Measurement and Modeling of Site-specific Nitrogen and Oxygen Isotopic Composition of Atmospheric Nitrous Oxide at Mace Head, Ireland

Abstract:

Global mixing ratios of atmospheric nitrous oxide (N₂O), a potent greenhouse gas, have increased nearly linearly from the beginning of the modern industrial period to today, with the current global average in excess of 325 ppb. This increase can be largely attributed to anthropogenic activity above the level of N₂O emissions from natural biotic sources. The effect of N₂O on Earth’s climate is twofold: in the troposphere, N₂O is radiatively active and chemically inert, while it serves as a reactive source of ozone-destroying nitrogen oxides in the stratosphere. The marked altitudinal divide in its reactivity means that all stages in the N₂O life cycle—emission, transport, and destruction—must be examined to understand the overall effect of N₂O on Earth’s climate. However, the understanding of the total impact of N₂O is incomplete, as there remain significant uncertainties in the global budget of this gas. Due to unique isotopic substitutions (¹⁵N and ¹⁸O) made by different N₂O sources and stratospheric chemical reactions, the measurement of N₂O isotopic ratios in ambient air can help identify the distribution and magnitude of distinct source types. We present the first year of site-specific nitrogen and oxygen isotopic composition data from the MIT Stheno-tunable infrared direct absorption spectroscopy (TILDAS) instrument at Mace Head, Ireland. Aided by the Stheno preconcentration system, Stheno-TILDAS can achieve measurement precisions of 0.10‰ or greater for all isotopic ratios (δ¹⁵N and δ¹⁸O) in ambient N₂O. We further compare these data to the results from Model for Ozone and Related Tracers version 4 (MOZART-4) simulations, including N₂O isotopic fractionation processes and MERRA/GEOS-5 reanalysis meteorological fields. These results will form the basis of future Bayesian inverse modeling simulations that will constrain global N₂O source, circulation, and sink dynamics better.