An economical tunable diode laser spectrometer for fast-response measurements of water vapor in the atmospheric boundary layer

Abstract. Water vapor in the atmospheric boundary layer poses 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 open-path, fast-response instrument for measurements of water vapor. The in situ method of tunable diode laser absorption spectroscopy in the shortwave 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 that enjoy widespread use for high-speed fiber-optic telecommunications. A pair of versatile, high-speed Advanced RISC Machine-based microcontrollers drive the laser and acquire and store data. High precision and reproducibility are obtained by tight temperature regulation of the laser with a miniature commercial proportional-integral controller. The instrument is powered by two rechargeable 3.6 V lithium-ion batteries, consumes 2 W of power, weighs under 1 kg, and is constructed from hardware costing less than USD 3000. The new tunable diode laser spectrometer (TDLS) agreed to within 2 % compared to a laboratory standard and displayed a precision of 10 ppm at a sample rate of 10 Hz. The new instrument is robust and simple to use, allowing users with little previous experience in laser spectroscopy 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.