MIT Center for Global Change Science
Submitted to the Dept. of Chemistry on August 20, 1992 in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Chemistry
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The hydroxyl radical plays a central role in the photochemistry of the troposphere. Successful measurement of ambient [OH] provides a useful test of current photochemical theory. In this work, an active titration method was developed as an independent approach to measurement of the local concentration of the tropospheric hydroxyl radical. Its feasibility was investigated through experimental assessment of laboratory analytical capabilities and both experimental and theoretical studies of the field requirements of the technique.
This chemical kinetic method is based upon the instantaneous release of two compounds into the open air. One of these, the titrant, must react nearly exclusively with OH. The second, the dispersant, is inert on the time scale of the experiment (a few hours). While atmospheric diffusion will cause the concentration of both to drop rapidly in an equivalent manner, their concentration ratio will change only as a function of the titrant reaction with OH. The change in the titrant to dispersant concentration ratio over time thus provides an indirect measure of ambient OH. By instantaneously introducing exotic species into the air, this technique addresses difficulties encountered with previous chemical measurements of [OH] in which continuous, uncontrolled releases, with interfering spatial gradients in relative concentrations, were monitored.
The titrant and dispersant species must meet the following requirements: low limits of detection, no normal presence in air, stability in storage, no significant reactivity besides titrant reaction with OH, low toxicity, and sufficient vapor pressures at ambient temperatures to facilitate rapid release. After a survey of potential compounds, perfluorocarbon rings were determined to be the best dispersant species. An optimal titrant species was not identified. Halogenated ethenes and cycloalkanes are acceptable, but both restrict the atmospheric conditions under which a successful measurement can be obtained.
A feasibility assessment for the technique was conducted. The analytical precision with which the titrant to dispersant concentration ratio can be measured as a function of concentration was experimentally measured. A gaussian puff model was used to predict the ambient release mixture concentration as a function of time for a range of atmospheric conditions. Our results indicate that the active titration method, using either the cycloalkanes or the halogenated ethenes, is expected to measure ambient [OH] with an accuracy of 40% and a precision of 30% at an OH level of 1 x 106 cm-3 during stable to neutral atmospheric conditions.
We determined that halogenated ethenes provide a better analytical precision at low concentrations than cycloalkanes, and the predicted uncertainty in the measurement of [OH] is therefore slightly greater for the cycloalkanes. However, the higher rates of reaction with OH and lower toxicity, the cycloalkanes were chosen as the titrants for further field studies. Each measurement of [OH] with this method at the accuracy necessary to test fast photochemical theory requires one month of chromatographic analysis following sample collection in the field. The active titration method therefore is useful as a test of theoretical knowledge concerning tropospheric photochemistry under a range, but not under all atmospheric conditions.