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When the Fourmile Canyon fire erupted west of Boulder, Colo. in 2010, smoke from the wildfire poured into parts of this city on the edge of the Rockies. The fire burned for 10 days, and plumes often drifted over the David Skaggs Research Center, which houses NOAA's Earth System Research Laboratory.
Within 24 hours of the wildfire’s ignition, Dan Lack, Ph.D., a researcher at the Cooperative Institute for Research in Environmental Sciences (CIRES), and NOAA colleagues opened up a particle sampling port on the roof of the government building, pulling in smoky air for analysis by two custom instruments inside. The spectrophotometers could capture exquisite detail about all particles in that air, including characteristics that might affect the particles' tendency to absorb sunlight and warm the surrounding air. CIRES is a collaboration of the University of Colorado and NOAA.
A wildfire near Boulder, Colorado
The Fourmile Canyon wildfire west of Boulder burned for 10 days in September 2010. With smoke plumes drifting over the NOAA Earth System Research Laboratory, scientists sampled the smokey air and subsequently discovered evidence of the heat-trapping effects of wildfire smoke particles. Credt: Dan Lack/CIRES, University of Colorado
Researchers know that overall, wildfire smoke can change the "radiative balance" in a region, sometimes leading to greater warming of the air and cooling of the surface, for example. But the details have been difficult to quantify, in part because of sparse observations of particles from real-world fires.
"When the fire erupted on Labor Day, so many researchers came in to work to turn on instruments and start sampling that we practically had traffic jams on the road into the lab," Lack said. "I think we all realized that although this was an unfortunate event, it might be the best opportunity to collect some unique data. It turned out to be the best dataset, perfectly suited to the new instrument we had developed."
Once the researchers began studying the data they collected during the fire, it became obvious that the soot from the wildfire was different in several key ways from soot produced by other sources -- diesel engines, for example.
"When vegetation burns, it is not as efficient as a diesel engine, and that means some of the burning vegetation ends up as oils," Lack said. In the smoke plume, those oils coated soot particles, and that microscopic sheen acted like a magnifying glass, focusing more light onto the soot particles and magnifying warming of the surrounding air.
Such a "lensing effect" has been shown in laboratory experiments and predicted by researchers, but the new study, published in the Proceedings of the National Academy of Sciences, is one of the first to directly measure the effect during an actual wildfire. Lack and his colleagues found that lensing increased the warming effect of soot ("black carbon" to scientists) by 50-70 percent.
The researchers also discovered that the oils coating the soot were brown, and that dark coloration allowed further absorption of light, and therefore further warming of the atmosphere around the smoke plume.
The additional warming effects mean greater heating of the atmosphere enveloped in dark smoke from a wildfire, and understanding that heating effect is important for understanding climate change. It's also important on shorter timescales, and close in: that extra heating can change the "thermal structure" of the air above and downwind of a forest fire. Such changes can affect cloud formation, turbulence in the air, winds and even rainfall.
The discovery was made possible by state-of-the-art instrumentation developed by NOAA, CIRES and other scientists, specifically a cavity ring down spectrometer and a photo-acoustic aerosol absorption spectrometer. Those instruments could capture fine-scale details about particles sent airborne by the fire, including their composition, shape, size, color and ability to absorb and reflect sunlight of various wavelengths.
"For the first time we were able to measure these warming effects minute-by-minute as the fire progressed," Lack said. "With such well directed measurements, we can look at the warming effects of soot, the magnifying coating and the brown oils and see a much clearer, yet still smoky picture of the effect of forest fires on climate."
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