Lightweight, low-power-consumption, inexpensive ozone sensors based on colorimetric chemical sensing would be developed, according to a proposal. Colorimetric chemical sensing is an established technique, but it has not been applied previously to sensing of ozone. The proposed ozone sensors could be incorporated into radiosondes for measuring tropospheric and stratospheric ozone concentrations; they could also be used to monitor ozone in a variety of indoor and outdoor environments near such ozone sources as electric-arc welding equipment, high-voltage laboratory instruments, photocopiers, laser printers, and electrostatic air cleaners.

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The Absorbance of a Chlorophyllin Film on a glass slide was measured before and after exposure to ozone. The fall of the 630-nm peak and the rise of the 500-nm peak can be interpreted in terms of ozone dosage.
An ozone sensor as proposed would include a transparent substrate (e.g., a glass or plastic slide) coated with an organic dye that changes color when it reacts chemically with ozone. The coated substrate would be illuminated by one or more light-emitting diode(s) or diode laser(s) of the appropriate wave-length(s), and the portion of incident light transmitted through the coated substrate at each wavelength of interest would be measured by a photodiode. The color change would manifest itself as a change in absorbance, and thus a change in the amount of transmitted light at each wavelength of interest. A change in absorbance at each wavelength of interest would be related to the degree of reaction and thus to the ozone dosage via the Beer-Lambert law:

A = αlc = ln(I/I0),

where A is the change in absorbance, a is the dosage-dependent change in the absorption coefficient of the dye, l is the thickness of the layer that contains the dye, c is the concentration of the dye in the layer, I0 is the intensity of transmitted light before exposure to ozone, and I is the intensity of transmitted light after exposure to ozone.

An ozone sensor should operate without interference by oxidizing substances other than ozone (e.g., halogens, SO2, and NO2). Therefore, the dye should be either one that does not exhibit color changes in the presence of the other substances, or else one for which the color change induced by ozone differs measurably from the color change(s) induced by the other substances. A promising dye of the latter type is chlorophyllin — a copper-containing, water-soluble derivative of chlorophyll.

Chlorophyllin exhibits an absorption peak at a wavelength of 630 nm. This peak diminishes in proportion to the degree of reaction (see figure), making it possible to quantify the ozone dosage via the Beer-Lambert law. In addition, the product of ozonolysis of an ethylene group on the chlorophyllin molecule gives rise to a smaller absorption peak at 500 nm; the corresponding absorption peaks induced by other oxidizing species occur at different wavelengths. Thus, one could use the 500-nm peak to verify that the observed color change was caused by ozone or, alternatively, one could identify the oxidizing species by measuring the different wavelength of this peak.

Because the color change would be irreversible and proportional to the cumulative exposure to ozone, a sensor of this type would be a dosimeter (as distinguished from a concentration meter). However, it may be possible to determine the instantaneous concentration of ozone from the rate of change of the absorbance.

This work was done by Margie Homer, Margaret A. Ryan, and Roger Williams of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category. NPO-20469