Mighty miniature microwave limb sounder sets sights on atmospheric ozone
Oct. 10 2022 – A new NASA instrument will help researchers study atmospheric chemistry in Earth’s stratosphere, paving the way for future insights into complex chemical reactions and air motions that impact the ozone layer, as well as climate, air quality, and even aspects of weather patterns.
The instrument, “Continuity Microwave Limb Sounder” (C-MLS), is a compact space-based remote sensor that views the atmosphere edge-on (a process known as “Limb sounding”), and measures the tiny, naturally-emitted microwave signatures emitted by molecules of atmospheric trace gases. With that information, scientists can determine how a wide variety of minor trace gases are distributed around the world, observe how those trace gases are transported throughout the atmosphere, and study how those gases interact with each other, as well as with cloud and aerosol particles.
NASA already has a Microwave Limb Sounder instrument (MLS) in low-Earth-orbit aboard the Aura satellite, launched in 2004. But C-MLS features a number of significant improvements. It can detect nearly all the molecules visible to Aura MLS while observing only two spectral regions – 320-360 GHz and 620-665 GHz – compared to Aura MLS’s five. This, along with the adoption of digital signal processing technologies, reduces power consumption and enables dramatic reduction in size and weight.
By viewing the atmosphere at an angle, limb sounding instruments obtain measurements with better vertical resolution when compared to instruments that observe the atmosphere vertically – also known as “nadir” sensors. Additionally, because measurements from limb sounders transect hundreds of kilometers of atmosphere, they can detect signals from many more molecules compared to nadir sounders. This provides a stronger signal for tenuous trace gases that play an important role in atmospheric chemistry.
Data from Aura’s MLS instrument. On the left, ozone in Earth’s stratosphere at an altitude of approximately 12 miles; on the right, chlorine monoxide – the primary agent of chemical ozone destruction. Note the white line on both images, which marks the area where chemical ozone destruction took place. (Image Credit: NASA)
Aura MLS weighs nearly 500 kilograms and consumes more than 500 watts of electricity, while C-MLS will weigh only 60 kilograms and consume just 80 watts of electricity. The lighter C-MLS instrument will help maintain a stratospheric composition record that began in the early 1990s for future generations of scientists.
For perspective, the microwave limb sounder onboard Aura relies on nearly 30 spectrometers each about the size of a brick, while C-MLS can accomplish nearly all the same observations with superior spectral resolution and bandwidth using only ten small chips that sit on circuit boards, each of which are the size of a credit card.
“Technology has advanced to the point where you could have a computer, a radio, and an analogue amplifier on one square of silicon,” explained Nathaniel Livesey, an atmospheric scientist at NASA’s Jet Propulsion Laboratory and the lead scientist for both Aura’s Microwave Limb Sounder instrument and C-MLS. “This is the kind of technology that’s inside modern cell phones. We’re capitalizing upon these advances by putting more and more of our subsystems inside custom chips.”
Traditionally, MLS instruments required a lot of energy-hungry hardware. “For the receiver and spectrometer electronics, we’re going from something the size of a small car to something the size of a carry-on bag, without sacrificing performance,” said Livesey. Smaller instruments are easier to manufacture and send into space, meaning C-MLS makes it less difficult for researchers to send new microwave limb sounders into orbit.
C-MLS will be especially useful for monitoring Earth’s Ozone Layer, a thin band of ozone gas in the stratosphere that shields Earth from the Sun’s most powerful UV rays. Mere trace amounts of certain gases containing chlorine can destroy large amounts of ozone, most notably in the Antarctic “ozone holes” which have formed each southern Winter and Spring since the 1980s.
The Montreal Protocol is the first internationally accepted agreement to phase-out the production of ozone-depleting substances that entered into force in 1989. While ozone is recovering as expected in some regions of the atmosphere, there’s a large amount of variability from year to year in other regions, which makes it difficult to discern just how well the ozone layer is recovering in other regions, notably over the Arctic.
“Indeed, ozone is still going down in some regions where we would have hoped to at least see a flattening off by now,” said Livesey. “What we’re seeing is within, but close to the edge of, the range predicted by models. We need measurements from sensors like C-MLS to tell us why ozone is behaving this way, and to ensure our understanding is correct.”
C-MLS will also observe water vapor in the stratosphere, an important climate variable. Data from Aura MLS allowed scientists to determine that the January 2022 eruption of the Hunga Tonga-Hunga Ha`apai volcano lofted so much water vapor to high altitudes that stratospheric humidity increased by 10%.
Livesey said C-MLS could contribute to future NASA missions dedicated to observing atmospheric chemistry. NASA’s Earth Venture and Earth System Explorer Programs, in particular, are ideal venues for launching and operating a mission with an instrument like C-MLS.
Development of the C-MLS was supported through the Instrument Incubation Program (IIP), a part of NASA’s Earth Science Technology Office (ESTO).
Gage Taylor, NASA Earth Science Technology Office