Überprüfung der historischen "Schönbein Methode" zum Nachweis von Ozon in einem Strömungsrohr

Abstract

Atmospheric ozone was measured during the second half of last century by means of the Schönbeintechnique. It employed a strip of filter paper impregnated withpotassium iodide and starch. Upon exposure to ambient air, the iodide was oxidized by the ozone to iodine and the well-known blue color from the complex betweeniodine and starch was used as a measure of the ozone concentration by comparison with a chromatic scale. It was known that the Schönbeintechnique was sensitive to the humidity of the air and possibly to the wind speed during exposure. We have tested the method in a flow tube for its response to O$_{3}$, wind speed, and relative humidity. In addition, the dependence of the coloration on exposure time was investigated. For O$_{3}$ concentrations below 10 ppb, the coloration increased proportinal with time for about 8 hours. Then, a plateau was reached that lasted for a few hours. Afterwards, the coloration disappeared with time of O$_{3}$ exposure due to losses of iodine from the paper. The initial slope and the coloration reached during the plateau depended on O$_{3}$ concentration and on humidity. Most important, the length of the plateau was considerably shorter and the decay of the coloration was much faster at higher O$_{3}$ levels. The results demonstrate that the Schönbeintechnique was ambiguous in the sense that, at the exposure times around 8 hours or more, as they were used during lastcentury, higher O$_{3}$ concentrations may have produced less coloration. Only under the assumption that the O$_{3}$ concentration during last century was around or below 10 ppb, the data could be used for evaluation of the O$_{3}$ distribution in the lower troposphere over Europe and the USA during last century. Moreover, quantitative assessments are difficult due to the poor standardization of the papers then used and because of the strong influence of humidity