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.