By: Karen Yuen
The common adage “good things will come to those who wait” cannot ring truer than for NASA’s Orbiting Carbon Observatory-2 (OCO-2) mission and team members. A little more than a year after launch, the team is proud to make their first milestone in demonstrating the capability and precision of the OCO-2 instrument in measuring atmospheric carbon dioxide (CO2) concentrations.
OCO-2 launched from Vandenburg Air Force Base in California on July 2, 2014, and officially began gathering CO2 data on September 6, 2014. Since then, the team has been working hard to make sure that the data being collected from space are as good as they can be. The OCO-2 data measurements are being compared to measurements from a selection of more than twenty ground stations around the world, that are a part of the Total Column Carbon Observing Network (TCCON). These TCCON sites make observations over an enormous range of topographies and latitudes, with stations located on six of the seven continents, and governed by ten different countries.
To date in comparison, the team has found that the OCO-2 measurements are consistently within 1.2 parts per million (ppm) of these global TCCON sites.
“It is very rewarding to see the instrument work as we hope, and the TCCON comparison is really the critical component to linking the satellite remote sensing data to the international standards of CO2 measurements. Our TCCON partners have provided an invaluable dataset for this fundamental step in our mission”, said Greg Osterman, OCO-2 Validation Lead.
OCO-2’s capability to obtain precise CO2 measurements is more important than ever, because atmospheric CO2 has been rising globally since the advent of the industrial revolution, from 290 parts per million (ppm) to just over 400 ppm today. Although CO2 is an important gas for life on this planet, its role as a primary greenhouse gas can tip the Earth’s fragile atmospheric balance, if not managed properly. The sharp rise over the last 140 years has alarmed the international science communities, and raised concerns to the overall effect and potential consequences to the Earth’s carbon cycle. The demonstration of OCO-2’s capability, is a tremendous achievement towards the confidence in using OCO-2 to study the much needed and anticipated global and regional carbon fluxes over time.
So how good is OCO-2? Imagine that you have 10-gallon fish tank in Los Angeles and you are about to add 1-2 drops of food coloring to the tank. Your best friend would be able to see the change in color from San Francisco, which is more than 600 km away! This is how precise OCO-2 is from 705km or 438 miles out in space.
OCO-2 works by measuring the sunlight reflected off the Earth's surface. The sunlight rays entering the instrument will pass through the atmosphere twice - once as they travel from the Sun to the Earth, and then again as they bounce off from the Earth's surface to the OCO-2 instrument. So OCO-2 does not measure CO2 directly; but actually, the intensity of the sunlight reflected from the presence of CO2 in a column of air that goes from space all the way down to the ground. This measurement is unique like a fingerprint, and can be used for identification because gases in the atmosphere absorb light energy at very specific colors or wavelengths.
Figure 1. The artist illustration showing how OCO-2 samples the CO2 molecules in an atmospheric column from the reflected sunlight, all the way down to the surface.
About a million soundings a day are made by OCO-2 and the satellite flies between 14-15 global orbits daily. Since OCO-2 cannot see through clouds and our Earth is a cloudy planet, OCO-2 needs to make a lot of soundings and look for the cloud-free passes among the many daily soundings. In fact, the mission makes 233 passes every 16 days to sample a near complete global picture.
But the first true test for OCO-2 was to show how well it measured against a ground measurement, and it could not be any ordinary ground station. OCO-2 targets specific TCCON sites, because the network is considered the “gold standard” by which all other satellite atmospheric measurements are compared. The Total Carbon Column Observing Network (TCCON) is a network of ground-based Fourier Transform Spectrometers that record spectra of the sun. From these spectra, accurate and precise column-averaged abundances of atmospheric constituents including carbon dioxide (CO2), methane (CH4,) Nitrous Oxide (N2O), Hydrofluoric Acid (HF), Carbon Monoxide (CO), Water (H2O), and semi deuterated water (HDO), are retrieved.
”TCCON traces its origin to the OCO mission”, comments TCCON Chair Paul Wennberg from the California Institute of Technology in Pasadena. “In its absence, validation of the OCO-2 mission would have been much more difficult, if not impossible”.
Other NASA instruments such as AIRS on Aqua and TES on Aura, as well as the Schimachy instrument on the European Satellite ENVISAT and the Japanese GOSAT mission also compare their data to TCCON. In turn, TCCON measurements have been carefully evaluated against internationally recognized surface observations made by the NOAA Earth System Research Laboratory, which are often used in illustrating the increase in atmospheric CO2.
After establishing this important milestone in comparison to TCCON data, the OCO-2 science team will continue their work by reviewing the collected data to look for expected trends and any anomalies. The team makes the OCO-2 data available publicly through the Goddard Earth Sciences Data and Information Service center or GES DISC.
“This is a significant milestone for the project, the science team has been busy evaluating the data and using the TCCON data in our larger effort of data evaluation, said Dr. Annmarie Eldering, Deputy Project Scientist for OCO-2 at JPL. “We are very excited to make the data available to the public and to see how the scientific community uses these important observations of carbon dioxide”.
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