Transcript MOPITT - A New View of the Troposphere

Gas Correlation Spectroscopy and
the MOPITT Instrument
August 30, 2002, L’Aquila, Italy
James R. Drummond
Department of Physics,
University of Toronto
[email protected] Tel: 416-978-4723
MOPITT
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Introduction
What does MOPITT do?
How does it do it?
How is the instrument designed?
Conclusions
MOPITT Team
• AGENCIES
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• Principal Investigator
University of Toronto
York University
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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
– James R. Drummond
Co-Investigators
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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
http://www.atmosp.physics.utoronto.ca/mopitt/home.html
http://www.eos.ucar.edu/mopitt/home.html
Why The Troposphere?
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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
It’s the bit we breathe!
Thermosphere
Mesopause
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Altitude (km)
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Mesosphere
Stratopause
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Stratosphere
Tropopause
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Troposphere
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Temperature (K)
300
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”
Tropospheric Chemistry
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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)
And Dynamics and Radiation..
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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
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
Problems
• CLOUDS!
– Penetration zero until the
microwave region
– Measure over the clouds
only
• Surface
– Large signal
– Irrelevant
– Heterogeneous
Space Measurement Techniques
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Active vs passive
– Virtually all passive
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Nadir viewing vs limb
viewing/occultation
Solar reflection vs thermal
emission
Spectroscopy
– UV-Visible, infrared
– Grating, FTS, correlation
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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
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...
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)
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
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)
Simple CR System
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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
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
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
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
MOPITT OPTICS
Cooler B
Cooler A
MOPITT - Scanning
MOPITT - Technology
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Uses cooled detectors
cooled preamplifiers
cooled optics
Active cooling
Digital Signal Processing
Length Modulators
Pressure Modulators
• LMCs are the two circular
object
• LMCs are made of Ti
• Windows are coated Ge
• “Gold” object is a
molecular sieve
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
MOPITT - Instrument
MOPITT Calibration
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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.
MOPITT - Testing
• MOPITT is a complex
instrument, requiring
much testing
• Correlation cell
performance needed to
be verified
MOPITT - Test Chamber
Terra Spacecraft
LAUNCH!!
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Failed launch attempt
– December 16, 1999
– Change of procedure
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Successful launch
– Dec 18, 1999, 18:57GMT
(10:57PST)
– 9 seconds before end of
window!
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Orbit:
– polar, sun synchronous,
– Altitude, 705km, inclination
98.4
– 10:43am desc. node
MOPITT Activation Timeline
MOPITT - First CO Map
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.
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
Gain Trend Through the Mission
LMC Pressures
20.4
20.2
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19.8
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19.6
19.4
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150 200 250
Ops days (MET-13)
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P re s s 1 (k P a )
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P re s s (k P a )
• 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
LMC Pressures
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– Actual [DIFFgain / AVEgain] <0.9%
– Simulated [DIFFrad / AVErad] <3%
– For channels 3 & 7 the actual
change is <1.2%
RADIOMETRICS IMPLIES NO LEAK
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20.4
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150 200 250
Ops days (MET-13)
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P re s s 1 (k P a )
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
P re s s (k P a )
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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