From National Institute of Standards and Technology (US) : “Monitoring the Oceans’ Color for Clues to Climate Change”

From National Institute of Standards and Technology (US)

April 22, 2021
B. Carol Johnson

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Ocean chlorophyll concentrations, MODIS-Aqua full mission July 2002 to January 2021.
Credit: Ocean Biology Processing Group, National Aeronautics Space Agency (US)/Goddard Space Flight Center(US)

“It is February 1994 and I am on the research vessel R/V Moana Wave off the coast of Lanai, Hawaii, with the team of the Marine Optical BuoY (MOBY) project. The water is incredibly blue, and I can’t help but be awestruck by the enormous energy, momentum, power and depth of the ocean as I watch the currents and the wind create what appear to be rising and falling pyramids of solid substance, no longer a liquid but a mighty living thing. It is against this backdrop that we work to deploy devices designed to determine the optical properties of the Pacific Ocean. As the instrument at hand, bright yellow and wing-shaped, is lowered over the port side into the water, its yellow wings appear green. This change, and indeed the blue color of the water itself, was indicative of what was in the water, which in the case of the open Pacific, was “not much.” “Much” meaning small particles in the water that scatter or absorb sunlight, changing the overall reflectance of the sunlit layer, and thereby the observed color.

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Phytoplankton. Credit:National Oceanic and Atmospheric Administration (US) MESA Project.

One class of these small particles is phytoplankton, a form of algae. They contain chlorophyll and practice photosynthesis as does any other plant, using solar radiation to convert carbon dioxide dissolved in the water into plant sugars, releasing oxygen and respiring a portion of the carbon dioxide. A portion of the carbon dioxide is eventually converted to sediment thanks to the grazing activities of marine life (and death), such as zooplankton, young fish or crustaceans and those that feed upon them.

Phytoplankton
are the basis of marine life, produce about half of the oxygen in the Earth’s atmosphere, and currently absorb much of the carbon dioxide we humans produce. Ocean temperature, currents, acidification, surface winds and nutrients can affect phytoplankton populations, life cycles and the amount of carbon dioxide they remove from the atmosphere, and so measuring them is critical to understanding climate change. A reasonable question to ask is “Will the oceans continue to remove significant amounts of human-produced carbon dioxide in the future?

We need to continually observe the color of the world’s oceans to address these questions, and ocean color satellites have been in continuous operation since the launch of NASA’s SeaWiFS mission in 1997. The idea is simple: Just as you can tell a desert from a forest by the color, so it goes with the oceans. But, while the idea is simple, detecting the color of the oceans through Earth’s atmosphere is not. The main problem is the atmosphere also scatters sunlight, and both sources of scattered light, from the ocean and the atmosphere, are detected by the satellite sensor. Because the portion from the atmosphere dominates, it is not possible, with the current status of preflight calibration and atmospheric modeling, to use ocean color satellites to derive the light scattered out of the oceans with the accuracy we need to determine the chlorophyll concentration and other quantities of interest.

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Divers inspecting MOBY. The buoy extends 12 meters (39.4 feet) underwater and has sensors at depths of 1 m (3.3 ft), 5 m (16.4 ft) and 9 m (29.5 ft). Credit: Moss Landing Marine Laboratories (US)(MLML).

The solution lies in a procedure called system vicarious calibration (SVC), which was pioneered by Dennis Clark at NOAA and colleagues from ships during the Coastal Zone Color Scanner satellite mission (1978 to 1986). Based on this experience, Dennis implemented the MOBY project with support from NOAA and NASA’s SeaWiFS and MODIS projects; MOBY has produced data from July 1997 to the present. MOBY is an optical system that measures light at different colors (wavelengths). This type of system is called a spectrograph. It is mounted on a tethered wave-rider buoy and measures the light incident on the surface and at three depths, and the backscattered light at four depths. MOBY is located about 20 kilometers from Lanai where the water is representative of the world’s oceans. Data are acquired daily as ocean color satellites fly over the location. The MOBY instrument, in collaboration with NIST, is extremely well characterized and extensively calibrated, and the results are traceable to the International System of Units (SI), the modern metric system. By providing accurate values for the oceanic portion of the light measured by the satellite sensor, MOBY provides a calibrated source for any ocean color sensor that observes this region of the ocean.

