March 2020, ScientificAmerican.com 45plants by flames is entrained in smoke and wafted into the upper
troposphere by the fire’s heat. As it climbs, some of the com-
pounds react with radicals until, after a cascade of reactions, what
started as nitrogen oxide can become peroxyacetyl nitrate (PAN),
a relatively stable molecule when the temperatures are cool
enough. As long as the smoke continues to drift in the cooler tem-
peratures of the upper troposphere, the nitrogen is locked up and
the ozone production process is essentially frozen.
But when the smoke begins to sink again into the warmer tem-
peratures at lower elevations, the PAN breaks down and nitrogen
oxide returns. Suddenly, hundreds or even thousands of miles
downwind from the fire, ozone can form in volumes toxic to
humans. This helps to explain why, during certain wildfire events,
ozone levels spike in Midwestern or even Eastern cities when
plumes born in the West drift eastward. Urban areas, already rich
in nitrogen oxide from cars and industry running on fossil fuels,
can jump way past their air-quality exceedance when wildfire
emissions blow into town on a hot summer day. These conditions
gave Seattle the world’s worst air quality at several points in 2018.
What Coggon and Warneke wanted to know is if there are oth-
er molecules emitted by fires that play a similar role as PAN. Dur-
ing their lab studies, they found catechols, the precursors to
nitroaromatics, which, oddly enough, are used to treat coughs. At
first it was not a particularly interesting find—just another mole-
cule among the hundreds of VOCs they had identified. But in the
two years after the lab work, Coggon developed a chemical model
that suggested nitro aromatics could play a key role in nitrogen’s
life cycle and therefore in ozone’s formation. “When they were
there, there was less ozone,” he says.
After looking at what he called back-of-the-envelope calcula-
tions based on the model runs, Coggon suspected wildfires should
produce significant volumes of nitroaromatics. These molecules
had never been investigated in this context. Thus, by modifying
an existing tool, Warneke and Coggon developed a device to ana-
lyze the concentration of molecules in the air every tenth of a sec-
ond. Called a proton-transfer-reaction mass spectrometer and
small enough to fit in a rack on the DC-8, this was the instrument
that tipped Coggon off to something remarkable during the flight. SIGNALS IN THE SMOKE
“WE’RE gEttiNg iNto it! We’re getting into it now!” Crawford said
over the plane’s communication system as the DC-8 began to
shake and beep. An hour and a half after leaving the North Hills
Fire in Montana, the DC-8, pitched into a steep descent, had
arrived at “GO HERE NOW”: the 14,000-acre Tucker Fire in the
shadow of Mount Shasta. When the plane entered the plume, the
light went orange and the smell of wood smoke filled the cabin.
Coggon sat behind the plane’s left wing staring at a screen with
data from the spectrometer. The chart measured the molecular
composition of hundreds of different VOCs, but Coggon’s eyes
were fixed on catechol, which was now at very high volumes and
ticking down rapidly. “This is even more stuff than we saw two
days ago!” he said. The spectrometer could not detect any nitroaro-
matics—just their precursor compounds. But Coggon had his sus-
picions about where the catechol was going. Suddenly, he was on
his feet, tottering between quakes of turbulence to Wyatt Brown, a
graduate student about a third of the way up the cabin. Brown was
running an instrument that could detect what Coggon’s could not:
submicron aerosols such as nitroaromatics. “Are you seeing it?”
Coggon asked. Brown pointed to the screen—nitrocatechols, a
type of nitroaromatic, had been unambiguously detected.
Coggon’s reaction was too colorful to print. Although he was
witnessing real-world confirmation of the chemistry he had seen
in the models, the troves of novel data were just the start of a
knotty process. Coggon later guessed it would take two years and
further studies to determine whether nitrocatechol was a nitro-
gen reservoir that, like PAN, locked up the element temporarily
and delayed ozone production, or whether it sequestered it per-
manently, halting the formation of ozone. Either theory had
potentially profound implications for forecasting ozone produc-
tion from smoke and therefore smoke’s impact on people.
Over the course of the campaign, such riddles grew common.
There was the house fire they had accidentally measured while
trying to sample biomass burns in Kansas, a case study that may
end up being particularly useful considering the increasing regu-
larity with which wildfires burn human infrastructure. There was
the low-intensity controlled fire in Florida’s pines that produced
gluts of ozone almost immediately after ignition, in contrast to a
high-intensity wildfire in Washington that appeared to produce
almost none. Warneke guessed, and hoped the data would bear
out, that the variability was from the Florida fire burning nitro-
gen-rich fuels on a bright sunny day with low smoke, whereas in
Washington, where the smoke reached 31,000 feet, chemical reac-
tions had been prevented by a column too dense for sunlight to
penetrate. Perhaps most vexing of all was the secondary forma-
tion of PM 2.5. On several fires they observed the volume of PM
2.5 dipping before increasing again. Were the same processes they
observed in the lab also at work in nature?
After an hour of crosshatching the Tucker Fire’s plume, the sun
dipped behind the Pacific Ocean. Out the jet’s window, the fire
was still visible on the ground, a long orange ribbon snaking
through the blackness. The DC-8 was running low on fuel. The
pilots banked a turn east toward Boise, and Crawford finally left
the cockpit. “As an individual emissions event, this was a drop in
the bucket,” he said. “But the details we can extrapolate from here
are going to be really valuable.”
Soon the scientists would turn to the less thrilling tasks of
organizing the data and preparing papers that might tune model-
ing and forecasting tools focused on health. On the distant hori-
zon those tools could “ideally ease regulations to make it easier to
light more prescribed fires,” Soja explained. But that night, awash
in the smell of smoke, the scientists shook hands and exchanged
congratulations. Somebody joked that Warneke had better have a
Gatorade bath ready for the team when they landed.MORE TO EXPLORE
Between Two Fires: A Fire History of Contemporary America. Stephen J. Pyne.
University of Arizona Press, 2015.
U.S. Particulate Matter Air Quality Improves Except in Wildfire-Prone Areas.
Crystal D. McClure and Daniel A. Jaffe in Proceedings of the National Academy of Sciences
USA, Vol. 115, No. 31, pages 7901-7906; July 31, 2018.
The Impact of Prescribed Fire versus Wildfire on the Immune and Cardiovascular
Systems of Children. Mary Prunicki et al. in Allergy, Vol. 74, No. 10, pages 1989-1991;
October 2019.
FROM OUR ARCHIVES
As Alaska Warms, Wildfires Pose a Growing Threat. Jane Wolken; ScientificAmerican.com,
May 31, 2019.
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