Pine
trees are one of the biggest contributors to air pollution. They give
off gases that react with airborne chemicals—many of which are produced
by human activity—creating tiny, invisible particles that muddy the air.
New research from a team led by Carnegie Mellon University’s Neil
Donahue shows that the biogenic particles formed from pine tree
emissions are much more chemically interesting and dynamic than
previously thought. The study provides the first experimental evidence
that such compounds are chemically transformed by free radicals, the
same compounds that age our skin, after they are first formed in the
atmosphere.

These
findings, published in the Proceedings of the National Academy of
Sciences, can help make climate and air quality prediction models more
accurate, and enable regulatory agencies to make more effective
decisions as they consider strategies for improving air quality.

“We
have been able to show conclusively that biogenics are chemically
transformed in the atmosphere. They’re not just static. They keep going,
they keep changing and they keep growing,” said Donahue, professor of
chemistry, chemical engineering, engineering and public policy, and
director of Carnegie Mellon’s Center for Atmospheric Particle Studies.
“Quite a few atmospheric models, which are commonly used to inform
research and policy, have been assuming that that doesn’t happen. What
we really need to have in the models is an accurate representation of
what’s really going on in the atmosphere, and that’s what this lets us
do.”

The
air that we breathe is chock-full of particles called aerosols. These
tiny liquid or solid particles come from hundreds of sources including
trees, volcanoes, cars, trucks and wood fires. The small particles
influence cloud formation and rainfall, and affect climate and human
health. In the United States each year, 50,000 premature deaths from
heart and lung disease are attributable to excess concentrations of
aerosols, especially particles less than 2.5 micrometers in diameter.

“There’s
a very, very strong body of data that establishes that fine particles
in the air we breathe have a significant bad effect on people. What is
less well understood is how the size and chemical composition of those
particles influences that effect,” Donahue said.

What
complicates matters is that the atmosphere is a highly oxidizing,
highly reactive place, which means that aerosols are transformed very
rapidly into particles that can have completely different chemical
compositions. Donahue and colleagues in the Center for Atmospheric
Particle Studies were the first to describe the chemical processes
involving free radicals that transform aerosols emitted by man-made
sources like diesel exhaust. But this mechanism didn’t explain what
happens to natural compounds when they enter the atmosphere.

“It
was too aggressive and made too much stuff, so the modelers simply
turned off biogenic aging entirely. This seemed a little extreme,”
Donahue said. He suspected that the biogenic particles would age too,
but in a different way.

Donahue,
together with colleagues in Germany, Sweden, Denmark and Switzerland,
set out to test this hypothesis using fake atmospheres called smog
chambers, which contain several cubic meters of air in an enclosed space
in the laboratory. They fed alpha-pinene, an aerosol released by pine
trees, and ozone into the smog chambers and then added hydroxyl (OH)
radicals, which are naturally occurring, highly reactive molecules that
drive reactions with other chemicals present in the air. The researchers
gathered data from four different smog chambers and fed it into a
computer model that they developed. They discovered that OH ages the
particles, altering their properties and concentrations and producing
three times more particulate matter than what was originally released
into the atmosphere.

“The
most intriguing part is that humans may influence the way that
chemistry plays out,” Donahue said. “The trees emit the stuff, but since
human activity changes the chemistry taking place in the atmosphere,
those changes can affect the amount and properties of the natural
aerosols. There is a lot of evidence that, even when organic gases come
from natural sources, the aerosol levels that come from them are
controlled by human activity. Our work shows one of the ways this can
happen.”

In
addition to Carnegie Mellon, the authors include researchers from the
Karlsruhe Institute of Technology, the Forschungszentrum Jülich, and the
Johannes Gutenberg University, Germany; the University of Gothenberg,
Sweden; the University of Copenhagen, Denmark; and the Paul Scherrer
Institute, Switzerland.  

This
research was funded by the EU project EUROCHAMP, the Swiss National
Science Foundation, the Swedish Research Council and the U.S. EPA STAR
program.

Aging of biogenic secondary organic aerosol via gas-phase OH radical reactions

Source: Carnegie Mellon University