The Science Of Wildfire Smoke : Short Wave Smoke from wildfires can travel huge distances. We've already seen smoke from the fires in California reach all the way to Minnesota. And with all that smoke comes possible risks to human health. So what actually is smoke? Jessica Gilman, an atmospheric chemist with the National Oceanic and Atmospheric Administration, explains what it's made of, how it behaves in the atmosphere, and smoke's role in climate change.
NPR logo

The Science Of Wildfire Smoke

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript
The Science Of Wildfire Smoke

The Science Of Wildfire Smoke

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript


You're listening to SHORT WAVE from NPR. For over a week...


UNIDENTIFIED REPORTER #1: Flames burning out of control.

SOFIA: ...Wildfires have raged through the western United States.


UNIDENTIFIED REPORTER #2: And it's not just the North Bay. Fire is burning in parts of Santa Clara County and the East Bay, along with Santa Cruz, San Mateo Counties. That's a CZU fire tonight.

UNIDENTIFIED REPORTER #3: With more than a million acres already on fire and dangerously limited resources, some firefighters are bracing for catastrophic consequences.

SOFIA: In multiple states, these are some of the largest fires we've ever seen.


UNIDENTIFIED REPORTER #4: And the Pine Gulch fire north of Grand Junction is now burning an area the size of Colorado Springs. It happened suddenly overnight, when it jumped to become the second largest wildfire in state history.

UNIDENTIFIED REPORTER #5: There are at least 10 wildfires burning across our state.


SOFIA: And all of that fire is leading to some states out west seeing alarming drops in air quality.

JESSICA GILMAN: I've been checking all the monitors, and many of them are, you know, just off the charts, certainly well into the unhealthy range.

SOFIA: Jessica Gilman's an atmospheric chemist for NOAA out in Boulder, Colo.

GILMAN: I have a dark blue Jeep parked in my driveway, and you can see the ash and the dust kind of starting to build up on the car as its...

SOFIA: Lately, she's been studying how the pandemic has affected air quality, with lots of people staying at home.

GILMAN: So now we got everything up and running to try to look at the air quality impacts.

SOFIA: But in true 2020 fashion...

GILMAN: (Laughter) Those measurements, while they were ongoing, are now being impacted by the smoke.

SOFIA: See; the damage from wildfires doesn't stop at all the fires themselves. The thing that gets glossed over a lot in all these conversations about the fires is all the smoke that can travel huge distances.

GILMAN: That's one of the interesting things with smoke, is that it doesn't stay where it was admitted; it travels with the wind and can impact large parts of the population well downwind of the fires.

SOFIA: With potential impacts on people's health, especially for those with preexisting heart or respiratory conditions like COVID-19. So today on the show, the science of wildfire smoke - what it actually is, how it behaves and how it perpetuates the cycle of hotter, larger wildfires. I'm Maddie Sofia, and this is SHORT WAVE, the daily science podcast from NPR.


SOFIA: I don't know about you, but when I hear the word smoke, it makes me think of, like, huge, thick plumes of different shades of gray sort of blanketing everything - you know, nothing too complicated. For somebody like Jessica, though...

GILMAN: Smoke is an incredibly complex mixture of different gases and particles. And if we look just at the gases, there are hundreds to thousands of different gases that are formed in biomass burning.

SOFIA: Biomass - we're talking things like trees and brush that burn up in a wildfire. When it comes to particles in smoke, there's also a huge range from larger ones in the form of ash and dust that can more quickly settle out of the sky.

GILMAN: But you also get really teeny-tiny particles on the order of a millionth of a meter in diameter, and those really small particles can stay in the atmosphere for a lot longer.

SOFIA: Yeah. And from the particulates side, the thing that people seem to be the most freaked out about is this PM 2.5...

GILMAN: Yeah (laughter).

SOFIA: ...Or this - the little particles that are super, super small.


SOFIA: And there seems to be a lot of that going on right now in California and, like, large parts of the West, right?

GILMAN: Yeah, so one of the primary hazardous air pollutants is particles that are called PM 2.5s. That has a overall diameter of 2 1/2 micrometers or less, and that's roughly about 50 times smaller than a single grain of salt.

SOFIA: Yeah.

GILMAN: So really, really small particles. The smaller particles, not only can they travel further distances, but they also have this unique ability to follow the sort of micro air currents and can bend around corners and edges and everything. And that means that if you're breathing in smoke, those larger particles are going to hit the back of your throat first, but the smaller particles can actually make it all the way down your throat and then deep into your lungs. And that's where they start to cause all kinds of different health effects.

SOFIA: One of the most interesting things about smoke is how it behaves, how it interacts with the different layers of our atmosphere, including the layer closest to us called the boundary layer. And how big that layer is, how thick it is, depends on temperature. So at night, when it's cooler, that layer condenses and comes back down in altitude. Also, with cooler temps and higher humidity at night, wildfires tend to die down.

GILMAN: And when they die down, that's actually when they produce quite a bit of smoke. And that mixing into a more shallow boundary layer just means you get a lot more smoke very close to the ground, particularly at night, especially if you're in a kind of a mountain valley, where it just starts to pool and accumulate. And it's not really diluted or moved out of your immediate area until the sunrise comes, that boundary layer starts to expand, the wind speeds pick up and kind of take the smoke away.

SOFIA: Sure. Yeah. I guess I didn't - I had no idea that, you know, in areas where there's wildfire burning, that the smoke actually kind of settles back down at night. And it makes me think about, like, you know, it's night, it's cool, you want to open a window, right? That can be problematic.

