Aerosol Ice Interface Transmission Spectrometer
The new optical particle spectrometer AIITS (Aerosol Ice Interface Transmission Spectrometer) is the next instrument in the Small Ice Detector (SID) family. Like SID3, it acquires two-dimensional forward scattering patterns from particles in the size range from about one to a few hundred micrometers (depending on variable settings). The patterns allow quantifying the phase, habit and fine surface features of large aerosol and ice crystals, which are frequently too small to be adequately characterised using traditional imaging techniques.
Two 2D-forward scattering patterns are recorded per particle using two high-resolution cameras. The cameras fire simultaneously, recording the scattering pattern via a beamsplitter. AIITS can be configured such that the cameras measure either perpendicular polarisations (i.e. P-polarisation with one camera, S-polarisation with the other) or to have a different gain setting on each camera to encompass a larger dynamic range. The incident beam can be either circularly or linearly polarised. Backscatter depolarisation is also measured. The camera and beam configuration must be selected pre-flight.
The probe was deployed on board the NASA Global Hawk aircraft during a recent ATTREX/CAST campaign in the Tropical Tropopause Layer of the eastern Pacific. The TTL is known to be of importance due to the presence of subvisual cirrus, which contributes to net climate radiative feedback. Knowledge of the processes involved in the creation and persistence of such clouds is limited due to sparse observational data