I was on the Wave in 1994 because Dennis had come to NIST a couple of years earlier for help with establishing traceability to the SI, which means ensuring the MOBY results are rigorously connected to the SI measurement system so that researchers around the world have the best possible ocean color reference. He had designed a robust measurement plan, with cross-checks and validation at every turn. Over the years, NIST has supplied radiometric sensors for the MOBY team to track its calibration sources in between the NIST calibrations, and we have deployed additional NIST radiometers and sources on occasion to validate the radiometric scales at the MOBY facility in Honolulu.

NIST has also played a role in characterizing the MOBY optical system. A good example is a problem Dennis presented early on: Independent, simultaneous measurements at the same wavelength and depth did not agree. Now, this is a problem, but really it is a good thing to find issues. In metrology, in order to assure ourselves we are getting the best (and hopefully correct) answer, it is good practice to measure the same thing with different approaches, in this case the backscattered light at the same wavelength with two different spectrographs. It took a while to figure out, but thanks to some laser characterizations and subsequent discussions, we identified stray light as the issue and developed and implemented an algorithm to correct the problem.

As you may have gathered, MOBY has been around for quite some time. It is an example of how collaborations really work, leading to a world-class product. We’re currently on our 67th buoy (a buoy has a deployment cycle of three to six months). We rotate two systems, calibrating and refurbishing one while the other is in the water. NOAA fully supports the MOBY project for its visible infrared imaging radiometer suite (VIIRS) calibration. Presently, under the leadership of Kenneth Voss (University of Miami; Dennis retired in 2005 and died in 2014) and execution of Moss Landing Marine Laboratories, and with NOAA support, we are implementing a new system design. The new optical system collects data from all depths simultaneously in order to reduce environmental sources of measurement uncertainty. A new carbon-fiber buoy structure, and new control, communication, and data analysis systems complete the system, which we call “Refresh.”

The MOBY team, with NASA funding, is developing a portable version termed MarONet. This system is identical to Refresh and enables deployment at a new location with recalibrations in Honolulu at the MOBY facility. The first MarONet will be deployed off Rottnest Island, Australia, in 2022. NIST’s role in MarONet is to supply a stable source and spectroradiometer system to validate any changes with shipment. In 2022, NASA will decide whether to continue the MarONet project as the primary SCV site for the upcoming Plankton, Aerosol, Cloud and ocean Ecosystem

(PACE) mission.

I would like to close with a few words about the MOBY team and how this work has been a core part of my career at NIST. Yes, I have been involved with other satellite sensors, performing on-site validation activities at the manufacturer facilities with my NIST colleagues for the NASA Earth Observing System program, NOAA geostationary satellites, ESA’s Sentinel-2 , the Orbiting Carbon Observatory and others.

I have had the opportunity to participate in validation of ground-based measurements of the Moon’s irradiance. But the field of ocean color has led to long-standing relationships with exceptional scientists, and I am so grateful for this experience.

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National Institute of Standards and Technology (US)‘s Mission, Vision, Core Competencies, and Core Values

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Background

The Articles of Confederation, ratified by the colonies in 1781, contained the clause, “The United States in Congress assembled shall also have the sole and exclusive right and power of regulating the alloy and value of coin struck by their own authority, or by that of the respective states—fixing the standards of weights and measures throughout the United States”. Article 1, section 8, of the Constitution of the United States (1789), transferred this power to Congress; “The Congress shall have power…To coin money, regulate the value thereof, and of foreign coin, and fix the standard of weights and measures”.

In January 1790, President George Washington, in his first annual message to Congress stated that, “Uniformity in the currency, weights, and measures of the United States is an object of great importance, and will, I am persuaded, be duly attended to”, and ordered Secretary of State Thomas Jefferson to prepare a plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States, afterwards referred to as the Jefferson report. On October 25, 1791, Washington appealed a third time to Congress, “A uniformity of the weights and measures of the country is among the important objects submitted to you by the Constitution and if it can be derived from a standard at once invariable and universal, must be no less honorable to the public council than conducive to the public convenience”, but it was not until 1838, that a uniform set of standards was worked out. In 1821, John Quincy Adams had declared “Weights and measures may be ranked among the necessities of life to every individual of human society”.

From 1830 until 1901, the role of overseeing weights and measures was carried out by the Office of Standard Weights and Measures, which was part of the U.S. Coast and Geodetic Survey in the Department of the Treasury.

Bureau of Standards

In 1901 in response to a bill proposed by Congressman James H. Southard (R- Ohio) the National Bureau of Standards was founded with the mandate to provide standard weights and measures and to serve as the national physical laboratory for the United States. (Southard had previously sponsored a bill for metric conversion of the United States.)

