Tiny particles in the air called secondary organic aerosols hang around a lot longer than previously thought. Credit: Pacific Northwest National Laboratory |
The fresh scent of pine has helped atmospheric scientists
find missing sources of organic molecules in the air—which, it could well turn
out, aren’t missing after all. In work appearing in the Proceedings of the National Academy of Sciences
Early Edition Online, researchers examined what particles
containing compounds such as those given off by pine trees look like and how
quickly they evaporate. They found the particles evaporate more than 100 times
slower than expected by current air-quality models.
“This work could resolve the discrepancy between
field observations and models,” said atmospheric chemist Alla Zelenyuk.
“The results will affect how we represent organics in climate and air
quality models, and could have profound implications for the science and policy
governing control of submicron particulate matter levels in the
atmosphere.”
Zelenyuk and colleagues at the Department of Energy’s
Pacific Northwest National Laboratory were able to measure evaporation from
atmospheric particles in a much more realistic manner than ever before. This
allowed them to show that they are not liquids, as has been assumed for two
decades, and to get an accurate read on how fast these particles evaporate.
What researchers previously thought takes seconds actually takes days.
Airy organics
Secondary organic aerosols are tiny bits of chemically modified organic
compounds floating in the air. They absorb, scatter or reflect sunlight, and
serve as cloud nuclei, making them an important component of the atmosphere.
For a couple of decades, researchers have interpreted
laboratory and field measurements under the assumption that these particles are
liquid droplets that evaporate fast, which is central to the way these
particles are modeled. However, to this day researchers have failed to explain
the high amounts observed in the real atmosphere. The never-ending search for
extra sources of organics has been frustrating for scientists studying these
aerosols.
To re-examine the assumption, researchers at PNNL used
equipment that could study the particles under realistic conditions. Zelenyuk
developed a sensitive and high-precision instrument called SPLAT II that can
count, size and measure the evaporation characteristics of these particles at
room temperature. Research and development for SPLAT II occurred partly in EMSL,
DOE’s Environmental Molecular Sciences Laboratory at PNNL.
SPLAT surprises
First, the researchers created secondary organic aerosol particles in the lab
by oxidizing alpha-pinene, the molecule that makes pine trees smell like pine.
Oxidation is the same thing that happens to iron when it rusts, and happens a lot
in the atmosphere when aerosols come into contact with gases such as ozone,
which is a pollutant when it is low in the atmosphere.
For comparison, the researchers also made particles from
other, well-understood organic molecules that are known to form solids or
liquid droplets, such as one called DOP. Lastly, they allowed these other
organic molecules and the pine-scented SOA particles to mingle to simulate what
likely happens in the outdoors.
Monitoring the various particles with SPLAT II for up to
24 hours, the research team found that DOP particles behaved as expected.
Organics evaporated from the particles quickly, and faster if the particle was
smaller, which is how liquid particles evaporate.
But the pinene-based particles did not. About 50% of
their volume evaporated away within the first 100 minutes. Then they clammed
up, and only another 25 percent of their volume dissipated in the next 23
hours. In addition, this fast-slow evaporation occurred similarly whether the
particle was big or small, indicating the particles were not behaving like a
liquid.
This lack of evaporation could account for the inability
of scientists to find other sources of atmospheric organics. “Our findings
indicate that there may, in fact, be no missing SOA,” said Zelenyuk.
Slowing spectators
In the world, the SOAs from pinene co-exist with other organic molecules, and
some of these slam onto the particle and coat it. Experiments with the
co-mingled SOAs and organic compounds showed the researchers that coated
particles evaporate even slower than single-source SOA.
Zelenyuk then tested how close to reality their lab-based
SOAs were. Using air samples gathered in Sacramento,
Calif., the team found the
behavior of atmospheric SOAs (whether from trees and shrubs or pollution)
paralleled that of the co-mingled pinene-derived SOAs in the lab and did not
behave like liquids.
The results suggest that in the real atmosphere, SOA
evaporation is so slow that scientists do not need to include the evaporation
in certain models. The researchers hope that incorporating this information
into atmospheric models will improve the understanding of aerosols’ role in the
climate.
