Quantitative Analysis of Atmospheric OH by Active Titration

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In the past dozen years atmospheric scientists have determined that the hydroxyl radical (OH) apparently plays a pivotal role in controlling the trace chemical composition of the lower atmosphere (troposphere). Hydroxyl radicals (a pair) are produced when an ozone molecule (O ) is decomposed by ultraviolet light to form an 3 energetic oxygen atom which then reacts with a water molecule. It is believed that the hydroxyl radical is the primary species responsible for converting a number of water insoluble reduced chemical compounds (e.g. methane, sulfur dioxide, carbon monoxide, nitrogen dioxide, ammonia and others) into condensable and/or water soluble oxidized species. It is this oxidation by OH which permits these compounds to be cleansed from the atmosphere by precipitation or the gradual settling of small solid and liquid particles. Our understanding of the atmospheric photochemical oxidation process has been developed primarily by laboratory and computer modeling studies. To date it has not been possible to evaluate the completeness of the theory of tropospheric photochemical cycles. To test this theory requires making direct atmospheric measurements of OH and its relationships to the compounds believed to be responsible for its creation and destruction. The detection of the hydroxyl radical in the ambient atmosphere has been a primary objective of atmospheric chemists for the past several years. The difficulty in measuring this species results from its exceedingly low concentration in the atmosphere. Theory predicts OH to be in concentrations one million to a hundred million times smaller than ozone, its parent compound (which itself has a tropospheric concentration of less than one part in ten million). The techniques presently available for measuring OH are not sufficiently sensitive to make the necessary measuremetns in the atmosphere to study the chemistry of this important species. In this project an experimental determination of tropospheric boundary layer hydroxyl radical concentrations using an active titration technique will be attempted. The method involves release of a mixture of two classes of gases: titrant gases which are _______ destroyed predominantly by reaction with OH on a time scale of a day or two; and dispersant gases which are inert essentially and used to __________ determine the dispersion rate of the mixture once it is released into the atmosphere. The time rate of change of the ratio of the titrant and dispersant gas concentrations can be related to the local OH concentration. Several unsaturated halocarbons as titrants and certain saturated halocarbons as dispersants will be measured and their concentrations measured with an electron-capture gas chromatograph.