/ January 2003
instrument reveals ultrafine aerosols
Scientists explore fundamental building blocks of the atmosphere
Ultrafine aerosols have profound impacts on human
health and on the atmosphere, but scientists are just now developing the
technology to examine them.
Take a deep breath of air and youll inhale more than a half-million
tiny particles that can affect both your health and the environment in
a variety of ways. Most of these ultrafine aerosols are smaller than 100
nanometersor a thousandth the diameter of a human hair. Although
they are a fundamental building block of the atmosphere, researchers trying
to study them have been stymied by a lack of tools to investigate such
tiny and complex particles.
Fred Eisele, Jim Smith, and Katharine Moore with the thermal desorption
chemical ionization mass spectrometer.
Now, thanks to a team of scientists from the Atmospheric Chemistry Division
and the University of Minnesota (UM), along with instrument designers
in the Atmospheric Technology Division, that may be changing.
The team has developed an instrument known as a thermal desorption chemical
ionization mass spectrometer (TDCIMS). This new device works by giving
aerosols an electric charge and then using an electric field to collect
them on a metal filament. Subjected to heat and chemical ionization, the
aerosols are turned into gas and transferred to a mass spectrometer where
they can be analyzed.
This technique allows scientists, for the first time, to measure the
chemical composition of particles as small as four nanometers.
The main thing about this invention is that just about anything
you can find with it has significance because almost nothing is known
about these particles, explains ACDs Jim Smith, one of the
lead scientists on the project.
Jim is working with ACDs Fred Eisele, ASP postdoc Katharine Moore,
and UMs Peter McMurry. At ATDs machine shop, Jack Fox and
his staff helped develop the instrument.
Why is this research important?
One reason is that ultrafine aerosols can trigger severe respiratory
problems because they are inhaled deeply into the lungs. In contrast,
coarser particles, such as the dust raised by vehicles traveling on unpaved
roads, are generally expelled by the bodys protective mechanisms.
Another is that ultrafine aerosols play a significant, if little understood,
role in global climate. These small particles form the nucleus upon which
cloud droplets form. Knowing more about their composition would improve
our understanding of how clouds form and the influences that pollution
may have on clouds and precipitation.
Aerosols also play a role in the chemistry of the atmosphere because
they provide surfaces on which certain gases condense and react. By studying
the smallest and typically the most recently formed aerosols, scientists
should be able to gain insights about how the presence of gases outside
the aerosol relates to the chemical composition of the aerosol and the
transformations taking place within it.
Ultrafine aerosols are emitted directly by sources such as motor vehicles
and power plants, and indirectly from the gases released by these same
sources. They also come from natural sources, including possibly trees
and other vegetationa process that the team also plans to investigate.
Using the newly developed spectrometer, Jim and his colleagues last
spring analyzed aerosols in the 4- to 20-nanometer range that were collected
in ambient air samples outside the Mesa Lab. As might be expected, the
particles were made up of a variety of chemicals, including nitrate, ammonium,
and sulfate, as well as several types of organic ions.
But when the instrument was packed up and shipped to Atlanta as part
of a collaborative experiment with UM and other institutions called the
Atlanta Aerosol Nucleation and Real-time Characterization Experiment (Atlanta-ANARChE),
researchers came up with very different findings. In Atlantas extensive
urban environment, the ultrafine aerosols analyzed by the spectrometer
had very simple chemical compositionsmostly ammonium and sulfate.
Why would ultrafine aerosols in Atlanta be simpler than those in Boulder?
Jim theorizes that the size of the aerosol, rather than the location,
may be the key. In Atlanta, the TDCIMS studied only aerosols of about
seven nanometers. At that tiny size, the surface of the particle resembles
more the tip of a pin than a flat surface and is less likely to attract
additional molecules. Since there are fewer gases in the atmosphere that
are sticky enough to stay on such a surface, the smallest
aerosols in the atmosphere may be chemically the simplest.
This theory, if proven true, has interesting ramifications. If one accepts
that the chemical composition of the aerosols is primarily responsible
for their toxicity, then this result could suggest that younger and smaller
aerosols pose less of a threat to human health because they have not yet
picked up toxic chemicals.
You might actually be able to predict how toxic an aerosol is
by its age and its size, Jim says. This is important for understanding,
from a health standpoint, how the evolution of ultrafine aerosols influences
their chemical acquisition.
Jim adds that future issues to be explored include reducing the size
range of the instrument to measure still smaller aerosols, as well as
looking at the rate of growth of a population of aerosols up to sizes
of hundreds of nanometers and how such growth relates to their composition.
Jim and Fred have also designed an ion trap mass spectrometer that they
plan to attach to the TDCIMS in order to study organic materials that
are in the process of transforming from gas into solid particles. This
will help them answer such questions as whether chemicals emitted by plants
contribute to the formation of ultrafine aerosols.
In the past few years, numerous researchers have observed the
formation of new particles in forested areas, but no one has been able
to prove irrefutably that the sources of these aerosols are the byproducts
of plant emissions, Jim says. We have a unique capability
with the TDCIMS to look directly into the aerosols and see what theyre
made of. David Hosansky
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