Sponsor
Portland State University. Department of Physics
First Advisor
Andrew Rice
Term of Graduation
Fall 2025
Date of Publication
9-29-2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy (Ph.D.) in Applied Physics
Department
Physics
Language
English
Subjects
biomass burning, El Nino, ethane, fossil fuel
Physical Description
1 online resource (xii, 179 pages)
Abstract
Atmospheric non-methane hydrocarbons (NMHCs) are relevant to both short-term environmental and long-term climate changes. NMHCs are emitted from fossil fuels, and the burning of biomass and biofuels. The global annually averaged lifetime of C2 NMHCs ranges from two months for ethane (C2H6), to two days for ethylene (C2H4). C2H6 is the most abundant NMHC in the atmosphere due to annual emissions of roughly 10–20 Tg (1 Tg = 1012 g), and its relatively long atmospheric lifetime. Multi-decadal trends of atmospheric C2H6 have been used to understand changes in fugitive fossil fuel emissions and the resulting implications for methane (H4) emissions. Prior studies of C2H6 identified a correlation between global C2H6 mole fractions and CH4 growth rate. Since there are relatively few time-series measurements of atmospheric NMHC mole fractions prior to the 1990s, new measurements and analysis of historic trends are restricted to ice core, firn air, or archived air samples. Work presented here contains a three-part study to investigate historic NMHC trends. This study includes analysis of prior datasets, the development of an NMHC analytical system and its application to urban air, and the quantification and analysis of the OHSU-PSU Air Archive-a rare collection of air samples collected at remote locations from 1978-1997.
Analysis of prior datasets includes the presentation of previously unpublished whole air sample measurements of NMHCs collected at approximately weekly intervals from 1982-1987 at six stations spanning the Northern and Southern Hemispheres. Samples originated from remote sites that primarily received Pacific Basin air masses, as determined by back-trajectory analysis. The four alkanes in this dataset, C2H6, C3H8, i- and n-butane (C2H4) had a mean measured mole fraction of (712 ± 22) ppt, (245 ± 13) ppt, (55.2 ± 4.4) ppt, and (87.6 ± 8.7) ppt, respectively. No discernible global secular trend for these NMHCs was found over this five-year period, but significant trends at some individual sites may have been influenced by fugitive fossil emissions in the Northern Hemisphere. A large multi-year enrichment anomaly occurs at all sites that is likely associated with the strong El Niño of 1982-1983, and appears to influence trends observed in the Southern Hemisphere. A comparison of measured and GEOS-Chem simulated C2H6 mole fractions estimated global emissions of (13.4 ± 0.4) Tg/yr. Despite interannual variability, the average global mole fraction determined from these measurements during the five-year span corroborates a multi-decadal decline in NMHCs from the 1980s into the early 2000s, when compared with recent datasets.
The development of an NMHC analytical system was part of a pilot study that included hourly mole fractions of GHGs: CO2, H4, N2O, and select NMHCs: C2H6, C2H4, C3H8, i-C4H10, and n-C4H10. Measurements took place in Portland, Oregon (OR), USA in the summer of 2024. Portland, OR is a medium sized urban center, located approximately 100 km from the Pacific Coast. Analytical systems were fully automated: CO2 was measured using a non-dispersive infrared gas analyzer; H4 and NMHCs were measured using gas chromatographs (GC) equipped with a flame ionization detector; and N2O was measured with a GC equipped with an electron capture detector. The NMHC system used a custom front-end pre-concentration system. NMHCs showed high variability, with C2H6 varying from 1.9 ppb at the beginning of summer to 4.1 ppb by the end of September. Each species had a pronounced diurnal cycle, generally reaching a maximum during 6 AM, and a minimum during 4 PM. Various time periods showed strong enhancements that were most often related to wildfire emissions transported by easterly wind trajectories. A comparison of the atmospheric H4 to C2H6 ratio (MER) outside wildfire events to the MER of regional natural gas samples indicated (37.8 ± 3.2) % of CH4 enhancement in Portland during the summer of 2024 was attributable to natural gas.
The final part of this work includes C2-C4 NMHC mole fraction measurements from the 1978–1997 OHSU-PSU Air Archive, including a rare collection of 154 whole air sampled at Cape Meares, OR, USA (CMO). NMHC mole fraction trends complement previous studies of the CMO archive samples which detailed mole fraction and isotopic composition of H4. Long-term trends of C2H6, C2H4, C3H8, and C4H10 were more variable during the 1980s and relatively stable during the 1990s. Secular trends of each species measured showed a multi-decadal decline of (1.9 ± 0.3) % per year for C2H6, (2.9 ± 0.5) % for C3H8, and approximately 6.7% per year for C2H4 and C4H10. Comparison of overlapping mole fraction measurements from this study with those from the Oregon Graduate Institute (OGI) suggests a 21.5% increase to OGI measured C2H6 to conform with contemporary calibration. Applying this calibration correction to OGI C2H6 increases its previous emission estimates from 13.4 Tg/yr to (16.3 ± 0.9) Tg/yr for 1982-1987.
Rights
© 2025 Alex Martin Smith
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Persistent Identifier
https://archives.pdx.edu/ds/psu/44348
Recommended Citation
Smith, Alex Martin, "A Quantitative and Analytical Study of Light Atmospheric Hydrocarbons in Remote and Urban Environments" (2025). Dissertations and Theses. Paper 6977.