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| OH Radical Measurement | |
One way of fighting air pollution is to learn more about the
Earth's natural anti-pollution defenses. A step in that direction
occurred unexpectedly in the early 1980s, when former Engineering
Experiment Station scientist Dr. Fred Eisele was developing a
sensitive atmospheric ion-measurement technique. In the process,
he discovered the technique could be adapted to measure the
elusive hydroxyl (OH) radical.
Considered the single most important cleansing agent in the
atmosphere, the OH radical acts
as an oxidizing agent and removes several
greenhouse gases and other pollutants from the atmosphere.
Continuously replenished during the daytime, but highly
reactive — the molecule lasts
only a fraction of a second before it combines
with other chemicals — and tiny even by molecular standards, the
OH radical defied in situ measurement for 20 years until Eisele.
In early 1989, Eisele modified his ion-sampling apparatus to
support the physical chemistry needed to detect the OH radical.
His measurement technique combines the highly reactive nature of
the OH radical with the extreme sensitivity offered by a mass
spectrometer.
Air drawn through a sampling tube is subjected to a rapid
succession of chemical reactions initiated by the addition of
isotopically labeled sulfur dioxide, which converts all of the
naturally occurring OH into isotopically labeled sulfuric acid.
To prevent the formation of new OH by certain elements in the air
sample, Eisele injects propane shortly after the initial OH is
titrated away.
The acid is ionized to form an isotopically labeled bisulfate
ion, which is then measured with a selected ion chemical
ionization mass spectrometer. Because the OH was converted into
the acid in a one-to-one ratio, the amount of sulfuric acid
reveals the ambient OH concentration.
This technique also provides the unique ability to measure
ambient sulfuric acid, which plays
a central role in aerosol nucleation and growth
— in this case by detecting the non-isotopically labeled
bisulfate ion. Researchers calibrate both of these measurements
by photo-dissociating a known amount of water vapor with a known
flux of 184.9 nanometer UV photons. Together, they form known OH
and sulfuric acid concentrations in the instrument's sample
inlet.
Georgia Tech file photo
Working at a field experiment site, GTRI scientist Dr. Fred
Eisele uses a sensitive atmospheric ion-measurement technique he
adapted to measure the elusive hydroxyl (OH) radical. The OH
radical is considered the single most important cleansing agent
in the atmosphere.
For more information, contact Dr. Fred Eisele, National Center for
Atmospheric Research 1850 Table Mesa Dr., Boulder, CO 80303.
(Telephone: 303-497-1483) (E-mail:
eisele@ucar.edu)
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