Validation

The OCO team is currently developing techniques that will verify the observations of column averaged dry air mole fraction (Xco₂) acquired from space. Ground based Fourier Transform Infrared Spectrometers (FTIRs) will acquire precise and accurate measurements of Xco₂ from a variety of locations. Very precise laboratory instruments will provide detailed characteristics of spectrographic lines of O₂and CO₂ to the precision required for proper interpretation of the acquired atmospheric spectral measurements. In addition, the OCO project will support the acquisition of in situ observations of the CO₂ mixing ratio from flasks, tall towers, and aircraft. Research scientists will employ all of these sources of information to calibrate, verify and improve the accuracy of the OCO space-borne measurements of Xco₂.

In situ Observations
In situ CO₂ measured from surface sites and aircraft will play a vital role in the validation of Xco₂ measured from space. Among the merits of these observations are:

• the ability to obtain high absolute accuracy
• the ability to profile vertical variability from aircraft
• the ability to measure horizontal variability from aircraft
• the ability to capture sampling biases over long time periods
• an insensitivity to the effects of airborne aerosols

To ensure their applicability for the validation of OCO data, the ground-based FTIR observations will measure the same absorption bands at a much higher spectral resolution and record measurements with very similar vertical sensitivity.

Laboratory Spectroscopy
Improved measurements of the positions, the strengths, the widths, and the shapes of the O₂ and CO₂ spectral lines are essential both to interpret the data collected by the OCO instrument as well as to meet mission precision requirements. The current state-of-the-art absolute accuracy of CO₂ and O₂ intensities is 1% or better. The OCO instrument will require accuracies of 0.2% or better.

Members of the OCO science team will record high quality spectra of O₂ and CO₂ using the 1-m Fourier Transform Spectrometer (FTS) from the McMath-Pierce solar telescope facility at the summit of Kitt Peak in Arizona. OCO science team members will incorporate the following data into their investigation in order to retrieve the required CO₂ and O₂ spectral line information at Kitt Peak:

• accurate knowledge of gas conditions from mass-spectrometry calibrated CO₂ samples,
• accurate pressure and temperature measurements,
• a large number of spectra under a range of measurement conditions,
• high signal to noise ratio spectra through use of linear detectors and long integration times, and
• retrieval procedures that use accurate instrument models and detailed theories based on molecular physics.