Instantaneous Air Temperature, H2O and CO2 Mixing Ratio Measurements by a Collocated Ultrasonic Anemometer/Thermometer and an Open-path Gas Analyzer

Ultrasonic anemometers/thermometers based on time of flight and speed of sound principle provide high accuracy and low-resolution wind and air temperature measurements. Ultrasonically derived air temperature is fast response and is immune to solar radiation errors but require corrections for water vapor and cross wind effects. Infrared gas analyzers based on the light absorption principle fundamentally measure the density of the constituent present in the optical path and are also fast response but require air temperature and water vapor compensation to account for air density effects. An integrated sensor combining a sonic anemometer/thermometer with a collocated open-path gas analyzer provides many advantages and is capable of synchronized measurements of 3D wind vector, speed of sound, water vapor and CO₂ density required for the calculation of true air temperature, H₂O and CO₂ mixing ratios. In a proof-of-concept laboratory experiment the acoustically derived air temperature measurements are compared to a thermistor probe with 0.1 degrees C accuracy over the range of -30 to + 50 degrees ambient temperature and humidity levels of 10 to 90% RH. Results show an accuracy of better than 0.3 degrees C and a resolution of 0.002 degrees C over the entire test range. Results from a field experiment comparing the instantaneous H₂O and the CO₂ mixing ratios from the integrated open-path sensor with a traditional closed-path analyzer show excellent agreement and suggest that the density effects can be compensated in real time point-by-point, eliminating the need for post-processing corrections. Our findings show that measuring air temperature, H₂O and CO₂ mixing ratio and vertical wind fluctuations in the same open-air sample volume have the potential to reduce the uncertainty in CO₂ flux calculations by eliminating the density and sensor spatial separation corrections.