Podcast: How Can We Solve Aviation's Big Contrails Problem?

Eliminating contrails would cut aviation's climate impact by half, but can it be done? Aviation Week editors are joined by NASA scientist Richard Moore who shares the latest research on a problem that some consider to be the low-hanging fruit of sustainability.

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Transcript

Guy Norris:

Welcome to Aviation Week's Check 6 podcast. I'm Senior Editor Guy Norris, and I'm here with my Aviation Week colleagues, Thierry Dubois, European technology editor and Bureau chief for France and Graham Warwick, executive editor for technology based in Washington DC. Today we're also joined by a special guest, Dr. Rich Moore, a research physical scientist at NASA Langley Research Center who specializes in aerosol cloud climate interactions and airborne field measurements and instrumentation. And the reason we're talking to Rich today is last year I was very lucky to fly with Rich and his colleagues on NASA's DC-8 research aircraft in an experiment to sample contrails, also known as condensation trails formed by Boeing's 737-10 ecoDemonstator, which was burning both regular Jet A fuel and sustainable aviation fuels or SAFs. So we've all seen trails, they form in clear, cold, humid air when water vapor produced by the combustion of fuel in the aircraft engines condenses usually upon soot particles or sulfur aerosols in the exhaust.

And when the ambient relative humidity is high, the resulting ice crystal plume can last for several hours and spread outward to form a layer of cirrus cloud. And from a sustainability perspective, recent research has shown that contrails could play a major role in aviation's contribution to global warming, and therefore mitigating the formation of those trails could dramatically help the industry's drive towards net-zero emissions. But is it really fair to call this the sort of low-hanging fruit of sustainability? I mean, it's obviously better than nothing, but there's obviously major challenges. So Rich, can we start with you? First of all, thank you for joining us. Really great to have you on the podcast. Could you say a little bit more perhaps about, first of all, what a contrail is? I know I've just given a very sort of contrail 101 thing there, but perhaps explain what it is and potentially also look at explaining what the climate impacts could be of contrails.

Richard Moore:

Yeah, and your intro was spot on Guy, I'm in Eastern Virginia and it's not uncommon for me to look out my office window and see these line shaped clouds streaking across the sky every morning as the commercial air traffic goes up and down the East Coast from New England down to Florida and back. And as the day progresses, it's pretty common to see those line shaped contrails spread out and form the cirrus clouds that then cover the sky in the afternoon. And it's kind of a fascinating thought experiment to think about that cloud that is in the atmosphere interacting with sunlight, interacting with the radiation that comes up from Earth's surface. And wonder if that airplane had not flown through the sky today, would that cloud have formed anyway or are we leaving a fingerprint on our world?

And so that's really what the heart of contrail and aviation research is right now, trying to use measurements, observations from satellites and sophisticated models to unravel that very complex question and understand the impact that aviation is having on the environment. And so the idea here is that those clouds that we've created from the aircraft interact with the sunlight that's coming in to the earth. They reflect some of the sunlight back into space and that has a cooling effect on climate. And they also trap some of the outgoing heat radiation from the surface, almost a blanket similar to the greenhouse effect. And that has a warming effect on climate and it's that interplay between the two that gives rise to the net climate impact of contrail or cirrus clouds.

Guy Norris:

And Thierry, I know you obviously want to jump in here.

Thierry Dubois:

Thank you very much Rich for your explanation. And is my understanding clear, I understand that the net effect of the cooling contrails and the warming contrails is a net warming impact. And I also understand, correct me if I'm wrong, I also understand that researchers have only found out recently that this net impact is a warming impact. Is that correct?

