44 Scientific American, March 2020ples and stuck them into a Teflon sack lit by ultraviolet lights to
simulate sunlight. They were interested in PM 2.5, which is emit-
ted by all fires. Long-term exposure can be deadly, even when lev-
els are below Epa limits. In 2017 and 2018, more than 10 million
people in the West were exposed to levels of PM 2.5 that exceeded
the Epa’s air-quality standards. In 30 years that number is expect-
ed to be closer to 82 million. By 2100 chronic inhalation of wildfire
smoke is projected to kill 40,000 people annually in the U.S. alone.
In the sacks, the initial output of PM 2.5 dissipated quickly and
particle levels decreased—as expected. But in some experiments,
after several hours certain chemicals began to condense. Like
beads of mercury pulling together, other particles settled on these
growing surfaces until PM 2.5 levels that had dipped just hours
before blossomed in a new form. Warneke was not sure what pro-
cess explained the re-formation of PM 2.5, but he thought he had
found a starting point. It increased most often in the presence of
catechol, a large molecule in a building block of wood that was
emitted by smoldering fires. Most intriguing about this discovery
was the idea that if they linked a fire’s temperature to PM 2.5 pro-
duction, it might then be possible to forecast a fire’s PM 2.5 output
from satellites that already measure fire intensity. He and Matt
Coggon, a research scientist at Noaa, also found that catechol may
play a key role in ozone formation related to wildfires.
Ozone decreases lung function after repeated exposure. It is
not a direct emission of wildfires; rather it forms when nitrogen
oxide, VOCs and sunlight mix in the right proportions. There are
always VOCs in smoke, and sunlight is a close associate of flames.
But nitrogen production in wildfires is nuanced. Smoldering burns
release ammonia, a nonreactive form of nitrogen, from plants.
Hot burns release nitrogen oxide, which is volatile. “The tricky
thing is that the chemistry in a plume is pretty hot,” Coggon says.
“It’ll transform even within an hour on big fires into something
that is very different from what was emitted initially.”
The reasons for these shifts have been well understood for
a lmost 20 years. In big wildfires, nitrogen oxide released fromEQUIPMENT at the base of operations in Boise, Idaho ( 1 ).
Mission scientists Carsten Warneke ( left ) and Jim Crawford ( 2 ).12© 2020 Scientific American