Fibre-coupled in-situ laser absorption spectrometer for the selective detection of water vapour traces down to the ppb-level

A new fibre-coupled in-situ absorption spectrometer based on near IR diode lasers was developed for applications in basic research and industry. Its ability to detect H2O vapour selectively, calibration-free and without the need of probe sampling enables for the first time humidity measurements even in multi-phase flows. Long-term tests at the cryogenic aerosol chamber AIDA indicated even in dense ice clouds a detection limit of 15 ppb (1sigma, Deltat = 2 s) when using a folded absorption path of 82 m. This simultaneously demonstrated the first precise determination of the dynamic vapour phase super saturation during ice cloud formation.