December 2002 / January 2003
New
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 you’ll 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 nanometers—or 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 ACD’s Jim Smith, one of the lead scientists on the project.
Jim is working with ACD’s Fred Eisele, ASP postdoc Katharine Moore, and UM’s Peter McMurry. At ATD’s 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 body’s 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 vegetation—a process that the team also plans to investigate.
Early findings
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 Atlanta’s extensive urban environment, the ultrafine aerosols analyzed by the spectrometer had very simple chemical compositions—mostly 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 they’re made of.” •David Hosansky
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