Date of Award

Spring 1-1-2014

Document Type

Thesis

Degree Name

Master of Arts (MA)

Department

Ecology & Evolutionary Biology

First Advisor

Alan R. Townsend

Second Advisor

James W.C. White

Third Advisor

Nichole Barger

Abstract

Nitrous oxide continues to increase in the atmosphere mainly due to heightened microbial production from fertilized agricultural systems. Our ability to constrain the anthropogenic influence on N2O production at a global scale is mainly hampered by both the spatiotemporally heterogeneous emission of N2O from soil, as well as uncertainties regarding the transport dynamics of N2O between the stratosphere and troposphere. Monitoring the stable isotopic composition of N2O in air can help to better constrain N2O emission sources, since many pools of N2O exhibit distinctly different isotopic signatures. The intramolecular position of 15N (β position 15N14N16O versus α position 14N15N16O) can be used in conjunction with the total 15N isotopic composition of N2O (δ15Nbulk-N2O) to further elucidate source fluxes, as N2O produced from biological sources exhibits unique site-specific isotopic signatures that are independent of the substrate value. A subset of 19 sites from the NOAA/ESRL Cooperative Sampling Network was measured in order to describe the global distribution and seasonality of N2O isotopomers. Simultaneous and continuous measurement of N2O mole fraction, δ15Nbulk-N2O, δ15Nα-N2O, and δ15Nβ-N2O was conducted using the Picarro G5101-i wavelength-scanned cavity ring-down spectrometer (CRDS) coupled with a quantum cascade laser capable of the mid-infrared wavelength detection needed for N2O. A significant (p < 0.05) negative correlation between mixing ratio and δ15Nbulk is exhibited across 13 annual (spring 2013-2014) site means, which is consistent with a stronger N2O source signal in the northern mid-latitudes. While significant isotopic differences within and between sites are observed at most sites, long-term measurement uncertainties greater than 0.50/00, as well as the use of discrete versus continuous sampling, may limit our ability to detect tropospheric trends. Suggested quality assurance protocols including water removal and cavity temperature monitoring may help to reduce these uncertainties with future CRDS users.

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