Atmospheric Water Vapor Measurement With Spectroscopy

Researchers have shown that a new mid-infrared spectrometer can precisely measure the ratios of different forms of water — known as isotopologues — in atmospheric water vapor through open air in a little over 15 minutes. The new open-path mid-infrared dual-comb spectrometer (DCS) uses near-infrared femtosecond laser pulses and specially designed waveguides to create broadband mid-infrared pulses in a compact package. Land-based water evaporation and plant transpiration can affect isotopologue ratios. These ratios help develop climate change models and understand how water is transported globally in the atmosphere.

Open-path sensing using dual-frequency combs may make atmospheric water vapor isotopologue sensing simpler and easier to apply in remote environments. Future vertical column measurements using combs might also improve satellites’ calibration procedures for isotopologue measurement. In addition, sensing atmospheric water vapor with dual combs can complement other emerging air quality applications of broadband mid-infrared spectroscopy.

Presently, scientists rely on networks of point sensors to analyze isotopologues in atmospheric water vapor. Although these networks are expanding, they require careful calibration to maintain accuracy over time and between sites. Detecting water vapor in an open-air path may eliminate the need for calibration and make capturing large-scale evaporation above reservoirs or over entire watersheds easier.

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