Eamonn McKernan

Operational validation of the MOPITT instrument optical filters

Abstract Optical bandpass filters in the Measurements of Pollution in the Troposphere (MOPITT) satellite remote sensing instrument selectivity limit the throughput radiance to absorptive spectral bands associated with the satellite-observed trace gases CO and CH4. Precise specification of the spectral characteristics of these filters is required to optimize retrieval accuracy. The effects and potential causes of spectral shifts in the optical bandpass filter profiles are described. Specifically, a shift in the assumed bandpass profile produces a relative bias between the calibrated satellite radiances and the corresponding values calculated by an instrument-specific forward radiative transfer model. Conversely, it is shown that the observed bias (as identified and quantified using operational MOPITT satellite radiance data) can be used to determine the relative spectral shift between the nominal (prelaunch) filter profiles and the true operational (in orbit) profiles. Revising both the radiance calibratio...

MOPITT validation using ground-based IR spectroscopy

MOPITT is a nadir-viewing gas correlation radiometer due to be launched aboard the EOS Terra platform. The feasibility of MOPITT data validation using ground-based sun-viewing spectrometers of moderate resolution is investigated. Several instruments with a spectral resolution of approximately 0.2 cm-1 are now operating in Russia and in China for the monitoring of CO and CH4. A spectrometer of this type has been tested and improved at the University of Toronto. It has also been compared with other spectroscopic instruments in field conditions. The results of these comparisons, and the prospects for further work are presented and discussed.

Dynamic spectroscopic measurements of the temperature and pressure cycles in a MOPITT pressure modulator cell

The temperature and pressure cycles inside a pressure modulator cell (PMC) of the type used for gas-correlation radiometry aboard the Measurements of Pollution in the Troposphere (MOPITT) satellite instrument have been determined from dynamic measurements of the spectral line shapes of the R(0) and R(18) transitions in the fundamental vibrational-rotational band of carbon monoxide. The line strengths and linewidths were used to calculate the temperature and pressure, respectively, with a temporal resolution of approximately 200 micros, or 1/100 of a PMC cycle. The results are compared with a thermodynamic box model.

Pre-flight testing of the MOPITT instrument

The Measurements Of Pollution In The Troposphere (MOPITT) instrument will monitor the global concentrations of carbon monoxide and methane. It will be flown on the Earth Observing Satellite, EOS-AM1, scheduled for launch late in 1999. This paper primarily describes the pre-flight testing conducted at the University of Toronto, Instrument Characterization Facility (ICF) and will also very briefly describe testing, post integration to the spacecraft at the Lockheed Martin, Valley Force integration and test facility and at the Vandenburg launch site.

MOPITT sensitivity studies: Computation of instrument parameter dependencies

A detailed radiative transfer model of the MOPITT instrument and the Earths atmosphere was developed and validated. This model simulates the various radiometric sources for the instrument (atmosphere, space, onboard and laboratory calibration targets) as well as a number of detailed internal features not considered in the operational MOPITT retrieval algorithm (optical imbalance, chopper emission, etc.). It was employed to establish sensitivity levels of MOPITT to the uncertainty in various instrument parameters. It should also prove useful in the development of successor MOPITT instruments, and related correlation radiometers. The sensitivity studies highlighted several critical parameters including the filter positions, gas cell lengths and pressures, and optical imbalance. MOPITT calibration events are shown to reduce the impact of instrument parameter uncertainties on target gas retrievals, but in-flight validation is likely required for MOPITT to achieve its stated accuracy objectives.

Intercalibration of medium-resolution grating spectrometers for MOPITT validation

The validation of MOPITT measurements of atmospheric carbon monoxide (CO) and methane (CH4) will require independent, simultaneous, co-located measurements from ground- and aeroplane-based instruments. Recently, a program of MOPITT validation measurements in Russia and Canada has been proposed. This program will use three (nearly) identical Russian-made medium-resolution grating spectrometers (known as Sarcophagus) capable of measuring the atmospheric column concentration of CO and CH4. Two of these instruments are located in Russia, and one in Canada. The similarity of these instruments provides the opportunity of acquiring a highly correlated validation dataset from diverse locations around the globe. As part of this program, we are proposing to inter- calibrate these instruments using a set of standard gas cells. These cells will be regularly shipped between the instruments for calibration and inter-comparison purposes. These measurements will be made relative to measurements from a very high-resolution Difference Frequency Laser Spectrometer (DFLS) located at the University of Toronto. In this paper we present the results of a test of this inter-calibration experiment using a single CO gas cell and involving Sarcophagus, a high resolution Fourier Transform Spectrometer (FTS) and the University of Toronto DFLS.