Typically, the smell of a campfire is at the top of the list of undesirable stimuli when you’re on a plane. But for wildfire researchers aboard a tricked-out C-130, that was the smell of sweet, sweet science in the summer of 2018. Loaded with a bevy of instruments, the chunky cargo plane crisscrossed through plumes from two dozen wildfires up and down the West Coast, sucking in smoke and spitting out data.
The mission: explore the peculiar transformations of wildfire smoke. Scientists are finding that the smoke you breathe downwind of a blaze can be dramatically different in its chemical composition than the smoke when it’s coming right off the flames. That could have big implications for how we assess wildfire smoke as a threat to public health, even for people who live thousands of miles away from the fire itself—modeling this summer performed by the National Oceanic and Atmospheric Administration found that the West’s historically awful fires spewed smoke that drifted clear across the country.
Wildfire smoke is made up of two components: gases and particulates. The gases include carbon monoxide and dioxide, while particulates are tiny bits of charred vegetation. When a wildfire burns intensely, its heat drives air upward, carrying all this muck high into the atmosphere, where winds sometimes blow the smoke thousands of miles. Among fire researchers, smoke at the source is known as “fresh,” but after a few hours, it’s known as “stale.” It can be up in the atmosphere for days, getting really stale, during which time the gases and particulates are reacting not only with each other, but also with sunlight and gases already present in the atmosphere. By the time the smoke from West Coast wildfires reaches the East Coast, it’s fundamentally transformed.
Truly characterizing that transformation requires flying through wildfire smoke with a tricked-out plane loaded with instruments for sampling the atmosphere. “Anything you can think of, we were trying to sample it in the smoke to get the most complete picture of what gets emitted in these wildfires, and how it changes as it goes downwind,” says University of Washington atmospheric scientist Brett Palm, lead author of a new paper describing the research in the Proceedings of the National Academy of the Sciences.
We’re talking dozens of instruments that Palm and his colleagues spent three hours calibrating before each of their 16 seven-hour flights. (Unlike a typical lab where the power is on all the time, you can’t let a C-130 idle all night to keep the instruments humming along.) Some sampled organic and inorganic gases, while others counted particles. They even had instruments that measured the absorption of light by those particles. The plane was also outfitted with an internal detector to make sure the scientists weren’t huffing carbon monoxide as they flew through wildfire plumes.
That said, the air wasn’t exactly fresh inside the cabin. “It smells like you're flying through a campfire,” says Palm. “It's an exciting way of doing science because the reactions are happening right in front of you. And you're measuring them happening in real time in the atmosphere.”
To understand what the team found, first we have to talk about gasoline and sugar. Drip a bit of gasoline on the pavement and you’ll smell it immediately, because it’s highly volatile—it evaporates quickly. To put it another way, it doesn’t want to stay condensed. Sugar sitting in a bowl on your table, on the other hand, isn’t volatile, so it stays condensed. “You don't really worry about your table sugar evaporating,” says University of Washington atmospheric scientist Joel Thornton, coauthor on the new paper. “Over time, it's a much stickier, lower-volatility molecule.” Sticky in this case means molecularly sticky—if you load a lot of oxygen into a molecule, you get strong bonds and less volatility.
And there’s plenty of oxygen to go around up in the atmosphere. What Thornton and Palm found is that the molecules in wildfire smoke also get sticky over time, like sugars, in a sense coagulating. More specifically, smoke is loaded with carbon from burnt vegetation, which oxidizes in the atmosphere. “It's this sort of addition of oxygen to the carbon backbone that makes the molecule in the atmosphere be stickier and more likely to be in the condensed phase, like sugar,” says Thornton.
This means that the primary particles—stuff that came directly off the wildfire—can create secondary particles in the plume by way of chemical reactions. The team could measure this aboard the aircraft with a device called a mass spectrometer, which calculates molecular weight. There are perhaps tens of thousands of organic compounds in wildfire smoke—for example, phenols, consisting of hydrogen, carbon, and oxygen. In the atmosphere, these phenols oxidize, gathering more oxygen, thus becoming stickier, developing over time into particles.
