Bridging the Gap: Frequency Combs for a Next-Generation GHG Monitoring Network

Dual-Comb Spectroscopy (DCS) has emerged as a candidate to augment the next-generation observation network. The eye-safe laser can measure methane, carbon dioxide, and water vapor at ranges between 1-10 kilometers, which is at the same spatial resolution of ghg satellites and chemical transport models. Its accuracy and spectral sampling allows for high-resolution interrogation of absorption lines, and its stability and ideal instrument line-shape allows for the DCS to operate with minimal drift. However, ghg measurements need to be accurate to within 0.1% in order to measure long-term trends in a future network.

To fully assess the uncertainties in long-path DCS systems, we systematically investigate the impact of spectroscopic uncertainties as well as those induced by temperature, pressure and water vapor variations. Here, we show that the accuracy of DCS retrievals are mainly limited by errors in pressure effects. This is mainly due to errors in pressure broadening parameters aliasing into the dry column density. In a multi-month field-deployment, we found that errors in commonly used spectroscopic databases (HITRAN 2008, 2016, 2020, TCCON) can cause more than 4% (80 ppb) disagreement on retrieved methane concentrations. We also show the importance of this discrepancy in measuring vertical gradients of atmospheric methane - a future DCS application. Overall, the DCS has the potential to accurately retrieve methane, CO2, pressure, and temperature variations along the surface and in the atmosphere with high-accuracy.