Sulfur dioxide measurements by correlation spectroscopy using an ultraviolet light-emitting diode

Gas sensing by optical methods employs traditionally conventional broadband radiation sources or narrowband lasers to take advantage of the specific absorption spectra of the studied gas. The gas correlation spectroscopy (COSPEC) technique is particularly simple and immune to spectral interference because it utilizes a reference gas cell to recognize and quantify the amount of the same gas in the sample volume. This technique can be used in monitoring industrial and volcanic emissions by employing various light sources, including thermal background radiation, broadband lasers, light emitting diodes (LED) and multi-mode diode lasers. Recently, LEDs with structureless emission in the UV region have become available. This paper demonstrates that LEDs can be ideal sources for gas absorption measurements combined with the gas correlation technique, especially suitable for industrial emission monitoring.The LED employed is centered around 300 nm and has an emission band width of about 12 nm covering about 8 prominent absorption peaks of SO₂.By using a 19-cm long sample gas cell, an Allan variance analysis predicted a sensitivity of 0.4 ppm (1.1 mg/m³) with an integration time of one minute. The obtained sensitivity is sufficient for industrial emissions monitoring where typical SO₂ concentrations range from tens to thousands of ppm. For atmospheric monitoring applications at ppb level long path absorption or a multi-pass cell can be employed. The system is immune from interfering gases, such as NO₂, and it is also largely insensitive to total pressure variations and light intensity fluctuations.