New PACE satellite 'game changer' for aerosols and plankton

Satellite to give atmospheric and ocean images in extraordinary detail

When I speak with Jeremy Werdell, Project Scientist for NASA’s Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission, it is 54 days and 7 hours out from the scheduled launch of the spacecraft from Cape Canaveral Space Force Station, Florida.

“PACE is a game changer,” says Werdell.

“With our heritage instruments, we’ve often known exactly what kind of data we’re going to get. But the information this mission will gather will be so astoundingly superior to what’s available now that not only am I excited just to play with it, it will enable us to really grow scientifically.” 

Werdell’s feeling exhilarated, nauseous – and as though he is “standing on the shoulders of giants”, in the sense that this particular mission will advance the ground-breaking work of similar missions conducted in the past such as Terra, Aqua, Landsat, and SeaWiFS.

“The foundational science from those missions captured the public’s imagination, which has supported continued investment,” Werdell says. “And what we learned from those missions also provided us with new questions, which we are now trying to answer with PACE.”

One of the main questions Werdell and his colleagues are trying to answer is to do with atmospheric aerosols. These are “any suspended form of particulate matter – dust, sea spray, industrial pollution,” says Kirk D. Knobelspiesse, an atmospheric scientist working on the PACE mission. “And they have a very complex relationship with clouds.”

Clouds govern a lot of energy that comes into the climate.

Dr Kirk D. Knobelspiesse

Even though there has been significant improvement in scientists’ understanding of some of the ways aerosols influence the climate in recent decades – for example, either by scattering or absorbing incoming solar radiation – the relationship between them and clouds is still shrouded in mystery.

In fact, according to a recent paper in Atmospheric Chemistry and Physics: “Atmospheric aerosols and their impact on cloud properties remain the largest uncertainty in the human forcing of the climate system.”

This is echoed by Knobelspiesse.

“We’re at the level where we are good at detecting the presence of aerosols and understand a lot of the processes involved, but it’s hard to identify what type of aerosols they are,” he says.

“If you look at the ensemble of climate models currently, although they’re all based on physics, they start to diverge a little when it comes to their projections for the future. And a lot of the reason for that is how they’ve parametrised the aerosol cloud interaction component…What we really need to do is speciation, getting into the chemical composition of aerosols and their sources, and how they interact with clouds.”

And why is that?

“Because clouds govern a lot of energy that comes into the climate,” says Knobelspiesse

PACE will achieve this aim thanks to several advanced scientific instruments onboard the satellite.

We’re talking terrabytes of data per day that will be collected.

Dr Kirk D. Knobelspiesse

Two of these instruments are polarimeters, which measure the angle at which sunlight is polarised. This reveals specific characteristics of whatever it is the light bounced off, and scientists can use this information to determine the size, composition, abundance and other traits of the particles in the atmosphere.

Crucially, the two polarimeters onboard PACE – HARP2 and SPEXone – will make different but complementary observations: while HARP2 will observe four wavelengths of light from up to sixty different angles, SPEXone will observe light at hyperspectral resolution from five angles.

Although this will offer a picture of Earth’s atmosphere in unprecedented detail and improve our understanding of air quality, it will do so in a very data intensive manner.

“We’re talking terrabytes of data per day that will be collected,” Knobelspiesse says. “So, a lot of our focus right now is on interpretation and developing algorithms to process what is going to essentially be a fire hose of data.” 

Alongside the two polarimeters onboard the satellite is PACE’s primary instrument – an optical spectrometer known as the Ocean Colour Instrument (OCI).  

I wouldn’t change anything. No matter what happens on launch day, there will be tears.

Dr Jeremy Werdell

Consisting of a rotating telescope, thermal radiators, a half-angle mirror and solar calibration mechanisms, the OCI is, according to NASA, its “most advanced colour sensor ever”: whereas previous ocean sensors detected only a handful of colours, the OCI is hyperspectral and can observe more than 100 different wavelengths.

This will make it possible for scientists to identify phytoplankton in the ocean by species from space for the first time, as the colour of the ocean is determined by the interaction of sunlight with substances such as chlorophyll which is found in most types of phytoplankton.

In turn, this will improve our overall understanding of the health of the ocean, which will have far-reaching real-world applications. For example, it will enable scientists to better predict harmful algal blooms, monitor fisheries, and track changes in marine resources.

Importantly, the data gathered by PACE will also augment the data being collected by NASA’s Surface Water and Ocean Topography (SWOT) mission, which is currently making the first global survey of Earth’s surface water and mapping, in unprecedented detail, the ocean’s topography.

“PACE is going to be exceptional at defining what’s actually in the water,” Werdell says. “But it doesn’t have any kind of capability to understand the hydrogeography of what’s going on in all of this. By pairing SWOT and its hydrographic physical oceanography data with the biological response to all of this, you’re really adding to the information you have.”

PACE is a game changer.

Dr Jeremy Werdell

In September, the PACE spacecraft underwent a vital month-long thermal test in a specialised chamber at NASA’s Goddard Space Flight Center, where it was subjected to extreme temperatures and pressures to verify its performance once it is launched and operational.

Much to Werdell’s relief, the spacecraft passed, paving the way for the launch in less than two months’ time. Between now and then, it will undergo final preparations at the Astrotech Space Operations facility in Florida, including comprehensive performance tests and data processing simulations, before it is attached to a SpaceX Falcon 9 rocket and launched into the sky.

“The next light the instruments will see will be from space,” Werdell says.

But even though their work is now “starting to wind down”, Knobelspiesse says he and Werdell are now starting to deal with the “sleepless nights” and “separation anxieties” that come from knowing they are about to say goodbye to something that has figured so prominently in their lives for so long.

“There were a lot of hard days and I’m not sure I’d do it again,” he says. “But I wouldn’t change anything. No matter what happens on launch day, there will be tears.”

Werdell concurs.

“Once we have the data, the fun begins. We hope to be working with it for many, many years to come. We’ve put so much blood, sweat and tears into this as a contribution to the scientific community, and I just cannot to start hearing about people from around the world writing papers using some of this data.

“That will be the real reward.”

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