Contrails: Clearing the Air 

Should you be scared of jet contrails, the white streaks billowing across the sky? John Lin, an associate professor of atmospheric sciences at the University of Utah, sat down with City Weekly to set the record straight on contrails. Lin, who studied biology and earth & planetary sciences at Harvard, is interested in pollution and environmental issues and moved here from Ontario in fall 2012—just in time to witness one of the state’s coldest winters and worst inversions in recent memory.

What are contrails?

The analogy I make is that on a cold day, you see your breath—it’s the same physics. The water in your breath is in gas form, but it condenses to form liquids. That’s the visible part of the contrail. For a contrail to form, it has to be cold enough, and the atmosphere has to be moist enough. You don’t see contrails when a plane takes off. You don’t see the contrails right near the ground. It has to be high enough—the atmosphere gets colder as you go up.

It is exhaust. There is other stuff, of course, coming from the engines, and some of it is not good for you. There is pollution associated with aviation.

How do jet emissions contribute to local pollution?

Aircraft is way high up, and this stuff takes a long time to get down for us to breathe in. By the time it does, it’s pretty diluted. That effect is much smaller. During takeoff and landing, there will be pollution—definitely the airport has an effect on our air. But aviation pollution is nowhere as big as our cars—just the sheer number of cars and people. That’s the reason why the inversions are so bad for us. The pollution gets really concentrated in a small volume of air.

The aircraft source of pollution is smaller compared to cars. Aircraft is categorized as off-road mobile: construction equipment, lawn mowers, snowblowers, trains and aircraft. On-road mobile [cars] is responsible for double the amount of off-road mobile pollution. PM2.5 is very complicated because you emit something, and then those things, chemically, form PM2.5. Usually, PM2.5 isn’t directly emitted, although our cars often directly emit PM2.5.

We can also look at something called Nox—carbon monoxide, also toxic. This is what forms ozone, which is more of a summertime problem. Non-road mobile forms 13 percent of Nox, and on-road mobile forms 34 percent. Essentially, every source that you look at, the non-road mobile is a small chunk of it. It’s a share but it’s not the lion’s share. It’s pretty clear that when we drive less, air quality will be better.

What are the long- and short-term effects of contrails?

The contrail won’t last. The contrail, initially, will be a straight, clear line, with very sharp edges. And then it will diffuse and mix out. That’s different from some other things we’re doing to the planet. Carbon dioxide stays in the atmosphere, whereas contrails are short-term. That being said, there’s no reason we won’t be flying aircraft for a long time, so it’s reasonable to think, given that aviation will continue and probably expand, what will happen. It’s a legitimate question that the public would wonder.

Can contrails turn a blue-sky day white and enable other contrails to stick around?

If you have enough planes, that effect will be there—the contrails will stay around for a while. A good illustration is the air shows, where the fighter jets fly around and you see all these contrails hang around. If you’re in a dense area with a lot of aircraft, that could be true.

Do contrails contribute to climate change?
Right now, science suggests that there is a warming from the contrails, but it’s likely small, compared to other things. The IPCC (Intergovernmental Panel on Climate Change) is the most authoritative panel on climate change. It’s a U.N.-sponsored panel involving scientists from around the world. And every five years, they come together and review the latest science to come up with our best understanding of the climate. This report broke down the things that we worry about affecting climate. Right now, the effect of contrails is small.

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• Rachel Piper
• John Lin

Cruising altitude is 10,000 feet, and the planetary boundary level, under where pollution is mixed, is usually 1,000. During the inversion, that boundary is especially shallow—only hundreds of feet. What you have during the inversion is warm air on top of cool air. It’s a stable configuration—the warm air is lighter, sitting on top of the cooler, dense air. That sits there like a lid. That’s when our PM levels go through the roof. The lid is very shallow, and the contrails are thousands of feet above.

Do contrails affect the weather?

Contrails can actually cause other clouds to disappear more quickly. The contrails are essentially competing for water with other clouds. And when the sun shines on a contrail, it has more of a tendency to penetrate, which has an overall warming effect. On a day-to-day basis, our temperatures fluctuate. A lot of that is driven by whether you get air from Canada or from the south—big weather systems. Or what’s the temperature of the Pacific Ocean? That’s a big effect. And local effects pale in comparison. In order to understand the effect of contrails, you would have to remove all the other factors. In psychology, you can get people in a room and try to control their settings. We can’t do that with the natural system. That’s the root of a lot of our uncertainties. We only have one Earth. We have one atmosphere.

Why do you think contrails have become such a source of intrigue for conspiracy theorists?

It’s very striking, visually, with that white straight line—it doesn’t look natural at all. Visually, it’s something that people get worried about.

What should we worry about instead of contrails?

We’re talking about two related but different issues. One is the pollution—health. The other is the climate. Often, they’re linked, but sometimes they’re separate.

I think a lot of the time, the public cares more about air quality than climate change. They feel there’s more controversy involved in climate change, although a lot of the science has been settled—it’s pretty clear that the climate is changing. But we direct the attention on the air quality. And if we can improve that, and improve the climate, that’s great. Politically, it’s pretty bipartisan—everybody wants better air quality. We don’t have to drag the climate into the debate. But as we improve air quality, we can often improve the climate.

When we burn fossil fuels, we emit PM2.5 and carbon dioxide at the same time. If we cut down on that—driving less and being more energy-efficient—there are win-win scenarios where we can improve our air quality and reduce the climate impact.

Rachel Piper Twitter: @RachelTachel