITT instrument using
    Correlation Radiometry (CR) techniques

15
Correlation Radiometry
  • Correlation Radiometry (CR) Uses a sample of the
    gas of interest as a filter to separate out the
    required emissions from the background.
  • It has been used on a number of spacecraft
    instruments.
  • MOPITT uses two methods of CR
  • Pressure modulation using Pressure Modulator
    Cells (PMCs)
  • Length modulation using Length Modulator Cells
    (LMCs)

16
Simple CR System
17
  • In a CR system the amount of the gas is modulated
    and the incoming radiation is passed through the
    gas cell.
  • Radiation is modulated at the cell frequency if
    it's optical frequency coincides with that of a
    spectral line of the gas in the cell.
  • Detection of the modulated part of the radiation
    is performed digitally and this is a better
    indicator of the spectrum of interest than the
    raw radiance.
  • Advantages are the reduction of required dynamic
    range and rejection of other gaseous emission
  • Disadvantage is the complexity of the resulting
    signal

18
MOPITT - Vertical Resolution
  • Varying the pressure in the correlation cell
    varies the distance from line centre of maximum
    response
  • This can be used to tune the correlation cell
    to different vertical levels

19
Wavelength Regions
  • CO fundamental band at 4.7um
  • Gives information on profile
  • But no sensitivity at ground
  • CO overtone at 2.3um
  • Sensitive to column
  • Only works in sunlight
  • CH4 at 2.2um
  • Same as CO overtone
  • Quite weak

20
Solar vs Thermal
  • Thermal radiation
  • Is there all the time
  • Is more complex to understand
  • Can give altitude information
  • Solar radiation
  • Only in sunlight
  • Simpler in concept
  • Tends to give column amounts

21
MOPITT OPTICS
Cooler B
Cooler A
22
MOPITT - Scanning
23
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24
MOPITT - Technology
  • Uses cooled detectors
  • cooled preamplifiers
  • cooled optics
  • Active cooling
  • Digital Signal Processing
  • Length Modulators
  • Pressure Modulators

25
  • LMCs are the two circular object
  • LMCs are made of Ti
  • Windows are coated Ge
  • Gold object is a molecular sieve

26
Pressure Modulator Cells
  • PMCs have been used since mid-1970s for
    stratospheric sounding
  • Have a continuous cycle of gas amounts using a
    piston and cylinder arrangement
  • Have a finite maximum pressure, so not suitable
    for lower troposphere

27
MOPITT - Instrument
28
MOPITT Calibration
  • MOPITT has a saw-tooth scan - takes 13 secs
  • A space view is taken every 10 scans
  • A two-point calibration is taken every 11 minutes
  • A long calibration is performed monthly.

29
MOPITT - Testing
  • MOPITT is a complex instrument, requiring much
    testing
  • Correlation cell performance needed to be
    verified

30
MOPITT - Test Chamber
31
Terra Spacecraft
32
LAUNCH!!
  • Failed launch attempt
  • December 16, 1999
  • Change of procedure
  • Successful launch
  • Dec 18, 1999, 1857GMT (1057PST)
  • 9 seconds before end of window!
  • Orbit
  • polar, sun synchronous,
  • Altitude, 705km, inclination 98.4
  • 1043am desc. node

33
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34
MOPITT - Activation Timeline
35
MOPITT - First CO Map
36
Decontamination
  • The MOPITT detectors operate at about 85K
  • They act as a magnet for gaseous contamination
    in the instrument, particularly water vapour
  • Since water absorbs at MOPITT wavelengths the
    gain drops
  • Periodically we warm the detectors up to clear
    off the decontamination
  • Over time the water vapour gets less and the
    decontamination rate slows.

37
Gain Trend for Thermal Channels
The gain trend within a decontamination cycle in
2000 Ave Gain 2.32 / 200 days, Dif Gain 2.79/
200 days
38
Gain Trend Through the Mission
39
LMC Pressures
  • Cells 2-4 - SEALED
  • Cell 1 transducer indicates steady pressure
    decrease - 0.6kPa/yr
  • IF there is a leak the it should be also seen in
    the radiometric signals

40
LMC Pressures
  • Cells 2-4 - SEALED
  • Cell 1 transducer indicates steady pressure
    decrease - 0.6kPa/yr
  • IF there is a leak the it should be also seen in
    the radiometric signals
  • Actual DIFFgain / AVEgain lt0.9
  • Simulated DIFFrad / AVErad lt3
  • For channels 3 7 the actual change is lt1.2
  • RADIOMETRICS IMPLIES NO LEAK

41
Conclusions
  • Sounding the troposphere is possible
  • Other talks with consider the issues of
    retrievals
  • We have a good dataset from MOPITT for 14 months
    of operation (until cooler failure)
  • We now have a new view of the earth which was not
    possible before
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