An humidity sensor developed by the Physikalisch-Technische Bundesanstalt (PTB), the SEALDH laser hygrometer, has proven its worth when used aboard an aircraft; it fulfils all pre-conditions to be used as a transfer standard for conventional humidity-measuring instruments. This would allow the quality of air humidity measurements in the Earth's atmosphere - and, thus, also climate model computations - to be improved.

Humidity measurements in the atmosphere are of essential importance, since water vapour, as the most important natural greenhouse gas, has a strong influence on the Earth's atmospheric radiation balance and, thus, decisively influences our climate. In addition, water is responsible for meteorological phenomena such as the formation of clouds and precipitation.

Hence, the atmospheric water content is an essential measurand in all climate models, but also when it comes to forecasting the weather; this measurand has to be determined with great accuracy if reliable predictions are to be made with regard to the weather and to the development of the climate.

However, measuring water vapour as far as into the upper atmosphere is not an easy task, which leads to air humidity measurements differing sometimes by more than 10 % when measured in different research projects, using alternative methods, even within the scope of demanding laboratory comparisons [1].

Cloud-, precipitation- and also complex climate model computations should, however, be based on measurement data which is as accurate as possible to have sufficient significance.

In order to improve the quality of atmospheric water vapour measurements and to provide better comparability, PTB scientists have developed the traceable laser hygrometer SEALDH. SEALDH stands for "Selective Extractive Airborne Laser Diode Hygrometer" and works according to the principle of tunable diode laser absorption spectroscopy (TDLAS).

Its use aboard an aircraft requires the system to be small, light and insensitive to vibrations; in addition, it must be able to perform rapid measurements and to work autonomously [2], e.g. to constantly monitor itself or to be able to resume operation itself after a fault after an unplanned shutdown. But SEALDH is even more than this: it is self-calibrating. This is a clear advantage compared to conventional hygrometers which have to be calibrated frequently - and often under adverse conditions (e.g. in an airplane hangar).

SEALDH has now been tested under field conditions within the scope of a scientific mission on a Learjet 35A, a former passenger plane that was adapted for research purposes; it was able to give proof of its performance during seven flights after having passed a blind comparison with several established aircraft hygrometers at the environmental simulation chamber of Forschungszentrum Julich [3].

During its in-flight operation, the laser hygrometer proved a detection limit in the ppm range, a very large measuring range between 25 ppm and 25,000 ppm of water volume fraction, as well as an excellent temporal resolution of clearly below 1 s.

Furthermore, PTB scientists have compared SEALDH with PTB's national humidity standard and attained a mean deviation of less than 2 % - without prior calibration. Based on these features, it will, in the medium term, be possible to use SEALDH as a transfer standard for the quality assurance of air humidity measurements in atmospheric research.

What does "traceable measurement" mean?
Numerous research groups all around the world are investigating climate-relevant processes in the atmosphere; they use different measurement procedures under different conditions.

To make their measurement results comparable, traceable measurements would make sense, i.e. the measurement uncertainty of a measuring device compared to the best possible standard must be known. In the medium term, SEALDH could be such a standard, since its capacity as a transfer standard has been demonstrated also under real in-flight conditions and compared to PTB's primary humidity standard.

[1] H. Saathoff, C. Schiller, V. Ebert, D. W. Fahey, R.-S. Gao, O. Mohler, and the AQUAVIT Team, The AQUAVIT formal intercomparison of atmospheric water measurement methods, Geophysical Research Abstracts, Vol. 10, EGU2008-A-10485, 2008,SRef-ID: 1607-7962/gra/EGU2008-A-10485, and D. Fahey, R. Gao: Summary of the AquaVIT water vapor inter-comparison: static experiments (2009).

[2] B. Buchholz, N. Bose, S. Wagner, V. Ebert: Entwicklung eines ruckfuhrbaren, selbstkalibrierenden, absoluten TDLAS-Hygrometers in kompakter 19" Bauweise. AMA-Science, ISBN: 978-3-9813484-0-8, 16. GMA/ITG-Fachtagung Sensoren und Messsysteme 201, 22. 5. 2012, Nurnberg, Germany, DOI: 10.5162/sensoren2012/3.2.3

[3] B. Buchholz, B. Kuhnreich, H.G.J. Smit, V. Ebert: Validation of an extractive, airborne, compact TDL spectrometer for atmospheric humidity sensing by blind intercomparison. Applied Physics B: Volume 110, Issue 2 (2013), pp. 249-262