Richard Moore:

You're right. If we add up all of the warming and cooling associated with contrail cirrus clouds in our large scale climate models that simulate the whole world, some are cooling, some are warming, and we see it's not the same all the time. So during the day for example, where we have that cooling effect from the sun lights, some contrails are cooling, but some are still warming. And then at night there's no cooling effect because the sun isn't shining down. And so all of the contrail are warm. And so when we do the bookkeeping and we add up the global accumulated warming and cooling, the overall effect on the globe is warming. And some of the early model simulations, even as far back as 20, 25 years ago began to show us that, but with great uncertainty. And so really what the advances over the last 5-10 years have been is being able to—for the first time—put an order of magnitude on that effect and still with great uncertainty start to drill down on how it compares to other climate forcers that we're concerned about, like greenhouse gas emissions.

Guy Norris:

So really we've sort of begun to identify this as a global, it's not just a potential, it's a real global warming impact. The focus therefore goes to what do we do about it. And from your perspective, Rich, obviously I've heard you talk before about this three-legged stool kind of impact. Is that right? Could you explain a bit about how we could go about mitigating the impacts?

Richard Moore:

Absolutely. Yeah. So our climate models are telling us that contrails are important and we think they're of similar importance to the CO₂ and the greenhouse gases being emitted from burning jet fuel in aviation. And so given that importance, given that motivation, we're looking for solutions that we can undertake in order to minimize the climate impact of contrail formation. And we have really three different levers that we can start to prove, we can change the engine technology in order to make the engines more efficient, burn less fuel, and be less conducive to forming contrails.

We can change the fuel that the engines are burning. And so if we can move from a dirtier or more soot producing fuel to one that produces less soot, then we're going to reduce those nuclei, those seeds for the ice crystals to form on. And then finally, if we can use another set of models, weather forecast prediction models to identify ahead of time when we're going to form a contrail and where, maybe we can reroute the planes in order to avoid flying through that airspace in the first place. And so it's this combination of technology, advanced fuels and operational measures that we're hoping will make a significant impact going forward in reducing contrail climate impacts.

Guy Norris:

That's really fascinating. Thierry has recently written a story for us really showing that this modeling of the atmosphere combined with root planning could give you a potential for helping that. And I know Graham has also delved into that through some of the US research programs. So Thierry, would you mind outlining the basis of what you were talking about with that story for us?

Thierry Dubois:

Absolutely. Several companies or startup companies or research bureaus that are willing to create a startup company are working on how to prevent the formation of contrails by improved weather forecast models, and those models predict where the contrails are likely to form, at what altitude, in what area they are likely to form. And one of those companies called Satavia, they're based in Cambridge, UK, already offers a product that has already been purchased by at least one carrier, one airline. And the idea is that the flight plane is... well, let's say a few hours before the flight takes off, the flight plan is modified to avoid any potentially contrail prone area. And they claim science validation has confirmed the relevance of their work and well, they're already in the practicalities of doing that, of preventing contrail formation. When I say practicalities, it means training pilots, working with air traffic controllers and conducting either real life or simulations, for instance, to manage the extra workload on air traffic controllers. So they are very advanced already.

Guy Norris:

Right. And Graham, I know that you've really been pursuing this too. I mean, one of the big questions is how realistic is it to implement across say a nationwide transport system?

Graham Warwick:

So actually I've got a question for Rich before I come to that. When we estimate that this has a potential impact about the same as direct emissions of CO₂, so actual CO₂ emissions. Is that based on the percentage global coverage of aircraft-induced cirrus that's there today? Or when we make that calculation, do we assume the entire world is covered in aircraft-induced cirrus or only because this doesn't happen globally? It happens in certain parts of the world. There's a lot of it over the US because there's a lot of air traffic, but I mean there are huge parts of the world where this doesn't happen, I presume, unless it does spread globally and you get this thin, thin... That's one thing I'm trying to understand is it looks like a local problem, but you're talking in terms of having a global impact.

