Ground-breaking research sheds light on how wildfire smoke changes as it moves downwind

Atmospheric scientists flew through wildfire smoke during the summer of 2018 to learn more about its composition and how it changes over time - photo: Brett Palm/University of Washington

After years of processing and analyzing data from his involvement in the 2018 field campaign Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen (WE-CAN), Dr. Brett Palm’s findings are shedding light on how we understand the ways smoke evolves during wildfires. Due to the difficulties of replicating wildfire smoke in a laboratory or controlled setting, researchers like Palm participate in field campaigns, which are large, multi-institutional and coordinated research efforts that aim to study wildfires as they happen. 

“We use research aircraft loaded up with a dozen or more different instruments and fly it through fresh, authentic wildfire smoke in the western United States so we can get a better idea of what compounds are getting emitted at the source,” Palm, a postdoctoral researcher at the University of Washington, said. “We follow the air downwind as it ages to see how smoke changes as it travels downwind, with the purpose of getting a better understanding of how it’s changing as it gets transported to population centers such as Northern Nevada and Northern California.”

This Lockheed C-130 Hercules cargo aircraft is outfitted with a variety of scientific instruments to analyze wildfire smoke – photo: Brett Palm/University of Washington

Going into the campaign, Palm and his colleagues had a hypothesis that wildfire smoke undergoes two chemical processes in the atmosphere as it moves downwind from the mountain-areas where they commonly occur.

“The research I wrote was focused on the evolution of smoke particles, where they can basically undergo two different processes as they get transported downwind,” Palm said. “They can either evaporate, like cloud droplets can evaporate, where the organic molecules become gases and leaves less smoke in that air. Then at the same time, you can have new particles being formed through chemistry. These two competing processes, either taking away small particles or forming new small particles, affect how much smoke actually gets transported downwind to where people live and affects air quality and human health.”

The idea of two simultaneously occurring chemical processes, net particle loss through evaporation or dilution, and net particle formation, has been difficult for researchers to understand in the past because of the lack of wildfire smoke data to support it.

“There have been a few measurements of isolated wildfires in the past that have shown that sometimes you get net particle formation and sometimes you get net particle loss, but we couldn’t really make sense of why that was happening,” Palm said. “But in this campaign, we had such a wide array of different measurements of different fires, we could understand the magnitudes of these two processes and show for the first time that they’re really happening at the same time and very rapidly in these fresh plumes.” 

Understanding how smoke evolves over time and as it moves downwind is important, particularly as it relates to the amount of particulate matter in the air. The Environmental Protection Agency (EPA) describes particulate matter as fine, inhalable particles with diameters measured at 2.5 micrometers or smaller. This measurement, referred to as PM2.5, is one of the driving qualifications that determines the EPA’s Air Quality Index standards.

“PM2.5 is a typical unit that means the particles are smaller than two and a half microns in diameter,” Palm said. “The reason that’s important is because those are the ones small enough to make it deep into your lungs and cause the majority of health effects.”

Consequently, by interpreting how smoke processes both the dilution and creation of particulate matter, prediction models can better inform public health guidelines during wildfire outbreaks. 

“In the atmospheric chemistry of wildfire smoke, [net particle] formation would mean more PM2.5 in the air, but evaporation would mean a loss of PM2.5,” Palm said. “So evaporation would be better for human health because it’s better to inhale them as gases than as particles, whereas formation makes [wildfire smoke] worse for human health.”

Atmospheric scientists sampled wildfire smoke over several western states during the summer of 2018 – photo: Brett Palm/University of Washington

In order to collect the wildfire smoke data they needed, Palm and his colleagues working on the WE-CAN campaign flew on C-130 cargo planes equipped with scientific instruments. 

“With wildfires, it’s really difficult to predict where the smoke is going to be and so you need an airplane to be able to find it,” Palm said. “So we would have about a dozen scientists on board operating all the instruments and a lead scientist working up in the cockpit with the pilots to decide where to fly based on where the smoke and wind are going.”

All of the research flights were conducted from Boise, Idaho, so that the scientists participating in the field campaign had access to wildfires all across the West. Their research flights ranged from wildfires in California and Washington, to Montana and Utah. 

“For our research, the amount of time the airplane could fly was something like six hours or seven hours,” Palm said. “Transit to a fire would take anywhere between half an hour to two hours, then a few hours to fly patterns through the smoke as it ages and repeating that as much as we can until we are running low on fuel and have enough to fly back to Boise.” 

The C-130 is effectively a flying laboratory for the analysis of wildfire smoke – photo: Brett Palm/University of Washington

To capture the data they needed during these research flights, Palm and his fellow researchers used highly-sophisticated instruments as the plane flew through smoke plumes. This did not come without its own challenges, however.

“The compounds that we’re looking at are really large and have a lot of oxygen, which makes them interact with surfaces really well,” Palm said. “If we stick a long tube out the window and you sample the air, [the oxidized compounds] are going to hit the surfaces of that tube and stick to it and mess up the measurement. So we had to design a special inlet to get these compounds into our instrument so it is capable of sampling them.”

The development of these research instruments served to be a breakthrough that was crucial in compiling the data necessary for the study. 

“We were measuring compounds that hadn’t really been reported in real ambient air from wildfires yet and so we were able to use those compounds to help interpret the evolution of the smoke and that is something that hadn’t been done before,” Palm said. “So really, for the first time, we had the right measurements in the right place with the ability to sample many wildfire plumes, and because of that, we were able to interpret it in a way that made sense.”

While the achievements of the 2018 WE-CAN field campaign warrant acknowledgement, Palm said that there’s still more research that needs to be done, particularly as it relates to how wildfire smoke evolves overnight.

“These two processes [of evaporation and formation] were balanced in most of the fires that we sampled in part because we were sampling in the daytime, while the sun was up and there was a lot of rapid photochemistry happening,” Palm said. “But if a fire is emitted during nighttime, there’s a lot of different chemistry happening because the sun is not available for photochemistry.

“So it’s possible that you won’t get as much particle formation at nighttime while you still have the dilution going downwind. So if you’re trying to predict air quality downwind in a city when the smoke is being emitted overnight, it’s really important to understand when these two processes are happening.” 

Scientists from the University of Washington collect data from within active fire plumes during the 2018 fire season – photo: Brett Palm/University of Washington

But due to the intensive needs to conduct research flights in field campaigns like WE-CAN, time will tell when researchers like Palm can explore how smoke evolves differently during the night, compared to what they found during their daytime research flights in 2018.

“[These field campaigns] usually go for a couple of months and are a very time-intensive and money-intensive form of research that’s done in the field of atmospheric science,” Palm said. “But it’s a really exciting way of doing science.”


Scott King writes about science and the environment for the Ally. Support his work.