Concern about the climatic effects of anthropogenic emissions of carbon dioxide (CO<sub>2</sub>) has resulted in a growing need, both scientifically and politically, to monitor atmospheric CO<sub>2</sub>. The development of a satellite instrument which could measure the global distribution of atmospheric CO<sub>2</sub> would greatly improve our understanding of the global carbon cycle and provide a means of monitoring regional sources and sinks. In this paper, we propose and analyse the potential of a nadir-viewing, satellite-based remote sensing instrument consisting of a multi-channel Gas-Filter Correlation Radiometer (GFCR) tuned to the 6300 cm<sup>-1</sup> (1.6 μm) and 5000 cm<sup>-1</sup> (2.0 μm) regions to globally measure the atmospheric CO<sub>2</sub> column. Although such an instrument would present some engineering challenges, we find that it could potentially measure the atmospheric CO<sub>2</sub> integrated-column to a precision of 1ppmv of CO<sub>2</sub> or better.
Satellite-based remote sensing of atmospheric CO<sub>2</sub> holds the promise to greatly improve our understanding of the processes which regulate atmospheric CO<sub>2</sub> and the global carbon cycle. However, the required precision and resolution of such measurements needed to characterise sources and sinks of CO<sub>2</sub> on regional scales presents strong instrument design challenges. One type of remote sensing instrument which has been proposed to measure the integrated-column concentration of CO<sub>2</sub> is a Gas-Filter Correlation Radiometer (GFCR). As a technique, a GFCR is a radiometer which uses a sample of the gas of interest as a spectral filter for that gas in the atmosphere. In this paper we present a "strawman" design for a GFCR satellite instrument to remotely sense atmospheric CO<sub>2</sub>. This design, which includes multi-pass CO<sub>2</sub> and O<sub>2</sub> gas cells with path lengths of up to 10 metres, demonstrates that such an instrument can be built within the constraints of a satellite environment.
A unique optical configuration of a Gas-Filter Correlation Radiometer (GFCR) is described. This configuration, known as a Simultaneous-View Correlation Radiometer (SVCR), was designed for surface-viewing nadir remote sensing of atmospheric trace gases in the near-infrared. It provides simultaneous measurement of both the gas-filter and correlation channels of the GFCR, minimising noise induced by geo-spatial variations in the surface reflectivity. We analyse the reduction in the sensitivity of the SVCR to noise in input radiance in comparison to a sequential gas-density state GFCR.
Concern about the climatic effects of anthropogenic emissions of carbon dioxide has resulted in a growing need, both scientifically and politically, to monitor atmospheric carbon dioxide. The development of a satellite instrument which could measure the global distribution of atmospheric carbon dioxide would greatly improve our understanding of the global carbon cycle and provide a means of monitoring regional sources and sinks. In this paper, we propose and analyze the potential of a nadir-viewing, satellite-based remote sounding instrument consisting of a simple filter radiometer tuned to the 6300 cm<SUP>-1</SUP> (1.6 micrometers ) region to globally measure the atmospheric carbon dioxide column. Such an instrument would be among the simplest of all potential remote sounding instruments to make this measurement. Retrievals by a radiometer instrument are modeled using high-resolution FTS spectra and compared with SFIT2 retrievals. We find that the proposed instrument has potential, and that the sensitivity is likely to be limited by our knowledge of the atmospheric temperature and uncertainty line strengths and widths.
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<SUP>-1</SUP> are now operating in Russia and in China for the monitoring of CO and CH<SUB>4</SUB>. 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.
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 describes the analysis of a twenty four hour data set that was recorded during the latter stages of testing at the University of Toronto Instrument Characterization Facility (ICF). This data set represents the best 'near real time' contiguous data available and it is being used to help understand the instrument behavior and characteristics, as well as with algorithm development with the goal of the University of Toronto team being to determine the gain, offset and noise parameters for all channels from the in-flight calibration system.
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.
The validation of MOPITT measurements of atmospheric carbon monoxide (CO) and methane (CH<SUB>4</SUB>) 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 CH<SUB>4</SUB>. 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.
The Measurements of Pollution in the Troposphere-Aircraft (MOPITT-A) instrument is being constructed at the University of Toronto, as a primary data validation tool for the Terra based MOPITT instrument. MOPITT-A is designed to operate aboard a NASA ER-2 research aircraft and as such must be rugged and field serviceable while maintaining the same characteristics as the satellite instrument. The resulting instrument is a hybrid of flight space components with commercial devices. Calibration data generated by both instruments, at the U of T Instrument Calibration Facility (ICF) will play a key role in data validation.
MOPITT is a satellite instrument which will be launched in 1998 on the EOS-AM1 platform of the Earth Observing System. The primary objective of the MOPITT instrument is to enhance our knowledge of the lower atmosphere by measuring atmospheric profiles of carbon monoxide (CO) and methane. Operationally MOPITT will employ a new form of correlation radiometer known as the length modulated radiometer (LMR). To date, the LMR has been successfully implemented in a ground-based remote sounding instrument measuring CO, and is currently being implemented on two airplane-based instruments known as MATR and MOPITT-A. The operating principle of the LMR is the modulation of a static gas cell path length by means of an optically inert filler material. This paper will describe aspects of the operation of an LMR. Topics that will be covered include a discussion of the sources of optical imbalance in the LMR and the radiometric calibration of the LMR with CO. An analysis of the sources of error in the radiometric calibration of an LMR will also be presented.