Richard Moore:

Yeah, you're absolutely right. And so when we emit carbon dioxide into the atmosphere, we know it has a lifetime. It's not removed from the atmosphere on average for decades or even centuries, whereas the contrails that we're forming today will likely dissipate in a few hours to date. So the climate impact that we're talking about is very different both over time but then also over space because the contrails are forming over the narrow flight regions and the carbon dioxide is mixing and well mixed throughout the world. And so when the climate scientists run these simulations, they need to take into account that varying time dependency and also the spatial, and they need a metric that accounts for that. And so the metric that we typically use in order to evaluate the relative importance of these climate impacts is called effective radiative forcing.

And the units of that are watts, energy, like a light bulb, per square meter of earth's surface. And so we're able to take by normalizing by the spatial coverage of these different elements in the model, we're able to put them on the same basis. And so what the models tell us is that there's a certain radiative forcing associated with greenhouse gas emissions, carbon dioxide emissions that's on a few tens of milliwatts per square meter. And again, from the models, the radiative forcing associated with contrails and contrails cirrus cloud is thought to be on the order of a similar few tens of milliwatts per square meter. But of course the uncertainty associated with that climate forcing from the contrails cirrus clouds is much bigger than from CO₂. That feeds into how we interpret the importance. So we're confident that it's warming and potentially important, but that we're not confident in the actual number.

Graham Warwick:

So I think what that highlights is the complexity of this. So there's clearly something here, but it's very, very complex both in its atmospheric chemistry, its dynamism, I mean region to region, time of day to time of day or altitude to altitude. So it's incredibly complex from that viewpoint, but also complex operationally because... So what Thierry is talking about is kind of like not long-range forecasting, but forecasting on a flight plan timeline, right? So you've got an airline operations center that is getting data either from forecasting mechanisms or maybe from aircraft reporting because a lot of aircraft give you weather data as they fly along and you identify these areas of ice, super saturation, whatever, where these things can form and then you plan to avoid them. If we really want this to work on a global scale and have a global impact, as Rich talks about, we've got to have a more organic, dynamic responsive system.

So ARPA-E, which is the advanced projects arm of the Department of Energy here in the U.S., is looking at what would come after what Thierry is talking about where you have a system on the airplane that is able to some extent look ahead and see what the state of the atmosphere is ahead of the airplane and take that information in. And then not only predict whether or not that atmospheric area ahead is going to cause for contrails to form, but even if a contrail forms it may just dissipate within minutes or it may not form for hours and then form, right? And then it may then persist for hours after it's formed. So they want eventually to get to a situation where an onboard system here on the airplane can dynamically detect an area of potential contrail formation, then predict that the contrail forms will persist and become a problem. It will spread and become aircraft-induced cirrus.

Google AI and American Airlines did a trial in the U.S. where they did this looking at satellite imagery, looking at weather forecasts, dynamically changing aircraft flight plans, and they got more than 50% reduction in observed contrails. And they looked at the satellite imagery and there was more than 50% reduction in contrails, but there was a fuel burn impact of having to reroute the airplanes. And it was about 2% on these trials. But if you take it across the fleet, it's only a fraction of 0.3 of a percent, they say over a fleet, over a large scale operation, but there is a fuel burn impact and therefore a direct emissions impact. So what ARPA-E is trying to do is avoid that bit of the equation. If you can avoid contrails and not unnecessarily burn fuel, then you've kind of doubled up on the potential saving there. So that's looking beyond what Thierry is talking about, which is this, how do we go today, use what we have today, use it intelligently and do something quick now.

Guy Norris:

Right. And Thierry, you just wanted to follow up with something on that?

Thierry Dubois:

Yeah, sure. About the extra fuel burn, the fuel burn caused by the change of altitude. For instance, the company I was talking about and other organizations in Europe have reached basically a consensus that extra fuel burn is a fraction. It's really a fraction. Its impact is fraction of the avoided impact of the contrails. In other words, you only have to reroute 5% of the entire fleet flying at a given time, and that avoids 80% of the contrails, which impact is much larger they say than they point 8 or one or 2% of extra fuel burn.

