March 2020, ScientificAmerican.com 43the age of fire had arrived, millions of dollars in funding for major
research campaigns followed. In addition to the DC-8, which could
fly at high elevations and over a great range, the FIREX-AQ team
outfitted nimble prop planes with air-quality sampling instru-
ments to fly lower and closer to columns, as well as rural commu-
nities inundated with smoke. They similarly outfitted trucks for
sniffing smoke on the ground. On the jet, they deployed lasers of
different wavelengths to map a smoke column in three dimen-
sions in real time; there was an instrument to sense acetonitrile, a
chemical known to be an indicator species of biomass burning,
while other sensors looked for black and brown carbon, submicron
aerosol composition, and a long list of other components. This
compilation of tools would measure particles and gases in as many
forms and sizes as the state-of-the-art technology could capture.
By determining at a finer resolution what is in smoke and the
processes by which its nastier products form, air-quality forecast-
ers could better predict the impacts of wildfire emissions on
human health. Knowing how smoke differs between types of fires
could also ease the burden of fire management, specifically when it
comes to lighting prescribed burns. These controlled, lower-inten-
sity fires mimic natural ones and are lit to reduce the amount of
fuel available for future wildfires. They are also notoriously hard to
ignite for social, environmental and regulatory reasons. The Epa
stringently regulates smoke from prescribed fires, despite the fact
that no field studies have demonstrated that emissions from lower-
intensity burns are just as toxic as those from raging wild flames.
“When it comes to smoke in the sky, it’s pay me now or pay me
later,” Soja says. She means that whether managers choose to
ignite fires on their own terms or let nature decide when fire-
adapted landscapes burn, the skies will be smoky. Yet some kinds
of smoke might be worse for human health than others. “We’ve
got to get an understanding of emissions factors so that people
can make better decisions in the field.”THE VARIABILITY OF VOCS
iN thE Fall oF 2016 the FIREX-AQ team went to Montana’s Fire Lab
to start peeling back the layers on emissions. To figure out what
became of smoke downwind and how it produced noxious aero-
sols and ozone, they had to understand its contents at the ignition
point. Maybe certain plants, when burned, created smoke with
more ozone and PM 2.5 than others?
The team collected ponderosa pines from Montana, lilac shrubs
from California, oak from Arizona and 18 other groups of species
regularly burned in the West. They dried and weighed the plants,
then spread them onto chicken wire woven underneath a massive
ventilator hood. They lit two fires with each fuel type: a smolder-
ing burn where the rising smoke seemed viscous like lava and a
hotter burn where the smoke stood up with the fire in salute.
What they found, surprisingly, was that the fire’s temperature
dictated emissions far more than did the kind of plant that was
burning. Certain volatile organic compounds (VOCs) were emitted
during low-temperature burns, whereas others showed up mostly
during high-temperature burns. The fire’s temperature could be
used to predict about 80 percent of those emissions, results that
were published in 2018 in Atmospheric Chemistry and Physics.
For some of those burns, the researchers captured smoke sam-INSIDE THE CABIN of the DC-8 ( 1 ). Post-
doc Xu Lu adds liquid nitrogen to a mass
spectrometer used to measure a suite
of gases abundant in wildfire smoke ( 2 ).
Wing-mounted particle counters ( 3 ).
When opened during flight, the MASTER
instrument images the fire through smoke
( 4 ). At base, Ph.D. candidate Vanessa Seli-
movic prepares air-sampling canisters ( 5 ).2 345© 2020 Scientific American