Measurements of N₂O and SF₆ Mole Fraction and N₂O Isotopic Composition between 1978 and 1997 in Archived Air Samples from Cape Meares, Oregon

Author

Terry Clinton Rolfe, Portland State University

Sponsor

Portland State University. Department of Physics

First Advisor

Andrew L. Rice

Date of Publication

Summer 8-8-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Applied Physics

Department

Physics

Language

English

Subjects

Nitrous oxide -- Oregon -- Cape Meares -- Measurement, Sulfur hexafluoride -- Oregon -- Cape Meares -- Measurement

DOI

10.15760/etd.7003

Physical Description

1 online resource (xi, 160 pages)

Abstract

Nitrous oxide (N₂O) is the third most important greenhouse gas (GHG) behind carbon dioxide (CO₂) and methane (CH₄). Sulfur hexafluoride (SF₆) does not add significantly to climate forcing by itself due to the low concentration in the atmosphere; however, it is one of the most powerful GHG known. Measurements of atmospheric N₂O made prior to mid-1990 have larger uncertainties than later periods due to advancements made in gas chromatography (GC) methods. Few atmospheric SF₆ measurements pre-1990 exist, especially in the northern hemisphere. Archived samples may be analyzed using updated measurement techniques to reduce uncertainty in past periods. Additionally, measurements of the isotopic composition of N₂O (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) can address questions regarding how specific sources contribute to the observed atmospheric composition and changes in time. This information also has been used to identify changing contributions of nitrification versus denitrification processes to the global N₂O budget, determined by the rate of change in the site preference (SP, defined as δ¹⁵N^α - δ¹⁵N^ß) of ¹⁵N. Here, we present the findings of 159 measurements of N₂O and SF₆ mixing ratio and N₂O isotopic composition from the OHSU-PSU air archive, containing samples collected from Cape Meares, Oregon between 1978 and 1997.

Based on these analyses, N₂O mole fraction at Cape Meares in 1980 is found to be 301.5 ppb and rises to 313.5 ppb in 1996. The average growth rate over this period is 0.78 ± 0.03 ppb yr^-1 (95% CI). Seasonal amplitude maximum and minimum is 0.34 ppb near April and -0.42 ppb near November respectively and are statistically different from one another (p<0.005) Measurements of N₂O were found to match well with previously reported values for Cape Meares from AGAGE and other comparable locations, suggesting that the N₂O in archived samples has stored well.

For SF₆, the mole fraction in 1980 was found to be 1.06 ± 0.05 ppt and increased to 3.91 ± 0.07 ppt in 1996. The average growth rate over this period is 0.17 ± 0.01 ppt yr^-1 (95% CI). Seasonality shows peak amplitude of 0.04 ppb near January and minimum amplitude of -0.03 ppt near July. There are no previous reported measurements of SF₆ from Cape Meares to compare against directly; however, comparisons against archive measurements from Cape Grim, Tasmania suggest these results are accurate.

Isotopic composition measurements of N₂O are made by cryogenically concentrating N₂O before analysis using an isotope ratio mass spectrometer operated in continuous flow (CF-IRMS). From replicate analysis of the working standard, precision in measurement of the measured isotopologues is 0.05‰, 0.10‰, and 0.28‰ for δ⁴⁵, δ⁴⁶, and δ³¹, respectively. When calculating the desired isotopic composition, these translate to 0.05‰, 0.10‰, 0.37‰ and 0.39‰ for δ¹⁵N_bulk, δ¹⁸O, δ¹⁵N^α, and δ¹⁵N^ß, respectively.

For archived samples from Cape Meares, no distinguishable seasonality is found in δ¹⁵N or δ¹⁸O while δ¹⁵N^α, δ¹⁵N^ß, and SP show statistically significant amplitudes. δ¹⁵N^α and δ¹⁵N^ß show nearly opposite phases to one another, with SP matching the phase of δ¹⁵N^α. These results suggest processes that contribute air enriched in N₂O mole fraction at Cape Meares in the spring also contribute enriched δ¹⁵N^α and depleted δ¹⁵N^ß, causing a positive SP. During the fall, processes that contribute air depleted in N₂O mole fraction also contribute depleted δ¹⁵N^α and enriched δ¹⁵N^ß, causing a negative SP.

Secular trends (except δ¹⁵N^ß) calculated by applying a linear fit to the deseasonalized data show negative trends statistically significant at high levels of confidence. Secular trends for δ¹⁵N and δ¹⁸O match well with previously reported values while secular trends for δ¹⁵N^α and δ¹⁵N^ß for Cape Meares are significantly different than those reported by other groups, appearing to be nearly inverted for δ¹⁵N^α and δ¹⁵N^ß. To address this inversion, the sensitivity of the numerically calculated δ¹⁵N^α and δ¹⁵N^ß on the scrambling coefficient was investigated and ruled out. We also investigated the possibility of an error in the numerical algorithm used to convert measured ⁴⁵R, ⁴⁶R, and ³¹R values into δ¹⁵N, δ¹⁸O, δ¹⁵N^α, δ¹⁵N^ß and found this too was not responsible for the inverted results.

A 2-box model of the atmosphere was used to investigate changes in measured atmospheric composition to characterize source isotopic composition. From the results of the box model, the magnitude of the pre-industrial natural source match well with previous literature. The isotopic compositions of the natural source match well with previously reported values, as do the modeled anthropogenic δ¹⁵N, δ¹⁸O, and δ¹⁵N^α. However, our modeled δ¹⁵N^ß is significantly enriched compared with previously reported values. Additionally, the modeled anthropogenic SP is significantly depleted than previously reported values. Assuming the laboratory measurements of intramolecular SP are globally relevant, these results suggest there have not been significant changes to the balance between contributions from nitrification and denitrification to the observed isotopic composition of N₂O during this period (1978 -- 1996). However, a more sophisticated model maybe needed to investigate this hypothesis.

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Persistent Identifier

https://archives.pdx.edu/ds/psu/29602

Recommended Citation

Rolfe, Terry Clinton, "Measurements of N₂O and SF₆ Mole Fraction and N₂O Isotopic Composition between 1978 and 1997 in Archived Air Samples from Cape Meares, Oregon" (2019). Dissertations and Theses. Paper 5124.
https://doi.org/10.15760/etd.7003