Guy Norris:

Right. And sort of jumping forward really to the next leg of the stool or the, I'm not sure the analogy is correct, but the DC-8 flight that you and I were on, Rich was primarily looking at the impact of SAF, sustainable aviation fuel and this new fuel chemistry on the formation potential for contrails as well as its emissions profile in general. But another, just before we do look at that, one of the interesting other experiments aboard the aircraft was obviously a way of paving the way for, and this is where NASA is unique in a way, in being able to look at space and atmospheric sort of influences. A potential way of being able to having a temperature and water vapor profile from a geostationary satellite that would cover great chunks of our space around the world. And in the case of the U.S., it would be NOAAs GeoXO new satellites, which would basically allow a sort of a profile through the atmosphere to allow remote sensing and provide this forecasting tool, that was right. Is that correct?

Richard Moore:

Absolutely, yeah. The flight campaigns that we're able to do, we do it on a large aircraft like a DC-8, and we're looking forward to our newest research aircraft in the NASA fleet, the new Boeing 777 that we'll be able to use for this research. And we fill it with instruments. And we were studying not only the fuel chemistry and emissions, as you said, the contrail microphysics, how many ice crystals, but using water vapor and temperature sensors being developed in collaboration with the firms in the ARPA-E program. And then the satellite piece, looking forward to the 2030s when we're expecting to have a constellation of geostationary satellites on orbit that will look down and give us the water vapor profiles throughout the national airspace. And so all of these observational pieces complement those forecast models that Thierry was talking about in trying to predict where we expect the contrails to form in real time or ahead of time. And then being able to verify using either sensors on the airplanes or from space, whether or not the model accurately reflected where the void regions, the atmosphere.

Guy Norris:

Right. And of course now moving a segue to the fuel burn, to the experiment where we were chasing that contrail, could you kind of give a thumbnail really, Rich, of what we were looking at there and perhaps the implications for the new fuel chemistries that are on the way with SAFs and what are we learning so far about its contrail mitigation potential or not?

Richard Moore:

Absolutely. And so we're talking about contrails, but we shouldn't lose sight of CO₂ and greenhouse gas because that continues to be the big climate impact from aviation. And as the fleet expands in the future, we're expecting that effect to over grow. And so sustainable aviation fuels are now across the world, across the national governments, a very big push in order to replace the fossil fuel sources of petroleum-based jet fuel with a biomass derived sustainably is jet fuel. And the goal then is to make some of those decisions that Thierry was talking about of rerouting the planes around the contrail forming regions, maybe it may seem a little easier to summon because if we're already burning SAF, then it's an easy decision to incur even a small fuel penalty to avoid forming that contrail. But if we're burning fossil fuel, CO₂, then we get into some of the more complicated trades that he was discussing.

And so that's really motivating adoption of these sustainable fuels over the coming decades. A nice aspect of the sustainable fuels that we see as a co-benefit is that the fuel chemistry has fewer of the soot precursors that form these soot particles that come out of the engines. And so even as we're transitioning the fleet to a net-zero CO₂ emissions, we're also expecting that the soot particle emissions from these engines is going to increase. And so that should give us a benefit for contrail formation as well. And so we were up in the skies over the Northwestern United States, flying behind a Boeing 737 MAX, burning a hundred percent sustainable aviation fuel. And we were studying the sub-particle reductions associated both with the SAF and the advanced lean-burn engines on the aircraft and how that translated into changes in the contrail properties.

Guy Norris:

Right. And of course, one of the things that was immediately apparent was our DC-8, bless it with good old dash two CFM56-2s was producing a significantly bigger contrail at the same altitude, more or less in the same atmospheric masses as the Leap 1s on the 737. So that'll take us to the final phase of what we're going to talk about engine technology, but did we see any impact on contrail formation from using SAF? Is the jury still out on that?

Richard Moore:

Well, that's a great question. And so we have the data, we're looking at it now, and we have a science team meeting where we're going to bring the scientists back. We're going to assemble in Washington DC about a month from now, and we're going to review the data. And so over the past six months after the flight experiment, livid we all returned to our home institutions and have been busily quality assuring and controlling the data set from the flight campaign. And we're going to take that data and we're going to all post it and we'll compare notes, we'll make sure that things look consistent.

