Direct single-mode fibre-coupled miniature White cell for laser absorption spectroscopy.

We present the design, setup, and characterization of a new lens-free fibre-coupled miniature White cell for extractive gas analysis using direct tunable diode laser absorption spectroscopy (dTDLAS). The construction of this cell is based on a modified White cell design and allows for an easy variation of the absorption length in the range from 29 cm to 146 cm. The design avoids parasitic absorption paths outside the cell by using direct, lensless fibre coupling and allows small physical cell dimensions and cell volumes. To characterize the cell performance, different H2O and CH4 concentration levels were measured using dTDLAS. Detection limits of 2.5 ppm ⋅ m for CH4 (at 1.65 µm) and 1.3 ppm ⋅ m for H2O (at 1.37 µm) were achieved. In addition, the gas exchange time and its flow-rate dependence were determined for both species and found to be less than 15 s for CH4 and up to a factor of thirteen longer for H2O.

Advancements, measurement uncertainties, and recent comparisons of the NOAA frost point hygrometer.

The NOAA frost point hygrometer (FPH) is a balloon-borne instrument flown monthly at three sites to measure water vapor profiles up to 28 km. The FPH record from Boulder, Colorado, is the longest continuous stratospheric water vapor record. The instrument has an uncertainty in the stratosphere that is < 6 % and up to 12 % in the troposphere. A digital microcontroller version of the instrument improved upon the older versions in 2008 with sunlight filtering, better frost control, and resistance to radio frequency interference (RFI). A new thermistor calibration technique was implemented in 2014, decreasing the uncertainty in the thermistor calibration fit to less than 0.01 °C over the full range of frost - or dew point temperatures (-93 to +20 °C) measured during a profile. Results from multiple water vapor intercomparisons are presented, including the excellent agreement between the NOAA FPH and the direct tunable diode laser absorption spectrometer (dTDLAS) MC-PicT-1.4 during AquaVIT-2 chamber experiments over 6 days that provides confidence in the accuracy of the FPH measurements. Dual instrument flights with two FPHs or an FPH and a cryogenic frost point hygrometer (CFH) also show good agreement when launched on the same balloon. The results from these comparisons demonstrate the high level of accuracy of the NOAA FPH.