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Ethane and propane help scientists understand the greenhouse gas methane

Kullkraftverk, Polen
Foto: Colourbox

A new study in Nature Geoscience unveils a need of revising previous ethane and propane emissions studies, as these emissions have been underestimated by more than 50%. Such revision could in turn improve our understanding of the forceful and related methane emissions, still largely enigmatic to atmospheric sciences.

Ethane and propane are the most abundant non-methane hydrocarbons in the atmosphere, and yet their emissions, distribution in the atmosphere, and trends in their atmospheric concentrations are insufficiently understood. Through chemical reactions in the atmosphere, ethane and propane affect the formation and loss of several air pollutants and greenhouse gases.

Comparing models and observations

– Atmospheric model studies underestimate the observed ethane and propane concentrations in the Northern Hemisphere, suggesting a lack of scientific understanding of the budget of these gases, said Stig Dalsøren, a former researcher at CICERO Center for International Climate Research, now at the Institute of Marine Research, and first author of the study.

Direct emissions at the surface are the only sources of ethane and propane to the atmosphere.

Fossilt etan som slippes ut fra ulike kilder ved lavere breddegrader, transporteres til Arktis og fanges med høy nøyaktighet og tidsoppløsning opp ved Zeppelinobservatoriet.
Fossile ethane emitted from various sources at lower latitude are transported to the Arctic and detected at Zeppelin with high accuracy and time resolution Photo: Emma Lee/WHYY

– In this new study published in Nature Geoscience we compare recent atmospheric measurement data from a number of sites with the simulations from an atmospheric chemistry transport model, and we show that a substantial upward revision of the emissions is needed, says co-author Cathrine Lund Myhre at NILU. She emphasizes that there has been large progress recently in the access to quality assured measurements of these compounds with high time resolution, particularly through the research infrastructure ACTRIS (https://www.actris.eu/).

Must include natural emissions

This ethane study was initiated after a flight and ship campaign outside Svalbard summer 2014 focusing on methane. This was a part of the MOCA project lead by Lund Myhre at NILU.

– In addition to study methane, we decided also to try to measure ethane and potential emissions from the ocean. We observed elevated mixing ratios of ethane, just above the sea surface at RV Helmer Hanssen west of Svalbard, she says. – Based on this we decided also to explore the ethane budget in more detail, and emissions from the ocean and geological sources.

Major detailed atmospheric model studies of ethane performed so far have neglected natural geologic emissions of hydrocarbons. Natural geologic emissions include mud volcanoes, gas seeps, diffuse exhalation from petroleum basins, submarine seeps, geothermal manifestations and volcanoes.

– We show that substantial geologic emissions are necessary to reproduce observations of preindustrial ethane concentrations. This provides an important constraint on both preindustrial and current natural emission budgets, says Stig Dalsøren.

New, more extensive datasets

For man-made sources, this study is the first to use recently published fossil fuel (oil, natural gas and coal) emission datasets.

– Compared to previous inventories, these new datasets are based on novel approaches, and more extensive databases considering country-specific circumstances, resulting in highly different geographical emission distributions and 2-3 times higher global total emissions, said co-author Gunnar Myhre, research director at CICERO.

Using the new fossil fuel emission datasets and adding the geologic emissions, the applied atmospheric model reproduces observed current ethane and propane levels in the Northern Hemisphere, including episodic fluctuations. This also results in substantially higher simulated surface ozone in some polluted regions in Asia.

– The improved correspondence with observed ethane and propane at numerous sites with model simulations using greater emissions suggests that the level of fossil (natural geologic + fossil fuel) methane emissions in current inventories may need re-evaluation, said Lund Myhre.

Understanding the contribution from different natural and anthropogenic emission sources is a critical precursor to design efficient measures to reverse ongoing atmospheric ethane, propane, and methane increases.

Etan som slippes ut fra ulike kilder ved lavere breddegrader, transporteres til Arktis og fanges med høy nøyaktighet og tidsoppløsning opp ved Zeppelinobservatoriet på bildet.
Ethane emitted from various sources at lower latitude are transported to the Arctic and detected at Zeppelin with high accuracy and time resolution Photo, Kathy Thompson/AGAGE
Ethane budget and the relevance for understanding of methane in the atmosphere

Ethane and propane share several of their most important emission sources with methane, their more famous brother in the hydrocarbon family. Methane is the second most important greenhouse gas and there is a strong upward trend in atmospheric concentration over the last decade. The cause of the trend is uncertain, but likely due to increases
in the emissions, though not clear which emissions.

Understanding the trend and methane’s role as a greenhouse gas is an important issue in climate science. The atmospheric budget of methane is therefore heavily studied, which has resulted in quite certain quantification of total emissions to the atmosphere. However, the contributions of individual sources (fossil fuel, waste, ruminants, wetlands, etc.) and the man-made versus natural footprint is quite uncertain since complex processes drive the emissions.

Due to a long atmospheric lifetime (~12 years), methane is quite well-mixed in the atmosphere and it is therefore hard to distinguish the emission sources and regions responsible for observed atmospheric concentrations. This is easier for the shorter-lived ethane (months) and propane (weeks). For sources methane, ethane and propane have in common, better quantification of ethane and propane emissions could also improve the understanding of methane emissions.

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Read the article here:

Dalsøren, S. B., Myhre, G.,
Hodnebrog, Ø., Myhre, C. L., Stohl, A., Pisso, I., Schwietzke, S.,
Höglund-Isaksson, L., Helmig, D., Reimann, S., Sauvage, S., Schmidbauer, N.,
Read, K. A., Carpenter, L. J., Lewis, A. C., Punjabi, S., & Wallasch, M.
(2018). Discrepancy
between simulated and observed ethane and propane levels explained by
underestimated fossil emissions
. Nature Geoscience. Advance online
doi: 10.1038/s41561-018-0073-0

Read also:

Ethane and propane emissions have been underestimated (news from CICERO)

Hodnebrog, Ø., Dalsøren, S. B., & Myhre, G. (2018). Lifetimes, direct and indirect radiative forcing, and global warming potentials of ethane (C2H6), propane (C3H8), and butane (C4H10). Atmos. Sci. Lett. 2018;19:e804.