And then we're going to release that data to the public in six months after that, which will be this November. So that'll be freely and publicly available for the world to see. So it's premature to answer the question on what is the impact of the fuels and engine technology on the contrail in a quantitative sense. But I think as you noted, Guy, anecdotally it was very apparent that the contrail form behind the 737-10 was much more diffuse and transparent than the older engine technology. And certainly we could see in real time that it was, there was almost no soot coming out of the Leap 1 engines. And so there is a benefit.

Guy Norris:

And Thierry, you wanted to jump in there. I think.

Thierry Dubois:

Is it correct to say that even though the engine... we find a technology and/or a fuel that prevents any soot from the exhaust of the engine that prevents any soot formation of the exhaust of the engine, is correct to say that we could have contrails anyway because the droplets also form around naturally present aerosols in the atmosphere?

Richard Moore:

Yes, you're absolutely right. And so as we're getting good at producing the soot particles that come out of the engines, we're starting to look at other particle sources that may start to fill in the gap and see these contrails. Because we're still emitting the water vapor in the combustion stream, and that water vapor, as it cools, wants to condense on something. And so we're looking to the natural particles that are in the upper atmosphere as one potential source. We're also looking at co-emitted particles from the engine turbo machinery that we've been able to conveniently ignore in the past.

And so in a soot rich engine exhaust flow, the soot are the nuclei for the ice crystals to form on. And other particle emission sources, sulfur from the fuels or small engine oil exhaust particles were too small to matter. Well, now as we've gotten rid of the soot, we're starting to see that they may become important, and we may have to be a little more deliberate about how we vent the engine oil, how we de-sulfurize the fuels for these low emitting engines. So the engine companies and the air freighters are keenly interested in that and thinking about how we can do a better job there too.

Guy Norris:

So I mean, as you delve deeper into this, Rich, in terms of fuel chemistry, I mean there are so many options right out there at the moment. Obviously some leading, more leading candidates e-fuels, for example, of these sustainable fuels. Will the next part of the research into then really force us to begin looking through this large matrix of fuel types to see what actually might work better than others?

Richard Moore:

Yeah, absolutely. And so we're interested in understanding how fuel chemistry will improve the environmental impact of aviation, but the primary guiding principle for the aviation industry is safety and operability of the engines. And so we want to make sure that even as these new fuels come online that are better for the environment, they're also working as expected in the existing engine fuel systems and engine technologies. And so there's a lot of work being done now to examine these fuels and certify them for those purposes. And so as the U.S. Department of Agriculture, as the Department of Energy spin up the Sustainable Aviation Fuel Production Program, they're putting the fuels through their paces in terms of their performance in the engine and the aircraft fuel systems, and then also looking at what emissions come out of the tailpipe.

Guy Norris:

And of course, as Graham always likes to point out, even if we're talking about these different SAFs, it's still pumping out CO₂ at the tailpipe of one sort, but it's a much smaller, it's a shorter cycle carbon emission, which is the key I think in that case.

Richard Moore:

Yeah, absolutely. We're talking about not zero CO₂, but net-zero CO₂, the idea that we're recycling modern carbon through the atmosphere and it's not lingering for decades or hundreds of years like carbon that we take out of the ground.

Guy Norris:

And I think the last area really is to the third leg, engine technology. We've already partially hit upon it with the idea of going to these higher bypass engines like the Leap versus the CFM56 for example, and the benefits of that. But one of the problems with these more advanced engine cycles is that they're burning fuel at higher combustion temperatures and higher operating pressures too. So the balancing factor here is important because that's inevitably generating, almost inevitably generating more NOx, nitrogen oxides. So there's a trade to make here, isn't there? Yet another part of the complicated picture of how we go forward? What's your comment on that, Rich?

