Ocean-air N[subscript 2]O trace gas fluxes in Halifax Harbour derived from open-path FTIR measurements

Abstract

Open-Path Fourier Transform Infrared (OP-FTIR) spectroscopy is an established technique for path-average measurements of atmospheric trace gas concentrations. We describe the novel application of OP-FTIR to measure ocean-air gas fluxes across 586 m of Halifax Harbour (northwest Atlantic coast), also using a 3-D sonic anemometer and two vertically separated retroreflector arrays. We present spectroscopic retrievals of nitrous oxide (N₂O) concentration, micrometeorological data characteristics, and flux-gradient method calculations of ocean-air N₂O flux. From Dec. 2020 to Apr. 2021 the calculated ocean sink of −10.81 kg_N2Oha^−1 agreed broadly with a global model of thermal (solubility-driven) N₂O flux. A powerful short-term ocean source event observed on Dec. 15-17 (+2.00 kg_N2Oha^−1 ) correlated with high winds and strong ocean cooling that plausibly caused surface overturning and the upwelling of N₂O-rich deeper waters. The method presented is well suited to high frequency monitoring of coastal N₂O fluxes, necessary to resolve complex physical, chemical and biological processes.