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

• Main Authors: Harris, Eliza (Contributor), Nelson, David D. (Author), Zahniser, Mark (Author), McManus, Barry J. (Author), Prinn, Ronald G. (Contributor), Ono, Shuhei (Contributor), Olszewski, William J. (Contributor), Potter, Katherine Ellison (Contributor), Whitehill, Andrew Richard (Contributor)
• Other Authors: Massachusetts Institute of Technology. Center for Global Change Science (Contributor), Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences (Contributor)
• Format: Article
• Language: English
• Published: American Chemical Society (ACS), 2014-12-22T17:27:28Z.
• Subjects: Article
• Online Access: Get fulltext

 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.