Richard Moore:

Yeah, you're absolutely right Guy. So there are, in addition to the contrails, we're focused on the engine operation and safety aspects. There are other pollutants that we're concerned about. These nitrogen oxides are potent greenhouse gases in their own right, so they can have a climate impact directly if we're flying through the surface atmosphere on approach or idling at the gates at airports. We're also very concerned about NOx emissions for our local air quality and health near airports. So we want to keep an eye on that as well. So that feeds into some of these more advanced engine controls like Wilson.

Guy Norris:

And I guess the last area really is the sort of the elephant in the room in terms of future fuels is hydrogen. I know that's obviously further off and maybe even at the midterm, perhaps only on a regional aviation level, but it does seem as if there is a consensus, particularly in Europe, that this would be the long-term solution for net-zero. And a lot of studies are beginning to show that some of these larger applications are potentially feasible. So I don't know if there's anybody wants to make any comments on that. But one of the potential questions is what is hydrogen's potential for contrail formation? And I think that's a huge question. Thierry, obviously you've got something to-

Thierry Dubois:

Yeah, that's a huge question mark hanging over the use of hydrogen for aviation. On the one hand, you have much, much more water vapor emitted at the exhaust of the engine. Of course, on the other hand, you have no soot. By essence, you have no soot in your exhaust. And Airbus is conducting flight tests. Currently, they have modified, I think it's glider or motor glider, they've modified the motor glider with a small hydrogen turbine engine, and they're doing measurements basically at the exhaust just following the motor glider. So hopefully the question mark will be answered. The question will be answered soon, hopefully.

Guy Norris:

And Graham.

Graham Warwick:

Yeah, so as Thierry says, it's hydrogen and water vapor problems. So the folks that are beginning the hydrogen are the fuel cell guys. So the United Universal Hydrogen, ZeroAvia, so they're starting with small airplanes, regional airplanes. They fly lower, they tend not to... they're not where they produce contrails, but they're looking further ahead. So they're looking at water management. How do you maybe capture some of that water vapor that comes out in their case, out of the fuel cell, and do you store it and dump it in a different phase of flight, or do you use it for something, right? I mean, can you recycle it within the airplane? But I also think that for just your turbine engine going forward, I think water vapor management is going to become something that we have to start thinking about in engine design in the long term.

I'll give you an example, MIT, this is not water vapor, this is NOx. MIT is actually working on a catalytic converter for a turbine engine so that when you go to these very high combustion temperatures, you can have something after the turbine that will remove the NOx before it gets, so you're trying to control this issue you've got, that the higher your performance, the higher the NOx potential becomes, usually put this catalytic converter. And then you've got these advanced water cycle engines like switch in Europe where you are actually going to reuse the water that's in the exhaust. It's going to go through some sort of heat exchanger and go back to somewhere else in the engine. So I do think if you look far enough forward water management, water vapor management is going to become part of the design constraints that we put on engines going forward.

Guy Norris:

Well, it sounds suspiciously like we're returning to the age of steam, which I'm all in favor of, but Rich, I'll leave you to take us to the finish line on this. What do you think are the priorities in terms of future research that we should really be looking to to pave the way really for that?

Richard Moore:

Yeah, and Thierry and Graham have brought up really some outstanding questions that we have yet to resolve. We do expect that hydrogen should not be a sooting fuel, but it's still a gas turbine engine. And so we're still expecting that there will be some particle emissions associated with the lubricants and the oils that we use in those turbine machinery. And so with the additional water vapor, I think that the jury is still out on whether or not hydrogen will be a more or less conducive to forming contrails.

Guy Norris:

Great. Well, thank you so much to Thierry in France and Graham in DC and you of course, Rich in Langley and greatly appreciate you being on the podcast with us today. That's a wrap for this week's Check 6 podcast. Special thanks to our doughty podcast producer in London, Guy Ferneyhough, of course. And to our listeners, thank you again for your time and join us again next week for another Check 6.