Frequency Comb-Based Remote Sensing of Greenhouse Gases over Kilometer Air Paths

TL;DR

This paper demonstrates a high-precision, remote sensing method using frequency comb spectroscopy to detect and monitor greenhouse gases over a 2-km open-air path, enabling accurate atmospheric measurements.

Contribution

It introduces a novel application of dual frequency-comb spectroscopy for remote greenhouse gas detection over kilometer-scale paths with high accuracy and resolution.

Findings

Achieved <1 ppm precision for CO2 in 5 minutes

Resolved spectra with <1 kHz frequency accuracy

Tracked rapid variations in gas concentrations

Abstract

We demonstrate coherent dual frequency-comb spectroscopy for detecting variations in greenhouse gases. High signal-to-noise spectra are acquired spanning 5990 to 6260 cm^-1 (1600 to 1670 nm) covering ~700 absorption features from CO2, CH4, H2O, HDO, and 13CO2, across a 2-km open-air path. The transmission of each frequency comb tooth is resolved, leading to spectra with <1 kHz frequency accuracy, no instrument lineshape, and a 0.0033-cm^-1 point spacing. The fitted path-averaged concentrations and temperature yield dry-air mole fractions. These are compared with a point sensor under well-mixed conditions to evaluate current absorption models for real atmospheres. In heterogeneous conditions, time-resolved data demonstrate tracking of strong variations in mole fractions. A precision of <1 ppm for CO2 and <3 ppb for CH4 is achieved in 5 minutes in this initial demonstration. Future portable systems could support regional emissions monitoring and validation of the spectral databases critical to global satellite-based trace gas monitoring.