Optical hygrometer using differential absorption of hydrogen Lyman-alpha radiation

An optical hygrometer based on the differential absorption of the H Lyman-alpha line has been designed and built for use in atmospheric field studies. The absorption is determined along two separate paths containing sample and dry reference air, respectively, hence eliminating undesired absorption by O(2). At the same time the differential absorption eliminates problems related to intensity variations in the light source output. The dynamic range of the instrument can be tuned by diluting the sample with reference air. Frequent adjustment of the zero offset allows detection of water vapor concentrations of ~3.1 x 10(14) molecules. cm(-3) (0.01 g. m(-3)). With a water vapor concentration of 3.1 x 10(15) molecules.cm(-3) (0.1 g.m(-3)) the relative error is 10%. At those concentrations normally encountered in the troposphere the measuring accuracy is 3%. The response time of the present instrument is <1 s. The absorption cross section of water vapor has been determined to be (1.64 +/- 0.045) x 10(-17) cm(2) (p(tot) = 700 Torr, t = 20 degrees C) at the Lyman-alpha wavelength.