Previous studies have underestimated global ethane and propane emissions by more than 50 percent, a new study in Nature Geoscience led by CICERO researchers shows. A better understanding of these emissions could also improve our understanding of 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.
“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.
“In this new study published in Nature Geoscience we compare the findings from simulations with an atmospheric chemistry transport model to measurement data and show that a substantial upward revision of emissions is needed”, Dalsøren said.
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.
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Understanding the trend and methane’s role as a greenhouse gas is an important issue in climate science. The atmospheric budget 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.
Major detailed atmospheric model studies 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”, Stig Dalsøren said.
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 greater emissions and highly different geographical emission distributions. The new fossil fuel emissions are about 2 (ethane) and 3 (propane) times higher than those in the dataset used in work for the next IPCC report”, 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. Chemical reactions between the hydrocarbons and nitrogen oxide emissions from cars and power plants form ozone and other air pollutants. High surface ozone levels are linked to increases in human mortality and damage to vegetation and crops.
“The improved correspondence with observed ethane and propane in model simulations with greater emissions suggests that the level of fossil (natural geologic + fossil fuel) methane emissions in current inventories may need re-evaluation”, said Gunnar 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.
- Dalsøren, Stig B. et al. (2018). "Discrepancy between simulated and observed ethane and propane levels explained by underestimated fossil emissions", Nature Geoscience.