The concept:

The aerosol and cloud simulation chamber AIDA at the Karlsruhe Institute of Technology (KIT) offers a large volume of constant H₂O mixing ratio that provides the basis for 3 intercomparison options: (1) in situ ambient instruments without inlets installed inside the AIDA chamber, (2) in situ instruments with inlets configured to sample from a heated manifold flow extracted from the chamber, (3) optical instruments installed outside the chamber that probe a portion of the chamber volume.  A further distinction is that instruments external to the chamber can be either inside or outside the chamber thermal enclosure. The national metrology institute (PTB) will provide intercomparision to a mobile transfer humidity standard at KIT to ensure traceability to primary humidity standards.

Objectives:

Intercomparison of state of the art and prototype atmospheric hygrometers with each other and with independent humidity standards at the aerosol and cloud simulation chamber AIDA to:

• determine and document sensitivity, precision, and accuracy of the different instruments. KIT will provide comparison to the AIDA water vapour standard (±5% in the range 5-500 ppm; APicT and MBW 373LX) and PTB will provide comparison to a transfer standard ensuring traceability to the national water vapour standard.
• verify the results of the AquaVIT1 intercomparion 2007 .
• provide a continued quality control.
• document the improvement of existing and the development of new instruments.

This includes:·          

• Intercomparison of open path (water vapour) instruments (TDL).
• Intercomparison of sampling (total water) instruments. 
• Quantify the influence of rapid changes in pressure, temperature, and water vapour on instrument response (Not generally possible for the instruments in option 3 above).
• Humidity measurement in dense clouds in AIDA providing intrinsic saturation reference (Not generally possible for the instruments in option 3 above).
• Pressure range: 50 - 1000 hPa.
• Temperature range: 189 - 243 K.
• Water vapour range: 1 - 1000 ppmv (1 atm).
• Relative humidity over ice range: 0 – 100 % (and supercooled water clouds).

New compared to AquaVIT1:

• Test of radiosondes inside AIDA.
• Intercomparison of calibration devices (in flight, lab type).
• Intercomparison with national water standard (PTB transfer standard) to determine the accuracy of the instruments as part of the EU project METEOMET.

Similar activities which could be combined with AquaVIT2:

• Intercomparison of instruments for water isotope ratio measurement.

Organisers: Ottmar Möhler (KIT), Harald Saathoff (KIT), Holger Vömel (DWD), Martina Krämer (FZJ), Ru-Shan Gao (NOOA), David Fahey (NOOA), Volker Ebert (PTB), Thomas Peter (ETH)

Timeframe: April 2^nd -19^th, 2013 (one week instrument installation and 2 weeks of experiments)