A Direct Method to Measure Tropospheric Ozone by Absorption Spectroscopy in the Ultra-Violet Region
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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.