New fast response photofragment fluorescence hygrometer for use on the NASA ER-2 and the Perseus remotely piloted aircraft

We have developed an in situ instrument to measure water vapor on the NASA ER-2 as a prototype for use on the Perseus remotely piloted aircraft. It utilizes photofragment fluorescence throughout the stratosphere and the upper to middle troposphere (mixing ratios from 2 to 300 ppmv) with simultaneous absorption measurements in the middle troposphere (water vapor concentrations ≳5×1014 mol/cc). The instrument flew successfully on the NASA ER-2 aircraft during the 1993 CEPEX and SPADE campaigns. The 2σ measurement precision for a 10 s integration time, limited by variation in the background from scattered solar radiation, is ±6% and the data were tightly correlated with other long-lived stratospheric tracers throughout the SPADE mission. Its accuracy is estimated to be ±10%, based on laboratory calibrations using a range of water vapor concentrations independently determined by both standard gas addition techniques and by absorption. This accuracy is confirmed by in-flight absorption measurements in the troposphere. The fast response time of the instrument, limited by the 4 Hz data sampling rate, was demonstrated when the ER-2 flew across its own exhaust plume. Improvements to the instrument planned for the Perseus aircraft are described which include the addition of an in-flight water vapor addition system, significant reduction in solar background, and weight reduction.