Nitrous oxide (N2O) isotopic composition in the troposphere: instrumentation, observations at Mace Head, Ireland, and regional modeling

Potter, K.
CGCS Report Series, Ph.D. Thesis, Department of Earth, Atmospheric and Planetary Sciences, MIT, 179 pages, Report Nr. 82

Nitrous oxide (N2O) is a significant greenhouse gas and main contributor to stratospheric ozone destruction. Surface measurements of N2O mole fractions have been used to attribute source and sink strengths, but large uncertainties remain. Stable isotopic ratios of N2O (here considered 14N15N16O, 15N14N16O, 14N14N18O, relative to the abundant 14N14N16O) linked to source and sink isotopic signatures can provide additional constraints on emissions and counter-balancing stratospheric sink. However, the isotopic composition in the troposphere has been regarded and measured as a fixed value, limited by insufficient measurement precision and few data.

This thesis provides the foundation for high-frequency, high-precision measurements and utilization of N2O tropospheric isotopic composition. This is achieved through the development of a new measuring capability with sufficient precision to detect the subtle signals of N2O isotopic composition in tropospheric air and uniquely fully-automated and high-frequency capable. This instrument was applied to produce the first set of tropospheric air observations gathered at a remote research station covering a full annual cycle, paired with air origin information, and providing a valuable assessment of tropospheric composition and its potential utility. The first regional model of tropospheric N2O isotopic composition was developed for further assessment of expected variability and utility of isotopic composition data.

The optimized fully-automated, liquid-cryogen-free pre-concentration device coupled to continuous flow IRMS resulted in 15N site-specific precisions markedly improved over other systems of 0.11 and 0.14‰ (1σ) for δ15Nα and δ15Nβ, respectively, and among the best bulk composition precisions of 0.05 and 0.10‰ for δ15Nbulk and δ18O, respectively. The high-precision, non-continuous flask observations of N2O15N site-specific composition (January 2010 to January 2011; Mace Head Atmospheric Research Station, Ireland) detected statistically significant signals on short-term and annual timescales, and when analyzed with air history information showed consistencies with source-receptor relationships. No seasonal cycle could be detected in the low-frequency observations, but regional model scenarios of the stratospheric seasonal signal resulted in amplitudes at the cusp of current measurement capabilities.

This thesis illustrated detectable variations in tropospheric N2O isotopic composition which can potentially reduce uncertainty in the N2O budget with high-frequency, high-precision observations, now feasible by the instrumentation developed here.

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