University of Delaware
Office of Public Relations
UpDate - Vol. 16, No. 29, May 1

              Invention measures minute aerosol particles

     A new device may help researchers more effectively
analyze environmental events such as global warming by
measuring the composition of individual aerosol particles as
small as 10 nanometers- roughly one order of magnitude
smaller than existing transportable instruments, say UD
researchers who recently filed a patent disclosure.
     Dubbed RSMS-II (for Rapid Single-Particle Mass
Spectrometer), the instrument analyzes particles "at the
critical early stages of their growth," before they
accumulate in clouds, says Anthony S. Wexler, associate
professor of mechanical engineering. Whether particles are
produced by human activities such as industrial combustion,
or by natural events including volcanic eruptions, aerosols
clearly play a key role in global climate changes, Murray V.
Johnston III, chemistry, notes.
     Atmospheric aerosols may have "direct" or "indirect"
effects on climate around the world, Wexler explains.
Specifically, he says, aerosols may counter global warming
directly, by scattering light. Or, they may indirectly cool
various regions of the Earth by accumulating in clouds that
act like a mirror, bouncing sunlight back into space.
Despite these regional cooling effects, the United Nations'
prestigious Intergovernmental Panel on Global Climate Change
expects average global temperatures to rise by 2 degrees
Celsius between 1990 and 2000.
     But, researchers struggling to predict global climate
change have had a tough time calculating aerosol impacts, in
part because measuring the composition of particles smaller
than 100 nanometers using traditional technologies is
difficult. Wexler estimates that each cubic centimeter of
air contains approximately 1,000 aerosol particles between
20 micrometers and 10 nanometers in size. (One nanometer
equals one billionth of a meter.)
     Understanding these tiny particles is a crucial step
toward explaining aerosol impacts on climate.
     "We want to look at aerosol particles before they grow
into clouds, and we need to measure the number of naturally
occurring aerosol particles in the atmosphere," Wexler says.
"If we don't know how many naturally occurring aerosol
particles are in the atmosphere, we can't figure out what
level of anthropogenic [human-generated] aerosols would
perturb the global energy balance."
     Aerosol particles smaller than two microns may have an
impact on human health, as well as the global climate, he
added. Very tiny aerosol particles can penetrate homes,
office buildings and people's lungs, he says.
     To measure very small individual particles, Wexler and
Johnston developed a technique for souping up a conventional
mass spectrometer, which generates a 'fingerprint' of
different chemicals. Specifically, the instrument produces a
graph showing the spectrum of ions, or charged atomic
fragments, based on their relative masses and abundance
within a sample.
     Samples analyzed in the RSMS-II are sucked through a
nozzle attached to a tube-like apparatus, Johnston says. As
the sample squeezes through the nozzle at high speed, gas
molecules are stripped away, while aerosol particles pass
into the tube, where they enter the path of an excimer laser
firing concentrated light pulses 100 times per second. The
laser zaps and disintegrates roughly one of every 100
aerosol particles, forming ions. The ions are then captured
for analysis in the mass spectrometer. "From the ions, we
can infer the chemical composition of the particles,"
Johnston explains.
     A key to the RSMS-II is a special ion focusing lens
that increases its "hit rate," or the number of particles
struck by the laser. The lens more effectively directs ions,
making it possible to capture fragments over a 4-centimeter
region, Wexler says. Other instruments capture ions only
within a smaller region, he noted.
     Equipped with wheels and small enough to be transported
"in a van or small truck," the RSMS-II should prove useful
for studying a variety of aerosols in the field, Johnston
says. Natural forms of atmospheric aerosols include, for
instance, soot from forest fires, sulfuric emissions from
volcanoes and marine photochemistry-which generates sulfuric
and methanesulfonic acids resulting from the oxidation of
dimethylsulfide, a metabolic byproduct of phytoplankton.
Sulfuric acid also forms from sulfur dioxide emitted by
combustion processes.
     Johnston and Wexler's work, conducted in the new $20
million Lammot du Pont Laboratory, could ultimately support
a wide variety of interdisciplinary research. In addition to
aerosol analysis, Johnston and his colleagues within the
Department of Chemistry and Biochemistry are investigating
DNA, polymers, proteins and soils by mass spectrometry.
Research described here was supported by the National
Science Foundation, the U.S. Environmental Protection
Agency, Electric Power Partners and the DuPont Co.
                                         -Ginger Pinholseter