# Presentation: 649904

## Gas Correlation Spectroscopy and the MOPITT Instrument 

- Gas Correlation Spectroscopy and the MOPITT Instrument

**James R. Drummond**
- Department of Physics,
- University of Toronto
- _[james.drummond@utoronto.ca](mailto:james.drummond@utoronto.ca)__    Tel:  416-978-4723_

- August 30, 2002, L’Aquila, Italy

**Notes:**
This talk is supposed to give a general introduction to remote sounding of the troposphere with a specific emphasis on MOPITT and the MOPITT methodology

## MOPITT

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

**Notes:**
We begin with a brief introduction and then deal with the question “what is important about the troposphere and why should we be interested in measuring it?”  Then we look as the specific measurements that MOPITT is designed to do, followed by some of the scientific and technical details on how it does it.

## MOPITT Team

**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

**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

**Notes:**
Just a note of thanks to the many people who have helped with MOPITT over the many years of development

## 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
**It’s the bit we breathe!**

**Notes:**
The text mostly says it all.  Despite the great interest in the middle atmosphere, the troposphere is the region where most of the mass and most of the chemistry happens.

**Notes:**
It is a very varied place.  Spatial scales from 0 to planetary scale.  Surface sources can be natural or artifical.  Interacts with the stratosphere at the top – anything coming from the surface to the stratosphere goes through the troposphere first.  There are clouds which influence the radiation, dynamics and chemistry in a seemingly random manner.  They also block the view of most surface imagers and tropospheric sounders

## 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

- 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..

- 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

**Notes:**
If we want to measure these phenomena from space we have to choose a technique which is appropriate.  The problem is to separate out the very weak spectrum from the stronger emissions

## Problems

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

## 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

**Notes:**
Comparison of space techniques

## 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

## 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_x000c_

- 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_x000c_

- 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

**Notes:**
Showing a single swath of MOPITT with the sidescan coverage of about 650Km and the individual 4 x 1 88 x 22km measurements

## Slide 23

## MOPITT - Technology

- Uses cooled detectors
- cooled preamplifiers
- cooled optics
- Active cooling
- Digital Signal Processing
- Length Modulators
- Pressure Modulators

**Notes:**
The picture is of the MOPITT detectors (4 x 1 ) in the middle.  The detectors are about 1mm square.  The preamplifer is in the top left of the assembly and the whole thing is cooled to 80K

## LMCs are the two circular object
LMCs are made of Ti
Windows are coated Ge
“Gold” object is a molecular sieve

- 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

- 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

**Notes:**
Our test chamber in Toronto where the instrument was tested.

## Terra Spacecraft

**Notes:**
Just to show where we are on the spacecraft and how small we are compared to the scale of the other instrumentation.

## LAUNCH!!

- Failed launch attempt
- December 16, 1999
- Change of procedure
- Successful launch
- Dec 18, 1999, 18:57GMT (10:57PST)
- 9 seconds before end of window!
- Orbit:
- polar, sun synchronous,
- Altitude, 705km, inclination 98.4
- 10:43am desc. node

## Slide 33

**Notes:**
Some of the personnel who built and tested MOPITT

## MOPITT - Activation Timeline

## MOPITT - First CO Map

**Notes:**
Hey people! – It works – I’m leaving John to show the latest and greatest results.

## 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.

**Notes:**
Post launch care and feeding activities are mainly restricted to calibrations and decontamination.

## 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

**Notes:**
For most of the mission – notice that the line gets flatter (less change in gain with time) as the mission proceeds.

## 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

**Notes:**
This is just to point out that we need to monitor the cell pressures and cell #1 appears to be leaking slightly.  However it could be that the monitor is drifting.

## 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] <0.9%
- Simulated [DIFFrad / AVErad] <3%
- For channels 3 & 7 the actual change is <1.2%
- RADIOMETRICS IMPLIES NO LEAK

**Notes:**
So we look at the gain plot which should also be drifting if the pressure is drifting – but it doesn’t, so it must be a small drift in the sensor

## 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