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

Abstract

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.