Instrument Details (View Schematic)
MOPITT operates by sensing infrared radiation from either the thermal range (4.7 µm for CO), or solar range (2.2-2.4 µm for CO and CH4). The use of solar channels enhances the instrument sensitivity to the atmospheric boundary layer. Detection of CO and CH4 is done using correlation radiometry (the detection of elecromagnetic radiation). The instrument contains both a pressure modulated radiometer (PMR) and a length modulated radiometer (LMR).
Length Modulated Radiometer (LMR)
- Being used for the first time on MOPITT
- Operates by changing the gas path thus modulating the gas amount
- Achieved by rotating a disk of calcium fluoride in the vacuum cell
- Advantages: Pressure/temperature/time behavior is simple and pressures can be higher than pressure modulated radiometers
- Disadvantages: Optical components are moved in addition to cell gas leading to potential imbalences
Pressure Modulated Radiometer (PMR)
- Operates by changing the cell gas pressure using a piston
- Advantages: Few moving parts
- Disadvantages: Complex pressure and temperature cycles and the upper pressure is limited
- Used to increase instrument sensitivity to the upper troposphere
Channels
MOPITT has 8 channels laid out on 2 optical benches. The high and low opacity signals from each channel are both averaged and differenced, resulting in 8 D and 8 A signals. Average signals are low at the line of interest, providing information about background radiance and interfering signals. Difference signals have only significant response at the line of interest, providing information about the gas of interest. The correlation cell pressure determines the portion of the pressure-broadened line wing that is sampled. Using PMRs and LMRs with different pressures thus provides information about the vertical profile. These are the weighting functions for the different cell pressures of the 6 CO channels.
| Channel # | Primary Purpose | Modulator Type | Cell Pressure (mb) | Cell Temperature (K) | Cell Length (mm) | Spectral Band | Center Wavenumber (cm-1) |
|---|---|---|---|---|---|---|---|
| 1 | CO | LMC1 | 200 | 300 | 2-10 | CO thermal | 2166 (52) |
| 2 | CO | LMC1 | 200 | 300 | -10 | CO solar | 4285 (40) |
| 3 | CO | PMC1 | 50-100 | 300 | 10 | CO thermal | 2166 (52) |
| 4 | CH4 | LMC2 | 800 | 300 | 2-10 | CH4 solar | 4430 (140) |
| 5 | CO | LMC3 | 800 | 300 | 2-10 | CO thermal | 2166 (52) |
| 6 | CO | LMC3 | 800 | 300 | 2-10 | CO solar | 4285 (40) |
| 7 | CO | PMC2 | 25-50 | 300 | 10 | CO thermal | 2166 (52) |
| 8 | CH4 | LMC4 | 800 | 300 | 2-10 | CH4 solar | 4430 (140) |
Scanning
MOPITT scans across the satellite flight track +/- 26.1 deg in 13 secs.
This is accomplished in the instrument, by sending a signal through a cell containing the target gas. The cell pressure or length is varied, which produces a modulation in cell opacity within the lines of the target gas, while the cell opacity at other frequencies remains constant. This identifies the gas. MOPITT is a nadir (the point directly below the instrument) sounding instrument, which provides a horizontal resolution of 22 km, but introduces a number of challenges, such as the need for acurately characterizing the surface contribution to the signal.