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

  • APicT (AIDA PCI in-cloud TDL), coupled to a multipath White cell with variable open optical path
  • SP-APicT (Single Path APicT), one time the diameter of AIDA as optical path
Extractive measurement of total water concentration
  • APeT (AIDA PCI extractive TDL), coupled to a multipath Herriott cell with fixed optical path length; the instrument is connected to AIDA via a heated stainless steel sampling tube in which all condensed water is evaporated

Condensed water concentration or ice water content

  • Difference between total water and in situ water vapor concentration

Spot pattern in Herriott cell

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