Wildfires burning in the West affect not only the areas burned, but the wider regions covered by smoke. Recent years have seen hazy skies and hazardous air quality become regular features of the late summer weather.
Many factors are causing Western wildfires to grow bigger and to generate larger, longer-lasting smoke plumes that can stretch across the continent. An analysis led by the 91爆料 looks at the most detailed observations to date from the interiors of West Coast wildfire smoke plumes.
The multi-institutional team tracked and flew through wildfire plumes from the source to collect data on how the chemical composition of smoke changed over time. A resulting , published Nov. 2 in the Proceedings of the National Academy of the Sciences, shows that smoke forecasts may incorrectly predict the amount of particles in staler smoke.
The new results could significantly change the estimate for particles in staler smoke, which could be the difference between 鈥渕oderate鈥� and 鈥渦nhealthy鈥� air quality in regions downwind of the fire.
鈥淲ildfires are getting larger and more frequent, and smoke is becoming a more important contributor to overall air pollution,鈥� said lead author , a 91爆料 professor of atmospheric sciences. 鈥淲e really targeted the smoke plumes close to the source to try better understand what’s emitted and then how it can transform as it goes downwind.鈥�
Knowing how newly generated wildfire smoke transitions to stale, dissipated smoke could lead to better forecasts for air quality. Communities can use those forecasts to prepare by moving outdoor activities inside or rescheduling in cases where the air will be unsafe to be outdoors, as well as limiting other polluting activities such as wood-burning fires.
鈥淭here are two aspects that go into smoke forecasts,鈥� said first author , a 91爆料 postdoctoral researcher in atmospheric sciences. 鈥淥ne is just where is the smoke plume going to go, based on dynamics of how air moves in the atmosphere. But the other question is: How much smoke gets transported 鈥� how far downwind is air quality going to be bad? That鈥檚 the question our work helps to address.鈥�
When trees, grass and foliage burns at high temperatures they generate soot, or black carbon, as well as organic particles and vapors, called organic aerosols, that are more reactive than soot. Fires can also produce 鈥溾�� aerosol, a less-well-understood form of organic aerosol that gives skies a brownish haze.
Once in the air, the organic aerosols can react with oxygen or other molecules already in the atmosphere to form new chemical compounds. Air temperature, sunlight and concentration of smoke affect these reactions and thus alter the properties of the older smoke plume.
The multi-institutional team measured these reactions by flying through wildfire plumes in July and August 2018 as part of WE-CAN, or the field campaign led by Colorado State University.
Research flights from Boise, Idaho, used a C-130 aircraft to observe the smoke. The study flew through levels of 2,000 micrograms per cubic meter, or about seven times the worst air experienced in Seattle this summer. Seals on the aircraft kept the air inside the craft much cleaner, though researchers said it was like flying through campfire smoke.
鈥淲e tried to find a nice, organized plume where we could start as close to the fire as possible,鈥� Palm said. 鈥淭hen using the wind speed we would try to sample the same air on subsequent transects as it was traveling downwind.鈥�
The analysis in the new paper focused on nine well-defined smoke plumes generated by the in southwestern Oregon, the Bear Trap fire in Utah, the Goldstone fire in Montana, the South Sugarloaf fire in Nevada, and the Sharps, Kiwah, Beaver Creek and Rabbit Foot wildfires in Idaho.
鈥淵ou can’t really reproduce large wildfires in a laboratory,鈥� Palm said. 鈥淚n general, we tried to sample the smoke as it was aging to investigate the chemistry, the physical transformations that are happening.鈥�
The researchers found that one class of wildfire emissions, phenols, make up only 4% of the burned material but about one-third of the light-absorbing 鈥渂rown carbon鈥� molecules in fresh smoke. They found evidence of complex transformations within the plume: Vapors are condensing into particles, but at the same time and almost the same rate, particulate components are evaporating back into gases. The balance determines how much particulate matter survives, and thus the air quality, as the plume travels downwind.
鈥淥ne of the interesting aspects was illustrating just how dynamic the smoke is,鈥� Palm said. 鈥淲ith competing processes, previous measurements made it look like nothing was changing. But with our measurements we could really illustrate the dynamic nature of the smoke.鈥�
The researchers found that these changes to chemical composition happen faster than expected. As soon as the smoke is in the air, even as it鈥檚 moving and dissipating, it starts to evaporate and react with the surrounding gases in the atmosphere.
鈥淲hen smoke plumes are fresh, they鈥檙e almost like a low-grade extension of a fire, because there鈥檚 so much chemical activity going on in those first few hours,鈥� Thornton said.
The authors also performed a set of inside a research chamber in Boulder, Colorado, that looked at how the ingredients in smoke react in daytime and nighttime conditions. Wildfires tend to grow in the afternoon winds when sunlight speeds up chemical reactions, then die down and smolder at night. But very large wildfires can continue to blaze overnight when darker skies change the chemistry.
Understanding the composition of the smoke could also improve weather forecasts, because smoke cools the air underneath and can even change wind patterns.
鈥淚n Seattle, there are some thoughts that the smoke changed the weather,鈥� Thornton said. 鈥淭hose kinds of feedbacks with the smoke interacting with the sunlight are really interesting going forward.鈥�
The research was funded by the National Science Foundation and the National Oceanic and Atmospheric Administration. Other co-authors are graduate students and and research scientist in the 91爆料 Department of Atmospheric Sciences; Lauren Garofalo, Matson Pothier, Delphine Farmer, Sonia Kreidenweis and Emily Fischer at Colorado State University; Rudra Pokhrel, Yingjie Shen and Shane Murphy at the University of Wyoming; Wade Permar and Lu Hu at the University of Montana; and Teresa Campos, Samuel Hall, Kirk Ullmann, Xuan Zhang and Frank Flocke at the National Center for Atmospheric Research in Boulder, Colorado.
For more information, contact Thornton at joelt@uw.edu or Palm at bbpalm@uw.edu.