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

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.