An Infrared Hygrometer for Atmospheric Research and Routine Monitoring

Abstract

The development and testing of a field durable, infrared differential absorption hygrometer is described. This noncontact hygrometer offers reliable operation in harsh environments while maintaining subsecond response speed. A modified exponential response function was derived from laboratory calibration data. Absolute humidity resolution is better than 1% for absolute humidities greater than 2.0 g m?3, and better than 6% for absolute humidities in the range of 0.1?2.0 g m?3. The lower limit of sensitivity is less than or equal to 10 ppmv. A field trial was conducted at White Sands Missile Range, New Mexico, utilizing a total of six humidity sensors and four temperature sensors. The infrared hygrometer was determined to be the most accurate humidity sensor in the field trial, with a dew/frost-point measurement error of less than 0.5°C over the dew/frost-point range of ?15° to 10°C. A combination of numerical modeling, laboratory measurements, and field data was used to investigate possible humidity measurement errors caused by hydrometeors or aerosols present in the sample volume or deposited on the external optical surfaces. The marine boundary layer was found to be the only natural environment capable of producing such errors.