Balloon-borne Atmospheric Water Vapor Measurement by Laser Absorption Spectroscopy

Abstract

The radiative balance of the Earth is particularly affected by the abundance of water vapor, not only in the lower troposphere, but also in the upper troposphere and lower stratosphere (UTLS). Understanding the mechanisms that control UTLS water vapor, as well as its long-term variation, is important for reliable projections of the evolution of the global climate system. However, these insights are currently limited by the lack of accurate, frequent, and comparable water vapor measurements in the UTLS. In this project, an open-path direct laser absorption spectrometer is developed, sufficiently small and lightweight to be deployed aboard of meteorological balloons. Its low weight (3.9 kg) and compactness (30 x 23 x 11 cm3) inherently reduces sources of contamination and allows flexible and frequent soundings. The instrument incorporates a quantum cascade laser as a rapidly tunable narrowband light source in the mid-infrared spectral region, where H2O exhibits strong absorption lines. To achieve enhanced robustness, while satisfying the stringent mass limitation, the optical and electronic concept is completely reconsidered with respect to laboratory-based spectrometers. This comprises the elaboration of a lightweight and compact multipass cell concept, called the segmented circular multipass cell (SC-MPC), which enables the reduction of the optical setup to its basic components only, i.e., the laser, the MPC, and the detector. Furthermore, the instrument incorporates custom-developed electronics for fast and power-saving laser driving, data-acquisition hardware based on field-programmable gate arrays (FPGA), dedicated software for controlling and monitoring, and a thermal stabilization system that suppresses the external temperature variation by almost four orders of magnitude. The spectrometer is a fully integrated, standalone system, operating autonomously for the duration of a balloon flight. The H2O concentration is retrieved by calibration-free evaluation of the spectral data, i.e., only relying on SI-traceable measurements and absorption line parameters provided by spectral databases. Laboratory-based characterization experiments demonstrate an agreement within 2% to a reference. In a first balloon-borne test campaign, the instrument was successfully deployed up to the UTLS in two consecutive flights. The comparison to CFH shows a mean deviation of 3% in the troposphere. At higher altitude, accumulated humidity within the enclosure of the instrument currently leads to interference with the atmospheric absorption signal. Despite this restriction, the tests demonstrate the feasibility of spectroscopic water vapor measurements in the UTLS at an effective temporal resolution of 1 Hz during ascent and descent.