At the same time, the smoke plume is diluting as it moves downwind. Some compounds evaporate away, and particulates fall out of the plume and land on the ground. “Then you can also have organic gases undergo reactions that add to the particle phase,” says Palm. “So you have competing processes happening that are affecting the amount of particulate amounts, organic particles, that get transported downwind.”
That is, the plume is at once dissipating and accumulating new particles through chemical reactions. That’s important when we’re considering human respiratory health, because it’s the particulate matter from wildfire smoke that works its way deep into the lungs. These researchers didn’t single out which particles may be of the most concern, but scientists already know for sure that wildfire smoke ain’t good for respiratory health. In particular, they worry about particles known as PM 2.5 (particulate matter 2.5 microns or smaller) which can cause eye and nose irritation and exacerbate existing chronic heart or lung problems. They can contain heavy metal solids like lead and cadmium, and polyaromatic hydrocarbons, some of which have been linked to cancers.
The new work shows that we can’t just expect wildfire smoke to nicely dissipate as it moves downwind, since chemical reactions continue to form new particles the whole time. “We were a bit surprised at how quickly chemical and physical changes are happening,” Palm says, “because we had this added ability to measure a lot of new compounds that hadn't been measured before with all just high-quality, innovative instrumentation.”
So why is this important to know? Because the West Coast’s wildfire problem is now America’s problem. While smoke is more hazardous near the wildfire, where it’s less diluted, it can still make its way clear across the country and fall out onto the East Coast. Models can show both where that smoke will end up, and how much of it actually reaches a particular region. But scientists are just beginning to explore—thanks to that high-quality, innovative instrumentation—how a plume not only dilutes, but in a way grows over time. “These results should help better model the amount of smoke that gets transported to cities like Seattle and San Francisco, and even to the Midwest and East Coast,” says Palm, “which can be the difference between modeling good air quality, and modeling moderate or slightly hazardous air quality.”
Is it a pain to have to set up instruments for three hours before each flight? You bet. But there’s just no way that scientists can faithfully replicate a wildfire in the lab and study smoke that way. Too many variables are at play: What kinds of vegetation (or, unfortunately, how many structures) a fire is burning; the intensity with which it burns, which determines how many organic compounds get released; or how weather like fog might further complicate the plume’s chemistry. These and a galaxy of other factors combine to create “fire regimes,” or the patterns of how wildfires burn across a particular landscape.
This also means that future flights through other plumes will find unique chemical profiles of smoke—every wildfire is singular. “To me, it looks like they're opening up new avenues for research,” says Rebecca Buchholz, an atmospheric chemist at the National Center for Atmospheric Research who wasn’t involved in this work. “And it'll be really interesting to look at other fires in other years, other times, maybe in different places around the world as well, to look and see how consistent their results are across different fire regimes.”
Australia’s wildfires, for instance, are chewing through a far different landscape than the fires in California. “You can have different compounds and different emission ratios of different particles and gases from different kinds of vegetation,” Buchholz adds. “So, for example, the emissions from grassland would be very different to the emissions from forests.”
The emissions, particularly all that carbon, of course have implications for climate change. But more subtly, a wildfire smoke plume interacts with light, particularly organic compounds called “brown carbon,” which absorb visible light, making the smoke look brown. Since this smoke cloud is dark, it would absorb more of the sun’s energy, heating the sky. A lighter plume, on the other hand, would reflect and scatter more light, cooling the sky. All this could in turn affect the local weather on shorter time scales, and potentially the climate on longer time scales.
“There's a lot of talk in the field about which is more important in terms of climate impacts: Is the scattering outweighing the absorbing, or is the absorbing outweighing the scattering?” asks Buchholz. “The significance of absorbing the light is it can have climate impacts. As it dilutes down downstream, that absorbing property dilutes, but it's still very important and needs to be quantified.” It’s especially important given that we’re already seeing the consequences of climate change in supercharged wildfires, which are burning more intensely and blackening ever more square milage.
Thornton and Palm’s new research was done on smoke plumes in the afternoon—next, they want to do night flights. This will allow them to better understand the role of the sun’s energy in the multitudinous chemical reactions firing throughout the plume as the smoke goes stale.