Up in Flames: Retrieving Clues About Air Pollution from Forest Fires

Orange skies and choking smoke covered California last summer. Wildfires aren’t rare in this state, but climate change has been making them increasingly severe—2020 was the worst Californian wildfire season on record. Recent research on wildfire patterns can offer insight into what kinds of pollutants they release into the atmosphere and may tell us more about how they impact our health and our planet. 

Researchers from the Department of Chemical and Environmental Engineering at Yale University found that forest fire emissions evolve in surprising ways over time. Associate professor Drew Gentner and his lab focus on studying air quality and atmospheric chemistry, particularly the chemical transformations that organic compounds undergo in the atmosphere, and what their ultimate impact might be. The Gentner lab collaborated with Environment and Climate Change Canada to take part in a flight campaign that focused on studying oil sands emissions in Canada. 

The team’s goal was to sample a forest fire’s emissions to study how the composition of its smoke plume evolved over time and distance. “Forest fires are an important factor in global air quality and the air quality of local regions, so the field is conducting more and more projects to study wildfire smoke,” Gentner said.

Catching Fire

The flight campaign—an airplane-based measurement program—was initially focused on studying oil sands emissions and not forest fires. But when a forest fire coincidentally erupted during the campaign, a plane was quickly dispatched to sample its emissions. “I think it was in the back of their mind that, if this happens, we’re going to go, and once they heard about it they capitalized on the opportunity,” said Jenna Ditto, a former doctoral student at Yale in the Gentner lab. “This type of sampling definitely needs quick thinking, and you have to be ready.”

The aircraft flew in zig-zag lines called transects along the emission plume as it traveled downwind and sampled it in five different places. Sample one was taken closest to the fire, while sample five was the farthest. The farther that the sample was from the wildfire, the older those emissions were. 

The plane was outfitted with instrumentation that measured gases, particles, and weather conditions in real time, while also collecting offline samples for later laboratory analysis. These offline samples were collected on small filters or glass tubes filled with absorbent materials that trap a mixture of compounds from the atmosphere.

Once collected, the samples were frozen at around negative thirty degrees Celsius to prevent chemical reactions from altering them during storage. When the team did an initial compound class analysis to see what materials they were dealing with, they found a surprising result: the quantity of compounds called CHONS—which contain carbon, hydrogen, oxygen, nitrogen, and sulfur—increased in particle-phase samples taken the furthest from the forest fire. “We saw that and thought: that’s very interesting, we’ve never seen this before. Why is that happening? Let’s look more into the functional groups,” Ditto said. Compound class analysis is usually done as just a first step to get a sense for what the data looks like, but it fortuitously led the team to some interesting results.

They also found that the quantity of CHO (carbon, hydrogen, oxygen) compounds decreased from samples one to four, moving away from the fire—the fifth sample was contaminated by emissions from a nearby oil sands facility, and was not considered. This seemed to indicate that these CHO compounds were the precursors for the CHONS compounds that were increasing in quantity. “It’s interesting because we previously didn’t really know about the formation of these CHONS compounds,” said Megan He, a current junior and Environmental Engineering major at Yale College who co-authored the study. 

Human Health

The discovery that CHONS compounds are a major component of forest fire emissions is important for future studies on how biomass burning affects human health. For forest fires in particular, understanding what compounds form as emissions evolve downwind helps us determine exactly what we are breathing in. “If you’re a couple hundred miles downwind of a fire, you’re not really exposed to the same type of chemical components as you would be if you were ten miles away from the fire,” Ditto said. 

However, the results of this study are not just limited to wildfires. They could also apply to developing areas that burn biomass for fuel. Biomass burning emits nitrogen species and fossil fuel combustion emits sulfur species, which are similar to those that were present in this particular forest fire plume. Therefore, the chemical reactions that created CHONS in the forest fire could be representative of similar chemical processes that occur in developing regions.

Getting a better picture of what compounds are being formed when something is burned can help create better models that in turn help to inform environmental policies. “If you were a part of the government, it would be helpful to know what the health impacts of the different compounds are,” He said.

Looking to the Future

Moving forward, the team believes that more studies should be done on how the compounds they observed in the fire plume affect the environment. Looking at their light absorption properties could yield important results, since having a lot of light-absorbing particles in the air changes how incoming solar radiation interacts with the planet––which, in turn, affects the climate.

Looking at oil sands emissions, such as those that contaminated the fifth sample, could also be an insightful next step. This was not only the original purpose of the flight campaign, but is also the focus of He’s current research. Oil sands are untapped sources of petroleum fuel in Canada, and the facilities built on top of them drill down to extract and then process that oil. In addition to the natural evaporations from the sands, this process releases a lot of emissions. “Previous studies have shown that the enhancement of secondary organic aerosol formation from these emissions is at a similar magnitude to those that are downwind of major cities, such as Houston or Toronto,” He said. “This remote area where it’s just trees and oil sands facilities has emissions that are comparable to major megacities, so that just shows how important they are in the grand scheme of things.”

Undergraduate Researchers Blaze Forward

This study was particularly special in that it involved undergraduate students in frontline air pollution research. In addition to He, Tori Hass-Mitchell, a former Yale undergraduate and current Yale Ph.D. student, contributed to this research. “I was really excited about the chance to involve undergrads in research that’s at the forefront of the field, and for them to interact with leading scientists from places like Environment and Climate Change Canada,” said Gentner.

As someone who is passionate about air quality research, He emphasized how much she enjoyed the rigor of the research process and the opportunity to apply what she had learned inside the classroom to the real world. “As we saw last year with Australia, forest fires in general are increasing, and they’re just going to keep becoming more frequent,” He said. “There’s so much of a mystery surrounding what is burned and what’s in the air, so I just want to keep figuring out what it is that we’re breathing right now, and what’s coming into our bodies. I guess long story short, it’s just the mystery of it that I love, and we have the tools to actually solve those mysteries.” 

Climate change is worsening, and it’s more urgent than ever that we find ways to mitigate humanity’s negative impacts on the environment. The research that is done in labs can have a powerful impact on current worldwide problems, and new voices—such as He’s and Hass-Mitchell’s—on topics like air quality can get us closer to finding solutions for them. 

Further Reading:

Ditto, J. C., He, M., Hass-Mitchell, T. N., Moussa, S. G., Hayden, K., Li, S., . . . Gentner, D. R. (2021). Atmospheric evolution of emissions from a boreal forest fire: The formation of highly functionalized oxygen-, nitrogen-, and sulfur-containing organic compounds. Atmospheric Chemistry and Physics, 21(1), 255-267. doi:10.5194/acp-21-255-2021

Author Bio: Anavi Uppal is a first-year student and prospective Astrophysics major in Pierson College. In addition to writing for YSM, she is one of Synapse’s outreach coordinators, and teaches science to elementary schoolers through Yale Demos. 

Acknowledgements: The author would like to thank Drew Gentner, Jenna Ditto, and Megan He for their time and enthusiasm about their research.