SAGE III Science Data Validation Efforts Begin
SAGE III Science Data Validation Efforts Begin
From its perch on the International Space Station, SAGE III is measuring stratospheric ozone as well as other gases and aerosols.
An orbiting science instrument whose legacy dates back 34 years continues to beam back data on Earth’s protective ozone layer – this time, from a perch on the hull of the International Space Station.
The Stratospheric Aerosol and Gas Experiment III (SAGE III), a NASA Langley Research Center-led mission, was launched on Feb. 19, 2017 and installed on the International Space Station during a 10-day robotic operation.
Since March 2017, the instrument has been measuring and collecting data on Earth’s sunscreen, stratospheric ozone, as well as other gases and aerosols, which are tiny particles in the atmosphere at all altitudes.
The SAGE III instrument makes these measurements through occultation, which involves looking at the light from the Sun or the Moon as it passes through Earth’s atmosphere at the edge, or limb, of the planet. The initial set of atmospheric data collected from the SAGE III instrument was released publicly in October 2017, and the first lunar data was released in January 2018.
Because the SAGE III instrument makes measurements through remote sensing - collecting data from some distance away - the science validation team cannot be sure if the data they are receiving is accurate without first validating it.
To do that, SAGE III science data must be compared to in-situ measurements, or measurements made by other instruments or systems that come in direct contact with the ozone, aerosol, or gas data being collected. These in-situ measurements are collected by the Network for Detection of Atmospheric Composition Change (NDACC), an international group, part of the National Oceanic and Atmospheric Administration, composed of research sites across the world collecting data on the Earth’s atmosphere.
“These sites have been vetted, validated, and have a long statistical history of making science measurements with their instruments,” said SAGE III Science Manager Marilee Roell.
The NDACC will collect these validated measurements through various methods, with two primary methods being through lidar - light detection and ranging - and sondes. Lidar is a ground-based measurement technique that uses a laser to shoot a beam into the Earth’s atmosphere, causing light to scatter by the atmospheric gases and particles. Being able to detect the distance to these gases and particles, the lidar can gather data on the Earth’s atmospheric composition.
Sondes are lightweight, balloon-borne instruments that are flown thousands of feet into the Earth’s atmosphere. As the instrument ascends, it transmits measurements of particle and gas concentrations by radio to a ground-based receiving station. Sondes are used daily across the globe to capture meteorological data, allowing people to check weather conditions each morning.
The science validation team is using NDACC ozone and aerosol lidar data, as well as ozone and water vapor sonde measurements, to validate science data collected from SAGE III.
“We want to match our vertical science product to an externally validated source. It helps the science community have confidence in our data set,” said Roell.
The team is working towards having an externally validated aerosol sonde to compare to the collected SAGE III data. This effort is in the preliminary stages of validating the aerosol balloon sonde against a suite of aerosol sounders, including lidar.
The team is working to validate science data with NDACC locations in Boulder, Colorado and Lauder, New Zealand, which fall within similar latitude bands in the northern and southern hemispheres. To be precise in validation efforts, the lidar or sonde measurement is taken at the same time and location that SAGE III is passing over and collecting equivalent data.
One of the most recent validation efforts took place in Table Mountain, California, and Haute Provence in France. Both locations include validated lidar systems, with lidar being operated by NASA’s Jet Propulsion Laboratory in Table Mountain, California.
Validation efforts were taken a step further by including a third source of measurements: NASA’s DC-8 aircraft. The aircraft, based out of NASA Armstrong Flight Research Center in Palmdale, California, operates as a flying science laboratory. It helps validate the accuracy of other remote-sensing satellite data, such as SAGE III, and can fly under the satellite’s path to collect the same measurements.
Validating the science data using this method required SAGE III, the NASA DC-8 aircraft, and the lidar system in California or France to be taking measurements at the same time and location. The science validation team worked to have all three systems line up while taking measurements and collected some coinciding science data.
NASA also created a validation website for other NDACC sites to use. The site displays SAGE III overpasses of NDACC sites that are three weeks out or less. These sites can choose to make lidar or sonde measurements at the same time as the instrument overpass, and compare them to SAGE III data collected to see if the two sets coincide. The validation team is pursuing additional NDACC sites to coordinate overpass timeframes when the sites may be taking lidar and sonde measurements.
The SAGE III team will present initial science validation data at the European Geosciences Union conference in Vienna, Austria this April.
SAGE III is the latest in a legacy of Langley instruments that go back to the Stratospheric Aerosol Measurement (SAM), which flew on the 1975 Apollo-Soyuz mission. SAGE II, operational from 1984 to 2005, measured global declines in stratospheric ozone that were later shown to be caused by human-induced increases in atmospheric chlorine. Data from it and other sources led to the development of the Montreal Protocol on Substances that Deplete the Ozone Layer.
After the passage of the protocol, SAGE II data also provided key evidence that the ozone layer was showing signs of recovery.
SAGE III, which launched to the station Feb. 19 from Kennedy Space Center in Florida, will continue to monitor that recovery, but with more of Earth’s atmosphere in its sights. SAGE II monitored only the stratosphere. SAGE III is monitoring both the stratosphere and the mesosphere, which is the layer directly above the stratosphere.
Ozone in the upper atmosphere acts as Earth’s sunscreen, protecting the surface from cancer-causing, crop-damaging ultraviolet rays. Atmospheric aerosols contribute to variability in the climate record.
Allison Leybold NASA Langley Research Center
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