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

Abstract

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 0/00 (1[sigma]) for [delta]¹⁵N[alpha] and [delta]¹⁵Nbulk, respectively, and among the best bulk composition precisions of 0.05 and 0.10 0/00 for [delta]¹⁵Nbulk and [delta]¹⁸O, respectively. The high-precision, non-continuous flask observations of N₂O ¹⁵N 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.