Ocean-air trace gas (N2O) fluxes derived from open-path FTIR atmospheric concentration gradient measurements

Open-Path Fourier Transform Infrared (OP-FTIR) spectroscopy is a well-established technique used to measure path-average trace gas concentrations [ppbv] in the atmospheric boundary layer.  Recently, the technique has been applied to derive trace gas fluxes from soils using the flux-gradient (FG) approach. We describe the novel application of OP-FTIR-FG to derive ocean-air fluxes of N₂O [kg N₂O ha^-1 h^-1] at a coastal site in the northwest Atlantic (2018, 2020-2021).  Details of OP-FTIR system deployment across Halifax Harbour (Nova Scotia, Canada) are presented and we describe the application of co-located 3-D sonic anemometer measurements to gas flux calculations.  Finally, we present a full error characterization of N₂O flux and a case study of episodic negative (into ocean) flux.  This high frequency (30 min) and spatially averaging (560 m) method is well suited to coastal monitoring of ocean-air N₂O fluxes and complementary to ocean-side measurements of N₂O in complex circulation and microbial environments.  The possibility of other gas flux detection by this technique will be discussed.