Ultrasonic Detector for Carbon Dioxide

70042S02-II The quantification of sources and sinks of atmospheric carbon dioxide would be greatly facilitated by the development of a new detector for this important greenhouse gas. In particular a lightweight CO2 sonde, similar to the widely used ozonesonde, could be used to estimate landscape-scale (approximately 100 km2) fluxes of CO2 from vertical profiles through the convective boundary layer. This project will develop a new CO2 detector based on the measurement of the speed of sound in air. The ultrasonic detector will be highly selective for CO2 and have a precision of one part in 3000 or better, corresponding to a detection limit of 0.1 ppmv. To compliment the new CO2 detector, a small, lightweight calibration system also will be developed. In Phase I, an ultrasonic detector for gas chromatography was modified, using a scrubber, to be selective for CO2. It was determined that, over a wide operating range, the signal was independent of pressure and flow rate. The best precision was obtained at the lowest flow rate (5 cc/min) and highest pressure (60 psig). Using measurements of real air samples, the instrument was compared with a conventional CO2 analyzer and found to agree to within 0.3 ?0.3 ppmv. In Phase II, a new, miniaturized, ultrasonic detector, specific for measurements of CO2 mixing ratios, will be developed and tested. The sample and reference cells will be in thermal contact to greatly reduce temperature effects on precision and accuracy. The new instrument will be compared with conventional instrumentation at ground monitoring sites and aboard research aircraft. Commercial Applications and Other Benefits as described by the awardee: The ultrasonic detector should result in two commercial products: (1) a new ultrasonic detector for atmospheric measurements with high precision and accuracy, and (2) a portable calibration system, which does not require compressed gas cylinders, for CO2 measurements.