GILMAN: It is, yeah. And that's true of most air pollution sources, but particularly so for smoke. And many of the western states, you know, even here in Colorado, it's not necessarily all that common that you have air conditioning.

SOFIA: Right.

GILMAN: It does cool down quite a bit at night, and so that is the time people would turn on fans, etc., to try to ventilate the house, get it cool. At night, of course, you're home (laughter) at night sleeping and breathing all through the night, and so, again, that's one way that you can be exposed to smoke that you might not necessarily think of.

SOFIA: And so I think it's important to remember, right? So we're looking at areas like California and Colorado. We're seeing them on fire. We're seeing the smoke. And all of the smoke doesn't just hang out there, right? Like, smoke really travels.

GILMAN: Certain smoke plumes can literally travel the world (laughter) and go to...


GILMAN: ...Really, really remote places. And, of course, with fires, you know, we're impacted here in the United States right now, but of course, that flips as we go to the next season and then the Southern Hemisphere. So fire is just a constant emission source, you know, across the globe. And as I said, as it gets emitted and the different layers of the atmosphere can stay in the atmosphere longer, and that just means it can get carried by the wind currents further and further downwind.

And so I've been looking at the different fire models and stuff that NOAA is producing and can see that right now, even though most of the fires are certainly on the West Coast, you know, up to 80% or more of the continental U.S. is seeing the effects of the smoke. So even, you know, my family, who lives in Ohio, can go out and see these red sunsets, potentially, from smoke that's being emitted out in California and Colorado. And so that smoke can just travel tens, hundreds of miles downwind from the source.

SOFIA: Yeah. Yeah. OK, so we have this smoke, right? And it's all over the West. You know, how does the smoke leave, Jessica? Like, how long are people in California, people where you live in Colorado, going to be living under these, like, poor air quality conditions? And, yes, I am asking you to predict the future for me, Jessica.


GILMAN: Well, that's what I'm best at, so...

SOFIA: Yeah.


GILMAN: The two things that will determine when, you know, residents - particularly of California, those most impacted by the smoke - will get some relief is, of course, when the fires go out. And with that, you'll need a change in the weather pattern - so some rain to help put out those fires. And even if the fires are going, you know, again, a shift in the wind pattern can help move some of that smoke out away from them, but all that means is somebody else will get impacted by that smoke.

So one of the things I always try to remind folks is that we all live downwind of somebody. So it might be great air quality where you're at, but, you know, if there's another emission source just behind you, it's going to impact your neighbors. And so in that regard, California might get some relief, but then maybe Idaho or Montana now gets inundated with more smoke there. So that's the sort of immediate way that you can reduce your exposure to the smoke. But in the atmosphere, the only way smoke is truly removed is if it's rained out of the atmosphere. And it's not necessarily destroyed; it's just removed from the atmosphere.

SOFIA: You know, the kicker is, though, when this smoke maybe clears up from the way that we can detect it, like, just by going out and being like, oh, I can breathe a little bit easier, it never just disappears, right? Like, you know, smoke feeds into this cycle of climate change, right?

GILMAN: Yep. The primary component is going to be related to those particles. And so particles are something that can both cool the climate as well as heat the Earth. And so that's where that size and color of the particles really comes into play. And so the white particles that you associate with clouds generally reflect radiation back to space. So that's a cooling effect, right? If you're under a cloud on a super sunny day, you immediately feel better and cooler when that cloud is overhead.

The other ways that those darker particles, the black soot - those are things that readily absorb radiation from the sun, which means when the sun goes down, they can also readmit that radiation back into our atmosphere. And that's what contributes to that - the global warming effect, the greenhouse gas effect, that is so important for climate change. So that's one way that the aerosols play into it.

SOFIA: Right. And all of these things kind of feed into - and this is simply put - but these things feed into a longer, hotter fire season. So it's kind of this garbage cycle, right?

GILMAN: (Laughter) Unfortunately, yeah. We call that a negative feedback cycle. And so...

SOFIA: Yeah, a garbage cycle - that's what I said (laughter).

GILMAN: So as those particles that are released from biomass burning may impact climate and climate continues to change, which could lead to more fires and so forth, you just get the, unfortunately, negative feedback, where it just continues down the wrong path, rather than trying to correct itself or balance itself out.

SOFIA: Yeah. You know, I feel like the wildfires and the smoke are very visual examples of climate change. I mean, do you think that these fires could impact how people are thinking about climate change and what needs to be done?

GILMAN: I hope so. I mean, there's many different, you know, really visual ways of seeing climate change with our own eyes. I mean, from the rising sea levels and daytime flooding that's happening in some of the coastal cities, to the amount of runoff that you see on the Greenland ice sheet to these huge, you know, ice shelves cleaving off of Antarctica. I mean, the signs are all around. The biomass burning is certainly one that impacts, you know, a large community of people out west. And as you mentioned, it's a very visceral response.

And then with climate change, you know, you often hear of global warming, and of course, fires represent that heat, and so that's certainly a connection there as well. And so, you know, I can only hope that people start to think about how much their lives will be changed as our climate continues to change and get warmer.


SOFIA: Jessica Gilman is a research chemist at the National Oceanic and Atmospheric Administration.

This episode was produced by Rebecca Ramirez, fact-checked by Emily Kwong and edited by Viet Le. I'm Maddie Sofia. Thanks for listening to SHORT WAVE from NPR.

[CORRECTION: An earlier version of this episode incorrectly characterized the relationship between smoke and climate change as part of a negative feedback loop. In fact, it is a positive feedback loop.]


Copyright © 2020 NPR. All rights reserved. Visit our website terms of use and permissions pages at for further information.

NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.