Direct Tunable Diode Laser Absorption Spectroscopy (dTDLAS) at AIDA
Direct tunable diode laser absorption spectroscopy offers many advantageous characteristics that fulfill the needs of water concentration measurements at AIDA:
- No need for calibration of the TDL instruments
- High sensitivity down to the ppb range
- High time resolution
Three hygrometers based on dTDLAS, which measure in the near infrared at a wavelength of 1.37 µm, are in operation at AIDA. Their task is to simultaneously determine the concentration of water vapor as well as total water in the simulation chamber with a time resolution of ~1 Hz. From the difference of these two measurements, the condensed water content can be calculated.
In situ measurement of water vapor concentration
Condensed water concentration or ice water content
Spot pattern on one of the Herriott cell mirrors of the APeT. A HeNe-laser is used to make the spots visible.
The following table summarizes the main properties of the three AIDA TDL-systems:
|Opt. path||Dynamic range||Achievable resolution||Application|
|APicT||23-230 m||<1-2000 ppm||15 ppb||Cold clouds|
|SP-APicT||4 m||10-12000 ppm||250 ppb||Warm, dense clouds|
|APeT||30 m||<1-1500 ppm||25 ppb||Total water|
For further information, please contact Dr. Harald Saathoff.
An intercomparison between the APicT, APeT and other water sensors is given by
Fahey et al., The AquaVIT-1 intercomparison of atmospheric water vapor measurement techniques, Atmos. Meas. Tech. Discuss., 7, 3159-3251, 2014
A short overview of the TDL hygrometers and the measurement principle is available in
J. Skrotzki et al., Simultaneous, Absolute In Situ Water-Vapor and Extractive Total-Water Detection in Cold Ice Clouds Using a Dual 1.37 µm TDL-Spectrometer, Poster on FLAIR 2009, September 6-11 2009, Garmisch, Germany
More details on the APicT can be found in
V. Ebert et al., Fasergekoppeltes In-situ-Laserspektrometer für den selektiven Nachweis von Wasserdampfspuren bis in den ppb-Bereich, Technisches Messen 72 (2005) 1 23-30