.[1] We introduce a strategy for measuring the column-averaged CO 2 dry air volume mixing ratio X CO2 from space. It employs high resolution spectra of reflected sunlight taken simultaneously in near-infrared (NIR) CO 2 (1.58-mm and 2.06-mm) and O 2 (0.76-mm) bands. Simulation experiments, show that precisions of $0.3 -2.5 ppmv for X CO2 can be achieved from individual clear sky soundings for a range of atmospheric/surface conditions when the scattering optical depth t s is less than $0.3. When averaged over many clear sky soundings, random errors become negligible. This high precision facilitates the identification and correction of systematic errors, which are recognized as the most serious impediment for the satellite X CO2 measurements. We briefly discuss potential sources of systematic errors, and show that some of them may result in geographically varying biases in the measured X CO2 . This highlights the importance of careful calibration and validation measurements, designed to identify and eliminate sources of these biases. We conclude that the 3-band, spectrometric approach using NIR reflected sunlight has the potential for highly accurate X CO2 measurements.

A variation of the modified Langley plot algorithm is reported here and applied to the retrieval of atmospheric water column abundance from a filtered sunphotometer. In this new methodology an absorption data base (LOWTRAN 7) is used to compute a water abundance versus transmittance curve of growth, rather than the square‐root dependence previously assumed. Validation of the technique is provided from an uncertainty analysis, and plans to further validate using Fourier transform spectrometers are detailed. The new sunphotometer technique is used to report total column water vapor during the First ISLSCP Field Experiment (FIFE), and comparisons are made with abundances retrieved via FIFE radiosonde observations. The sunphotometer data can best be utilized, however, to in turn validate data from airborne or in‐orbit measurements of water vapor. With these flight sensors, horizontal and topographic variability within the scene can be viewed. An example of the airborne data set is given using an image from the airborne visible infrared imaging spectrometer (AVIRIS), as acquired on August 31, 1990.

[1] The column-averaged volume mixing ratio (VMR) of CO 2 over Kitt Peak, Arizona, has been retrieved from high-resolution solar absorption spectra obtained with the Fourier transform spectrometer on the McMath telescope. Simultaneous column measurements of CO 2 at $6300 cm À1 and O 2 at $7900 cm À1 were ratioed to minimize systematic errors. These column ratios were then scaled by the mean O 2 VMR (0.2095) to yield columnaveraged vmrs of CO 2 . These display similar behavior to the Mauna Loa in situ surface measurements. During the period 1977 -1995, the column-averaged mixing ratio of CO 2 increased at an average rate of 1.49 ± 0.04 ppmv/yr with seasonal variations of $7 ppmv peak-to-peak. Our retrievals demonstrate that this remote technique is capable of precisions better than 0.5%.

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