Water Vapor Cross-Sensitivity of Open Path H2O/CO2Sensors

When measuring the flux of CO2 with an open-path infrared absorption sensor, cross-sensitivity by water vapor is a source of concern. This is particularly true if the flux is small, such as over the sea. In this paper some possible mechanisms for cross-sensitivity are discussed. Pressure broadening by water vapor, and absorption and scattering by water adhered to the optics are the processes of major concern. Two types of open-path sensors were tested in a climate chamber for cross-sensitivity—one was constructed by the National Oceanic and Atmospheric Administration Atmospheric Turbulence and Diffusion Division (NOAA/ATDD) in Oak Ridge, Tennessee, and the other by the Royal Netherlands Meteorological Institute (KNMI). For the NOAA sensor it was found that cross-sensitivity is absent below 50% relative humidity, but with increasing relative humidity the CO2 concentration apparently decreases. A possible explanation for this effect is scattering of radiation by water adhered to the optics. In the case of the KNMI sensor, the CO2 concentration apparently increases with increasing absolute humidity. This can be explained by pressure broadening. With this sensor the optics are heated and interference by water on the optics is not likely.