11 Apr 2024
 | 11 Apr 2024
Status: this preprint is currently under review for the journal AMT.

An Economical Tunable-Diode Laser Spectrometer for Fast-Response Measurements of Water Vapor in the Atmospheric Boundary Layer

Emily Wein, Lars Kalnajs, and Darin Toohey

Abstract. The high spatiotemporal variability of water vapor in the atmospheric boundary layer possesses a significant measurement challenge with abundances varying by an order of magnitude over short spatial and temporal scales. Herein, we describe the design and characterization of an economical and flexible fast-response instrument for measurements of water vapor the atmospheric boundary layer (ABL). The in-situ method of tunable-diode laser spectroscopy (TDLS) in the mid-infrared was chosen based on a heritage with previous instruments developed in our laboratory and flown on research aircraft. The instrument is constructed from readily available components and based on low-cost distributed feedback laser diodes (DFB) that enjoy widespread use for high-speed fiber-optic telecommunications. A pair of versatile, high-speed ARM-based microcontrollers drive the laser and acquire and store data. High precision and reproducibility are obtained by tight temperature regulation of the laser via a miniature commercial proportional integrating (PI) controller. The instrument can be powered by two rechargeable 3.5 V lithium-ion batteries, consumes less than 5 W, weighs under 1 kg, and is comprised of hardware costing less than $3,000. The new TDLS agrees within 2 % compared to a laboratory standard and displays a precision of 10 ppm at a sample rate of 10 Hz. The new instrument allows users with little previous experience in instrumentation to acquire high quality, fast-response observations of water vapor for a variety of applications. These include frequent horizontal and vertical profiling by uncrewed aerial vehicles (UAVs), long-term eddy covariance measurements from fixed and portable flux towers, and routine measurements of humidity from weather stations in remote locations such as the polar ice caps, mountains, and glaciers.

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Emily Wein, Lars Kalnajs, and Darin Toohey

Status: open (until 20 Jun 2024)

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Emily Wein, Lars Kalnajs, and Darin Toohey

Model code and software

TDLS Arduino Sketches and Extraction codes Emily Wein

Emily Wein, Lars Kalnajs, and Darin Toohey


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Short summary
We describe a low cost and small research grade spectrometer for measurements of water vapor in the boundary layer. The instrument uses small Arduino microcontrollers and inexpensive laser diodes to reduce cost while maintaining high performance comparable to more expensive instruments. Performance was assessed with intercomparisons between commercially available instruments outdoors. The design's simplicity, performance and price point allow it to be accessible to a variety of users.