Preliminary Results from the First Year of Operations of the Orbiting Carbon Observatory-2 (OCO-2) (Invited Presentation)

The Orbiting Carbon Observatory-2 (OCO-2) is the first NASA Earth Science mission designed to measure atmospheric carbon dioxide (CO₂) with the precision, accuracy, resolution, and coverage required to quantify CO₂ emission sources and natural sinks on regional scales over the globe. OCO-2 was successfully launched from Vandenberg Air Force Base in California on 2 July 2014. After completing a series of spacecraft check-out activities and orbit raising maneuvers, it joined the 705 km Afternoon Constellation (A-Train) on August 3, 2014. Its 3-channel imaging grating spectrometer was then cooled to its operating temperatures and a series of calibration and validation activities was initiated. Since early September of 2014, this instrument has been routinely returning almost one million soundings each day over the sunlit hemisphere. Usually, over 10% of all soundings are sufficiently cloud free to yield full-column estimates of the column-averaged CO₂ dry air mole fraction, X_CO2, with single-sounding random errors are between 0.5 and 1 ppm at solar zenith angles as large as 70 degrees, as expected.

As the data set has grown over the first year of operations, global maps of X_CO2 from OCO-2 have started to resolve some of the most robust features of the atmospheric carbon cycle. The largest pole-to-pole gradients in X_CO2 are seen In in early May, as land plants in the northern hemisphere are starting to rapidly absorb CO₂ from the air to form new leaves, branches, and roots. Between mid-May and mid-July, this intense CO₂ drawdown decreases the X_CO2 over much of the northern hemisphere by 2% to 3% (8 to 12 parts per million out of the ambient 400 part per million background concentration). During this season, X_CO2 enhancements co-located with intense fossil fuel combustion and biomass burning can be seen in individual OCO-2 ground tracks, but are much less obvious in global maps. In the fall, when northern hemisphere biosphere slows down and the pole-to-pole gradient in X_CO2 almost vanishes, CO₂ enhancements from human activities are much more obvious. OCO-2 retrievals indicate that fossil fuel emissions increase the X_CO2 by almost 1 ppm above the background in the eastern U.S. and by almost twice that much in eastern China. Biomass burning in the Amazon, south-central Africa, and Indonesia also produce X_CO2 enhancements in this range (1-2 ppm) in October. As the season progresses, the X_CO2 enhancements associated with biomass burning in Africa move north of the equator. As noted above, these are some of the most robust features of the global carbon cycle and were expected. As the carbon cycle science community continues to analyze OCO-2 data, information on regional-scale sources (emitters) and sinks (absorbers) as well as far more subtle features are expected to emerge from this rich, high resolution, global data set.

Over its first year in orbit, the OCO-2 operations and observing strategy have been optimized to improve the data quality and coverage. For example, in the late spring of 2015, the spacecraft's orbit track was adjusted to better align the ground footprints of the nadir OCO-2 soundings with the cloud and aerosol soundings from CloudSat and CALIPSO to provide additional data for validating the OCO-2 cloud and aerosol retrievals. The glint/nadir observing strategy was also modified to produce more regular coverage of the ocean and high latitude continents. The initial strategy acquired only glint or nadir observations over the entire sunlit hemisphere for a complete, 16-day, ground track repeat cycle, and then used the other observing mode in the next 16-day cycle. This approach optimized the coverage of oceans and continents on monthly time scales, but produced 16-day long gaps in the coverage of the ocean while in nadir mode, and limited coverage of high latitude continents while in glint mode. In early July of 2015, this observation strategy was modified to alternate between glint and nadir observations on alternate orbits. This approach yields more continuous coverage of the entire sunlit hemisphere every day. Additional modifications are being planned to always acquire glint observation on predominately ocean orbit tracks, to further increase the data yield over the ocean.

The OCO-2 team began delivering Version 7 data products to the Goddard Earth Sciences Data and Information Services Center (GES-DISC) in early June 2015, for distribution to the science community. These products include calibrated, geo-located spectral radiances (Level 1 products), and retrieved geophysical quantities, including spatially resolved estimates of X_CO2, surface pressure, and solar-induced chlorophyll fluorescence (Level 2 products). These products are currently being validated to identify and correct regional scale biases. This presentation will summarize the mission status, principle features of the data product, and near-term plans.