Open-path measurement of stable water isotopologues using mid-infrared dual-comb spectroscopy

Abstract. We present an open-path mid-infrared dual-comb spectrometer (DCS) capable of precise measurement of the stable water isotopologues H₂¹⁶O and HD¹⁶O. This system ran in a remote configuration at a rural test site for 3.75 months with 60 % uptime and achieved a precision of <2 ‰ on the normalized ratio of H₂¹⁶O and HD¹⁶O (δD) in 1000 seconds. Here, we compare the δD values from the DCS to those from the National Ecological Observatory Network (NEON) isotopologue point sensor network. Over the multi-month campaign, the mean difference between the DCS δD values and the NEON δD values from a similar ecosystem is <2 ‰ with a standard deviation of 18 ‰, which demonstrates the inherent accuracy of DCS measurements over a variety of atmospheric conditions. We observe time-varying diurnal profiles and seasonal trends that are mostly correlated between the sites on daily time scales. This observation motivates the development of denser ecological monitoring networks aimed at understanding regional and synoptic scale water transport. Precise and accurate open-path measurements using DCS provide new capabilities for such networks.