RAPID: Low-cost, Lightweight Instrument for In Situ Measurements of Water Vapor on High-Altitude Balloons

This award supports development of a hygrometer, or water vapor sensor, for use in the tropical tropopause layer (TTL), the transition zone between the troposphere (the domain of clouds and precipitation) and the stratosphere (the cold and dry region above it which contains the ozone layer). TTL water vapor is of interest for several reasons including its role in the formation of cirrus clouds, which have a strong effect on earth's radiative energy balance, and the fact that much of the water vapor in the stratosphere enters through the TTL.

The hygrometer developed under this award is intended for use in an instrument package previously developed under AGS-1643022 called RACHuTS, for Reel-down Aerosol Cloud Humidity and Temperature Sensor. RACHuTS is a shoebox-size package weighing less than 2kg which is raised and lowered on a 2km nylon cable attached to the gondola of a research balloon.

RACHuTS is intended to study the formation of cirrus clouds, collecting the temperature, humidity, and cloud particle observations needed to observe the formation of cirrus clouds due to the supersaturation of ambient air. The hygrometer replaces the FLASH-B hygrometer originally used for RACHuTS, which is no longer available.

The requirements for the hygrometer are quite challenging as it has to measure water vapor over a range of 2 to 500 parts per million with a resolution less than 5% over the whole range and a response time of one second. It also has to operate unattended in the TTL environment for 2-3 months. In order to fit the RACHuTS package it has to weigh less than 700 grams and have an average power consumption less than 3 watts.

The hygrometer developed in this project uses wavelength modulation spectroscopy to meet these challenges, taking advantage of a distributed feedback infrared laser developed for telecommunications, an open multi-pass Herriott cell, and low-cost microcontrollers marketed to hobbiests. The hygrometer can be developed quickly because it is similar to existing hygrometers including the University of Colorado Second Generation Laser Hygrometer (CLH-2).

The project includes a test flight on a balloon launched in 2023, and the hygrometer should be available in time for use in the 2024 deployment of the Strateole-2 long-duration ballooning campaign.

The work has scientific broader impacts due to the value of RACHuTS observations for addressing a variety of questions regarding TTL processes. In addition, work under this award fosters the development of a new class of low-cost, lightweight instruments that can be used in a variety of configurations and applications. Potential applications include specialized "dropsondes" released from "hurricane hunter" aircraft and smaller, lower sensitivity versions for use in citizen science.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.