27 August 2012
NOAA instrument uncovers first direct evidence of "lensing" and other heat-trapping effects of wildfire smoke particles.
When the Fourmile Canyon fire erupted west of Boulder in 2010, smoke from the wildfire poured into parts of the city including the site of the David Skaggs Research Center, which houses our lab and scientists from the Cooperative Institute for Research in Environmental Sciences (CIRES) and NOAA.
Within 24 hours, a few researchers immediately opened up a particle sampling port on the roof of the building and started pulling in smoky air for analysis by two custom instruments inside. They became the first scientists to directly measure and quantify some unique heat-trapping effects of wildfire smoke particles.
"For the first time we were able to measure these warming effects minute-by-minute as the fire progressed,” said CSD's Dan Lack, lead author of the study published in the Proceedings of the National Academy of Sciences.
The researchers were also able to record a phenomenon called the “lensing effect,” in which oils from the fire coat the soot particles and create a lens, causing them to absorb more of the sun’s radiation and heat up the surrounding air. This can change the “radiative balance” in an area, sometimes leading to greater warming of the air and cooling of the surface. While scientists had previously predicted such an effect and demonstrated it in laboratory experiments, the Boulder researchers were 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.
"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."
The instruments called spectrophotometers can capture exquisite detail about all particles in the air, including characteristics that might affect the smoke particles’ tendency to absorb sunlight and warm their surroundings. While researchers know that overall, wildfire smoke can cause this lensing effect the details have been difficult to quantify, in part because of sparse observations of particles from real-world fires.
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.
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 fire, and understanding that heating effect is important for understanding climate change, Lack said. 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 CIRES, NOAA, and other scientists, specifically a cavity ring down spectrometer and a photo-acoustic aerosol absorption spectrometer, Lack said. 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.
"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," Lack said.
Daniel A. Lack, Justin M. Langridge, Roya Bahreini, Christopher D. Cappa, Ann M. Middlebrook, and Joshua P. Schwarz, Brown carbon and internal mixing in biomass burning particles, PNAS, doi:10.1073/pnas.1206575109, 2012.
Biomass burning (BB) contributes large amounts of black carbon (BC) and particulate organic matter (POM) to the atmosphere and contributes significantly to the earth’s radiation balance. BB particles can be a complicated optical system, with scattering and absorption contributions from BC, internal mixtures of BC and POM, and wavelength-dependent absorption of POM. Large amounts of POM can also be externally mixed. We report on the unique ability of multi-wavelength photo-acoustic measurements of dry and thermal-denuded absorption to deconstruct this complicated wavelength-dependent system of absorption and mixing. Optical measurements of BB particles from the Four Mile Canyon fire near Boulder, Colorado, showed that internal mixtures of BC and POM enhanced absorption by up to 70%. The data supports the assumption that the POM was very weakly absorbing at 532 nm. Enhanced absorption at 404 nm was in excess of 200% above BC absorption and varied as POM mass changed, indicative of absorbing POM. Absorption by internal mixing of BC and POM contributed 19(± 8)% to total 404-nm absorption, while BC alone contributed 54( ± 16)%. Approximately 83% of POM mass was externally mixed, the absorption of which contributed 27(± 15)% to total particle absorption (at 404 nm). The imaginary refractive index and mass absorption efficiency (MAE) of POM at 404 nm changed throughout the sampling period and were found to be 0.007 ± 0.005 and 0.82 ± 0.43 m2 g-1, respectively. Our analysis shows that the MAE of POM can be biased high by up to 50% if absorption from internal mixing of POM and BC is not included.