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Method

The fluorescent hygrometers for water vapor in situ measurements in the upper atmosphere with balloon, high-altitude aircraft, rocket were created and applied for atmospheric studies.1-8 The fluorescent method1 is based on the photodissociation  of  H2O molecules at wavelengths below 137 nm and subsequent fluorescent relaxation  of the exited OH* radical produced. For Lyman-alpha dissociation of water vapor the process can be expressed as

The electronically excited OH* radical either fluoresces at 310 nm

or is quenching by collisions with air molecules

 A photomultiplier with an interference filter measures the intensity of fluorescence which is signature of the parent molecules of . The intensity of fluorescence is obtained as

In determining the photon flux, absorption by oxygen has to be taken into account. At air pressure  below  10hPa the quenching by air and oxygen absorption are negligible1 and the fluorescence intensity becomes

Thus, under condition with negligible oxygen absorption, the fluorescence gives a direct measurements of the atmospheric water vapor mixing ratio in this case. The coefficient  in (6), representing optical properties is different for each hygrometer, thus a laboratory calibration of the instrument is necessary.

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