Ratiometric method for ozone absorption cross section measurement

Abstract

Ultraviolet absorption spectroscopy has been practised for ozone concentration measurement because the method of measurement is fast and reliable. Ratiometric method corrects intensity drift of light source for improvement of measurement stability. However, ozone absorption cross section may not be determined via ratiometric method because of limitation of existing Beer–Lambert law. Absorption cross section defines strength of absorption, which is an important parameter for calculation of ozone concentration. Firstly, optical path length of gas cell that suits dynamic range of ozone monitor in this work (less than 1000 ppm) is determined. Based on spectralcalc.com simulation, gas cells of optical path lengths of 5 cm, 10 cm and 20 cm are optimized for concentration measurement from 494.1 ppm to 988.1 ppm, 247.0 ppm to 494.1 ppm and 123.5 ppm to 247.0 ppm respectively. Secondly, Beer–Lambert law deviation is observed when long gas cell of optical path length 10 cm is used to measure high ozone concentration from 357 ppm to 971 ppm. Typically, ozone is sampled using strong absorption wavelength for high sensitivity measurement. When strong absorption wavelengths cause saturation, linearity of measurement is preserved by sampling ozone using weak absorption wavelength 279.95 nm. Thirdly, temperature and pressure stability of ozone absorption cross section are verified using spectralcalc.com simulation. Finally, a novel equation is established based on Beer– Lambert law for measurement of ozone absorption cross section via ratiometric method. The equation is verified for ozone concentration measurement from 450 ppm to 989 ppm using short gas cell of optical path length 5 cm, sampling wavelength 260.99 nm and reference wavelength 377.05 nm. The equation is attractive to researchers in areas of absorption spectroscopy and optical gas sensor because ratiometric method is gaining popularity for high stability ozone concentration measurement.