Evaluation of compact hygrometers for continuous airborne measurements

Continuous water vapour measurements in the troposphere and lowermost stratosphere are crucial for the understanding of global water transport processes and climate impact. In the course of the ‘Development and Evaluation of Novel and Compact Hygrometer for Airborne Research’ (DENCHAR) project, water vapour instruments (WaSuL, WVSS‑II, ICH) have been partly newly developed and/or extensively tested in the laboratory as well as onboard of research aircraft. For the blind intercomparisons of the instruments, an MBW DP30 frostpoint mirror and the established FISH Lyman‑α instrument (calibrated versus the MBW DP30) served as reference hygrometers in the laboratory and during the flights, respectively. All DENCHAR-instruments show very consistent behaviour in their respective measuring ranges with deviations of less than about 5–10 %, in‑flight with respect to FISH and in the laboratory with respect to DP30 in the range between 100 and 1000 ppmv, as well as among each other in the range of 100 to 20000 ppmv. At mixing ratios below 100 ppmv, differences between the instruments appear which depend on the individual response times and calibrations. In summary, the WaSul, ICH and WVSS‑II hygrometers can be recommended for continuous water vapour measurements at mixing ratios larger than 10, 30, and 30 ppmv, respectively. For an accurate, reliable, and stable measurement of lower water vapour mixing ratios with a compact hygrometer suitable for autonomous operation, either these instruments need to be improved or new technology would have to be developed. In addition to the instrument evaluation, the performance of different water vapour inlet systems is addressed by comparing a forward-facing Rosemount (TAT housing) and a wall plate inlet. From all in‑flight tests, no measurement artifacts caused by specific characteristics of an inlet could be identified, i.e., forward-facing Rosemount and wall plate inlets can be recommended for use on aircraft for water vapour mixing ratios above 30 ppmv.