Modelled column-averaged CO2 (XCO2), June to August 2019 mean from an atmospheric transport simulation: high values over the northern mid-latitudes, low over the summer land biosphere.

CO₂ Measurement

From a spectrum to a CO2 number: the three measured OCO-2 bands of one sounding over Railroad Valley, Nevada, and the five quantities retrieved from it (XCO2 422.89 plus or minus 0.43 ppm, surface pressure 858 hPa, per-band reflectance, aerosol optical depth 0.071, quality good). Click to enlarge.

What OCO sees: measured spectra of one OCO-2 sounding in the three bands at 0.76, 1.61 and 2.06 micrometres, above laboratory reflectance spectra of snow, dry grass, desert soil, conifer forest and water from 0.4 to 2.2 micrometres with the three bands marked. Click to enlarge.

The OCO instruments record spectra of reflected sunlight at near infrared wavelengths where both CO₂ and O₂ absorb.

To determine the ratio of the amounts of CO₂ and dry air between the ground and space (called XCO₂) the spectra are analyzed using a 'retrieval model' that calculates how much CO₂ and O₂ best fits the absorption seen in the spectra.

A significant challenge to determining XCO₂ is that the light detected by the OCO instruments passes through the atmosphere twice - once on the way down toward the surface to where it is scattered or reflected and then back to space where OCO records the light. To account for scattering and reflection by both the surface and particles within the atmosphere a sophisticated algorithm is needed to resolve different types of surfaces (ocean and land) and types of aerosol (e.g. dust or smog particles).

Only when clouds are not present and aerosol is low can good measurements of XCO₂ be made. Fortunately, because the instruments record as many as 72,000 observations on the sunlit side of each orbit, the OCO instruments acquires a large number of high quality measurements, even in those regions where clouds and aerosols are present.

To Learn More:

To determine how much dry air the reflected sunlight has passed through, OCO also records absorption by oxygen (O₂). The concentration of O₂ is constant, well known, and uniformly distributed throughout the atmosphere. OCO records absorption by O₂ in the so-called 'A-band' near of 0.76 µm (760nm). Observations from this band are used to measure the amount of air that the sunlight passes through on its way to the spectrometer.

OCO three spectrometers record spectra of reflected sunlight in three narrow near Infrared (NIR) wavelength bands – O₂ (near 0.76 µm), CO₂ (near 1.61 µm) and CO₂ (near 2.06 µm). The OCO instruments simultaneously measure the reflected sunlight intensity at these wavelengths from light scattered from the same location on the Earth's surface. The CO₂ band at 1.61 µm is sensitive to the CO₂ throughout the lower atmosphere. The CO₂ band in the vicinity of 2.06 µm is more sensitive to CO₂ near the surface. Together, using the data from all three spectrometers it is possible to determine how much CO₂ is in the atmosphere, how much aerosol is present, and how the scattering by these aerosols changes with wavelength (which depends on the size of the particles). All this information is essential for accurate measurement of XCO₂.

The strategy OCO uses for measuring XCO₂ was designed by Zhiming Kuang, a Caltech graduate student (now Professor at Harvard) working with Prof. Yuk Yung. You can read the original 2002 manuscript by Zhiming and his colleagues from Caltech and JPL here.

CO2 Measurement

CO₂ Measurement