Development of a Spectroscopic Technique for Continuous Online Monitoring of Oxygen and Site-Specific Nitrogen Isotopic Composition of Atmospheric Nitrous Oxide

Abstract

Nitrous oxide is an important greenhouse gas and ozone-depleting-substance. Its sources are diffuse and poorly characterized, complicating efforts to understand anthropogenic impacts and develop mitigation policies. Online, spectroscopic analysis of N[subscript 2]O isotopic composition can provide continuous measurements at high time resolution, giving new insight into N[subscript 2]O sources, sinks, and chemistry. We present a new preconcentration unit, “Stheno II”, coupled to a tunable infrared laser direct absorption spectroscopy (TILDAS) instrument, to measure ambient-level variations in [superscript 18]O and site-specific [superscript 15]N N[subscript 2]O isotopic composition at remote sites with a temporal resolution of <1 h. Trapping of N[subscript 2]O is quantitative up to a sample size of ~4 L, with an optimal sample size of 1200–1800 mL at a sampling frequency of 28 min. Line shape variations with the partial pressure of the major matrix gases N[subscript 2]/O[subscript 2] and CO[subscript 2] are measured, and show that characterization of both pressure broadening and Dicke narrowing is necessary for an optimal spectral fit. Partial pressure variations of CO[subscript 2] and bath gas result in a linear isotopic measurement offset of 2.6–6.0 ‰ mbar[superscript –1]. Comparison of IR MS and TILDAS measurements shows that the TILDAS technique is accurate and precise, and less susceptible to interferences than IR MS measurements. Two weeks of measurements of N[subscript 2]O isotopic composition from Cambridge, MA, in May 2013 are presented. The measurements show significant short-term variability in N[subscript 2]O isotopic composition larger than the measurement precision, in response to meteorological parameters such as atmospheric pressure and temperature.