The WV TDL spectrometer provides 1 Hz measurements of water vapor using near-infrared (1370 nm) laser absorption spectroscopy with second harmonic detection. Output from the fiber-coupled near-infrared diode laser is split into two optical paths that probe the same temperature-regulated sample cell: a short path (13.4 cm) for H2O mixing ratios greater than ~700 ppmv, and a long path (389 cm) for H2O mixing ratios from 1 to ~1000 ppmv. The instrument, mounted on the inside face of the top panel of the UCATS enclosure, utilizes the GC power distribution and data acquisition systems. Air is drawn from a tee in the heated GC inlet tubing, using a small downstream diaphragm pump, to provide sample flows of ~1L min-1 to the hygrometer. Water vapor mixing ratios for both optical paths are computed by a microprocessor within the instrument, using vendor-defined calibration parameters and standard pressure and temperature conversions. Mixing ratio values and vital housekeeping data (e.g., cell pressure, cell temperature, laser power, spectral line position) are output via serial port to the GC computer for logging. Calibration is checked in the home laboratory against a cryogenic frost point hygrometer, and in the field using gravimetrically-prepared standards of water vapor in zero air.
Data Interval: 1 s
Accuracy: ±3-5%
Precision: ±2-3%
UCATS
Tunable Diode Laser Spectrometer for Water Vapor (WV)