Evaluation of a photoacoustic detector for water vapor measurements under simulated tropospheric/lower stratospheric conditions.

Although water vapor is one of the most important and certainly the most variable minor constituent of the atmosphere, accurate measurements of p(H20) with high time resolution are difficult, particularly in the cold upper troposphere/lower stratosphere. This work demonstrates that a diode laser-based photoacoustic (PA) water vapor detector is a viable alternative to current water vapor sensors for airborne measurements. The PA system was compared with a high-quality frost point hygrometer (FPH) and with a Lyman-alpha hygrometer in the pressure range of 1000-100 hPa at frost point temperatures between 202 and 216 K. These conditions were simulated in a large environmental chamberfor 14 h. Simultaneous measurements with the three instruments agreed within 6%. Nitric acid vapor interferes with the FPH measurements at low frost point temperatures but does not affect the other instruments. The sensitivity of the PA system is already sufficient for measurements in the upper troposphere, and straightforward improvements can extend its useful range above the tropopause. Rugged construction, extreme simplicity, small size, and potential for long-term automatic operation make the PA system potentially suitable for airborne measurements.