President Theodore Roosevelt appointed Samuel W. Stratton as the first director. The budget for the first year of operation was $40,000. The Bureau took custody of the copies of the kilogram and meter bars that were the standards for US measures, and set up a program to provide metrology services for United States scientific and commercial users. A laboratory site was constructed in Washington DC (US) and instruments were acquired from the national physical laboratories of Europe. In addition to weights and measures the Bureau developed instruments for electrical units and for measurement of light. In 1905 a meeting was called that would be the first National Conference on Weights and Measures.

Initially conceived as purely a metrology agency the Bureau of Standards was directed by Herbert Hoover to set up divisions to develop commercial standards for materials and products. Some of these standards were for products intended for government use; but product standards also affected private-sector consumption. Quality standards were developed for products including some types of clothing; automobile brake systems and headlamps; antifreeze; and electrical safety. During World War I, the Bureau worked on multiple problems related to war production even operating its own facility to produce optical glass when European supplies were cut off. Between the wars Harry Diamond of the Bureau developed a blind approach radio aircraft landing system. During World War II military research and development was carried out including development of radio propagation forecast methods; the proximity fuze and the standardized airframe used originally for Project Pigeon; and shortly afterwards the autonomously radar-guided Bat anti-ship guided bomb and the Kingfisher family of torpedo-carrying missiles.

In 1948, financed by the United States Air Force the Bureau began design and construction of SEAC: the Standards Eastern Automatic Computer. The computer went into operation in May 1950 using a combination of vacuum tubes and solid-state diode logic. About the same time the Standards Western Automatic Computer, was built at the Los Angeles office of the NBS by Harry Huskey and used for research there. A mobile version- DYSEAC- was built for the Signal Corps in 1954.

Due to a changing mission, the “National Bureau of Standards” became the “National Institute of Standards and Technology (US)” in 1988.

Following September 11, 2001, NIST conducted the official investigation into the collapse of the World Trade Center buildings.

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NIST is headquartered in Gaithersburg, Maryland, and operates a facility in Boulder, Colorado, which was dedicated by President Eisenhower in 1954. NIST’s activities are organized into laboratory programs and extramural programs. Effective October 1, 2010, NIST was realigned by reducing the number of NIST laboratory units from ten to six. NIST Laboratories include:

Communications Technology Laboratory (CTL)
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Hollings Manufacturing Extension Partnership (MEP), a nationwide network of centers to assist small and mid-sized manufacturers to create and retain jobs, improve efficiencies, and minimize waste through process improvements and to increase market penetration with innovation and growth strategies;
Technology Innovation Program (TIP), a grant program where NIST and industry partners cost share the early-stage development of innovative but high-risk technologies;
Baldrige Performance Excellence Program, which administers the Malcolm Baldrige National Quality Award, the nation’s highest award for performance and business excellence.

NIST’s Boulder laboratories are best known for NIST‑F1 which houses an atomic clock. NIST‑F1 serves as the source of the nation’s official time. From its measurement of the natural resonance frequency of cesium—which defines the second—NIST broadcasts time signals via longwave radio station WWVB near Fort Collins in Colorado, and shortwave radio stations WWV and WWVH, located near Fort Collins and Kekaha in Hawai’i, respectively.

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The Center for Nanoscale Science and Technology (CNST) performs research in nanotechnology, both through internal research efforts and by running a user-accessible cleanroom nanomanufacturing facility. This “NanoFab” is equipped with tools for lithographic patterning and imaging (e.g., electron microscopes and atomic force microscopes).

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As part of its mission, NIST supplies industry, academia, government, and other users with over 1,300 Standard Reference Materials (SRMs). These artifacts are certified as having specific characteristics or component content, used as calibration standards for measuring equipment and procedures, quality control benchmarks for industrial processes, and experimental control samples.

Handbook 44

NIST publishes the Handbook 44 each year after the annual meeting of the National Conference on Weights and Measures (NCWM). Each edition is developed through cooperation of the Committee on Specifications and Tolerances of the NCWM and the Weights and Measures Division (WMD) of the NIST. The purpose of the book is a partial fulfillment of the statutory responsibility for “cooperation with the states in securing uniformity of weights and measures laws and methods of inspection”.

NIST has been publishing various forms of what is now the Handbook 44 since 1918 and began publication under the current name in 1949. The 2010 edition conforms to the concept of the primary use of the SI (metric) measurements recommended by the Omnibus Foreign Trade and Competitiveness Act of 1988.