The principal science objective of the OCO-3 mission is to retrieve a global geographic distribution of CO2 sources and sinks. The OCO-3 mission will not, however, directly measure CO2 sources and sinks. Instead, sophisticated computer-based data assimilation models that use column averaged dry air CO2 mole fraction (Xco2) data will infer the location of these sources and sinks.
To get the representative values of Xco2, or the amount of CO2 in the measured space, the OCO-3 instrument will measure at a given location, the intensity of reflected sunlight off the Earth's surface at specific wavelengths. Gas molecules in the atmosphere absorb the sunlight at specific wavelengths. So when light passes through the Earth's atmosphere, the gases that are present leave a distinguishing fingerprint that can be captured. The OCO-3 spectrometers, working like cameras, will detect these molecular fingerprints. Then the absorption levels shown in these spectra, like a captured image, will tell us how many molecules were in the region where the instrument measured. The OCO-3 measurement approach will concentrate on gathering data near the Earth's surface, where almost all of the CO2 sources and sinks are located.
One of the challenges to get to these sources and sinks, is that the light detected by the instrument must penetrate through all of the atmosphere. If you can picture the image of Earth from Space, that image will always include white swirls over the land and ocean. The presence of clouds and optically thick aerosols or uneven terrain such as mountains can block the light, and create an incomplete measurement of the complete atmospheric column. To reduce any uncertainties, the OCO-3 instrument will acquire a large number of densely-spaced samples. Each sample will cover an area of about 3 km2 when the instrument is looking straight down (nadir), along the spacecraft's ground track. The OCO-3 instrument can gather as many as 72,000 soundings on the sunlit side of any orbit. With measurement footprints of this size and density, the OCO-3 instrument will get an adequate number of high quality soundings, even in those regions where clouds, aerosols and topographic variations are present.
OCO-3 mission designers selected three specific Near Infrared (NIR) wavelength bands – O2 (Oxygen) A-band Weak CO2, and Strong CO2. The OCO-2 instrument will measure intensity over all three of these bands at the same location on the Earth's surface simultaneously. Each of the three selected wavelength bands provides specific information to measurement accuracy. The weak CO2 band wavelength, in the vicinity of 1.61 µm, is most sensitive to the CO2 concentration near the surface. Since other atmospheric gases do not absorb significant energy within this spectral range, the 1.61 µm band measurements are relatively clear and unambiguous.
To make sure that we have an accurate derivation of Xco2, we also do a comparative absorption measurement of a second atmospheric gas, O2. The concentration of molecular oxygen O2 is constant, well known, and uniformly distributed throughout the atmosphere. Therefore, O2 is the best candidate for reference measurements. The O2 A-band wavelength channel, in the vicinity of 0.76 µm, will provide the required absorption spectra. The O2 A-band spectra indicate the presence of clouds and optically thick aerosols that preclude full column measurements of CO2. Observations from this band will be used to infer the total atmospheric pressure, as well as to measure of solar light path length as it passes through the atmosphere.
Last, but not least, the strong CO2 wavelength channel, in the vicinity of 2.06 µm, will provide a second and totally independent measure of the CO2 abundance. The 2.06 µm band spectra are very sensitive to the presence of aerosols. The ability to detect and mitigate aerosol presence enhances the accuracy of Xco2. The 2.06 µm band measurements are also sensitive to variations in atmospheric pressure and humidity along the optical path. These variations in pressure and humidity have a known impact on Xco2.
OCO-3 will fly on the International Space Station and complete approximately 16 daily orbits to provide global coverage. On each orbit, the Observatory path will cross the equator at approximately 1:35 PM local time. Acquisition at this time of day is ideal for spectroscopic observations of CO2 that use reflected sunlight as the high sun maximizes the measurement signal-to-noise ratio. Furthermore, since Xco2 measurements tend to be near their daily average value at this time of day, the Observatory data will be highly representative of the region where they were acquired.