A Direct Method to Measure Tropospheric Ozone by Absorption Spectroscopy in the Ultra-Violet Region
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Authors
Freese, Robert
Issue Date
1974
Type
Thesis
Language
en_US
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Abstract
The Air Quality Act of 1967 placed a new and greater
emphasis on the research and development of air pollution
detecting and monitoring systems. By 1971, enough
progress in this field had been made to allow the Environmental
Protection Agency to identify a number of air
pollutants and set standards for their maximum allowable
concentrations· (1). Ozone was declared to be a pollutant
and the federal standard for total oxidants (essentially
ozone) was set at 80 parts per billion (by volume).
Several methods have been developed to measure the
concentration of ozone in the ambient air. Regener (2)
introduced one of the first reliable methods to measure
tropospheric ozone, using the principle of chemiluminescence.
In this approach, ozone reacts with Rhodamine B
mounted on silica gel. Light is given off as a linear
function of the amount of ozone reacting at the surface
of the gel. and can be measured by a photomultiplier
tube. Saltzman and Gilbert (3) developed a different
technique, based on the oxidation of a 1% neutral buffered
KI solution and co1ormetric detection of the
iodine produced. Other chemical methods are reviewed
by Hodgeson (4) and are mainly variations or improvements
on the above mentioned techniques.
The main problems encountered with the chemical
methods include the need for an accurate ozone calibration
source, the chemical instability of ozone, and the interference from gases and other substances in the atmosphere.
Chemical techniques measure ozone indirectly
and are therefore plagued by the additional difficulty
of transporting the ozone to the appropriate reacting
surface without altering the concentration in the process.
In an effort to avoid these problems, a physical
method to directly measure ozone concentrations was
developed (5) (6). based on the absorption of light by
ozone at 2531 X. Current methods using this technique
employ the use of a double-beam system, where calibration
is obtained by passing the second beam (at the
identical wavelength) through a cell void of ozone.
Difficulties associated with these methods include
electrical and thermal instability, and the necessity for
an ozoneless cell. Problems traceable to light scattering
and underlying absorptions are also encountered.
In this work, a single beam, ultra-violet absorption
method was developed, eliminating the need for an ozoneless
cell and improving stability. Ozone concentration
measurements were made by the comparison of. the intensities
of the 2537 X and 2894 X emission lines of a
mercury high pressure lamp, thereby reducing the effect
due to light scattering and underlying absorptions. A
preliminary report of this method is given. Ambient air
measurements using a 550 meter absorption path length
are reported. Variations in the technique are briefly
discussed and improvements in the apparatus are suggested.
Description
iv, 34 p.
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