The retrieval of a global geographic distribution of CO2 sources and sinks is the principal science objective of the OCO-2 mission. The OCO-2 mission will not, however, directly measure CO2 sources and sinks. Instead, sophisticated computer-based data assimilation models that employ column averaged dry air CO2 mole fraction (Xco2) data will infer the location of these sources and sinks.
To ascertain representative values of Xco2, the OCO-2 instrument will measure the intensity of reflected sunlight off of the Earth's surface at specific wavelengths. Gas molecules in the atmosphere absorb radiation at characteristic wavelengths. Thus, as light passes through the Earth's atmosphere, the presence of these gases leaves a distinguishing fingerprint on the residual radiation. The OCO-2 spectrometers will detect these molecular fingerprints, and the level of absorption displayed in these spectra will be indicative of the abundance of molecules in the region where the measurement was acquired.The OCO-2 measurement approach will concentrate on gathering data that reflect chemical abundance near the Earth's surface, where almost all of the CO2 sources and sinks are located.
OCO-2 mission designers selected three specific Near Infrared (NIR) wavelength bands. The OCO-2 instrument will measure intensity over all three of these bands at the same location on the Earth's surface at simultaneously. Each of the three selected wavelength bands provides a specific contribution to measurement accuracy. The weak CO2 band with wavelengths 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.
Accurate derivation of Xco2 using space-based readings of the CO2 absorption requires comparative absorption measurements of a second atmospheric gas. The concentration of molecular oxygen (O2) is constant, well known, and uniformly distributed throughout the atmosphere. Thus, O2 is the best candidate for reference measurements. The O2 A-band wavelengths in the vicinity of 0.76 µm will provide the required absorption spectra. Light at this wavelength is just beyond the red end of the spectrum that is visible to the human eye. 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 the length of the path of solar light as it passes through the atmosphere.
The strong CO2 band with wavelengths 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 the presence of aerosols 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 measure.
The coverage capability of an orbiting observatory ensures a continuous, uniform measure over all regions of the Earth. The Observatory will fly in a polar, sun-synchronous orbit, providing global coverage with a 16-day repeat cycle. On each orbit, the Observatory path will cross the equator at approximately 1:18 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. Coordination of the orbit with the A-train facilitates carbon cycle science by enabling the integration of OCO-2 observations with those of other instruments that fly aboard the Aqua and Aura spacecraft. Among these measurements are the temperature, humidity, and CO2 retrievals from Atmospheric Infrared Sounder (AIRS), the cloud, aerosol and ocean color observations as well as carbon source and sink measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS), and the CH4 and CO retrievals from Tropospheric Emission Spectrometer (TES).
This diagram displays the spectral reflectance of common Earth surfaces at the wavelength of the three OCO-2 channels.