Development of a Highly Sensitive Laser-Induced Fluorescence Instrument for Isotopologue-resolved Measurements of Atmospheric Nitric Oxide. 

Nitrogen (N) oxides (NOx= NO + NO2) are central to both climate and air quality, acting as short-lived climate forcers while driving the formation of harmful pollutants such as tropospheric ozone (O3) and particulate matter. Although fossil fuel combustion dominates global NO emissions, natural sources—including terrestrial and aquatic ecosystems—contribute an estimated 20–30% to the atmospheric NO budget.1 These natural sources remain poorly constrained, limiting confidence in surface ozone projections under future climate scenarios. Global and regional models are thought to under-estimate terrestrial contributions, particularly from soils, due to poor representation of these sources in emission inventories. Freshwater ecosystems have been considered negligible and are entirely absent from the IPCC AR6 assessment. Improved characterisation of natural NOx emissions requires new measurement approaches capable of distinguishing between different emission sources.Isotopic analysis of N species provides a powerful tool for source attribution and process identification, as different emission pathways can imprint distinct isotopic composition. We present the development of a laser-induced fluorescence (LIF) instrument for highly sensitive, isotopologue-resolved measurement of atmospheric nitric oxide. The instrument uses a Cavity PQS Nd:YAG Cr⁴⁺:YAG microlaser with precise wavelength control near 214.8 nm to selectively excite ¹⁴N¹⁶O, ¹⁵N¹⁶O, and ¹⁴N¹⁸O by probing distinct rovibrational transitions. Through off-resonance fluorescence detection, the instrument is designed to enable differentiation of isotopologues without signal overlap, providing near-simultaneous, real-time quantification. We expect to achieve sub-pptv sensitivity, enabling measurements of natural NOx emissions in remote environments. Future perspectives include the isotopic fingerprinting of biotic and abiotic NO emissions, with broad applicability in aquatic and terrestrial biogeochemistry studies.[1] Jaeglé et al. (2005) Faraday Discussions DOI: 10.1039/B502128F