SEALDH‑II: An airborne, autonomous, calibration-free TDLAS Hygrometer. First in-flight results with metrological links to the German Primary Humidity Standards

Abstract:

The accuracy and comparability of airborne hygrometers remains under debate often only reaching ±10% under static lab conditions or even >±30% in flight. This often limits atmospheric models as well as other species sensors which need to be corrected for water vapor dilution or cross sensitivity. Further challenges come from the huge dynamic range (2–40000 ppmv), the strong spatial gradients (up to several 1000 ppmv/s). and the difficult and error-prone calibration processes, very often without any direct links to the accurate global metrological water vapor scales, which are defined via national primary humidity generators.

To fill this gap and provide “traceable”, i.e. metrologically validated, airborne hygrometers we developed the Selective Extractive Airborne Laser Diode Hygrometer, SEALDH, which uses dTDLAS with a special, calibration-free data evaluation to circumvent the need for frequent field calibrations and to ensure high accuracy and comparability by a holistic data quality assurance concept in combination with extensive metrological validations at national primary standards. Thus SEALDH-II can be used in a calibration-free field sensor mode (with an absolute, metrologically defined uncertainty of 4.3% +- 3ppmv). The response time is mainly limited by the gas flow and significantly below 1 sec, yielding precision down to 0.08 ppmv (1σ, 1sec) measured at 600 ppmv and 1000 hPa. Its excellent long-term stability, <1% over 18 months, and the metrologically validated range from 5 to 30 000 ppmv makes it also well suited as a hygrometric field transfer standard. SEALDH-II has been recently operated without any failures for over 50 hours on several airborne science missions (DENCHAR, AIRTOSS-I, and AIRTOSS-II). In addition SEALDH participated in the representative, international comparison AquaVIT‑II.

The performance during these missions and the value of a traceability will be demonstrated and discussed.