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Ethane: Sources and Sinks

Key Terminology:

Familiarity with the following terms will be helpful:

Trace gas - a gas present in the atmosphere in very small amounts.  N2 and O2 make up approximately 99% of the gas in the atmosphere, followed by water vapor, argon, and carbon dioxide.  Other gases such as ethane and methane are present in extremely small percentages, and are considered trace gases.
Parts per trillion by volume (pptv) - this is a common unit of concentration for extremely trace gases; ethane concentration is typically measured in pptv.
Parts per billion by volume (ppbv) - a common unit of concentration for other trace gases; methane concentration is typically measured in ppbv.
Mixing ratio - the moles of the gas of interest divided by the moles of all other gases.  For trace gases such as ethane, this is effectively the same as the mole fraction.
Teragram (TG) - 1012 grams.  Although ethane is a trace gas, at a global scale the amount released annually must be measured in teragrams.

Significance of ethane in the atmosphere:

After methane, ethane is the second most common hydrocarbon in the atmosphere.  Ethane is also significantly shorter-lived than methane; while a methane particle released into the atmosphere has an average lifetime of 9 years, ethane has an average lifetime of only 2 months.  Ethane is also important because it shares common sources and sinks with methane.

Examining the vertical mixing ratio of ethane in the atmosphere in the following figure, we can note that the mixing ratio is largest at the surface (where, we will see shortly, ethane sources are located) and decreases with elevation.  We can also note that mixing ratios appear significantly larger above 25oN than they do in the tropics and southern hemisphere (less than 25oN):

Sources of atmospheric ethane:
Ethane comes from a number of sources, but the primary sources is the venting and flaring of fossil fuels (note that this is not the combustion of fossil fuels; burning additional fossil fuels will not necessarily increase global ethane concentrations, while releasing unburned natural gas into the atmosphere will).  Relatively minor but still significant contributors include the burning of biomass; for example, forest fires cause the burning of significant biomass.  Biofuels are also a significant and increasing contributor to global ethane.  Geologic and oceanic microbic activity also contributes to the amount of ethane in the atmosphere, but due to the high degree of uncertainty concerning the magnitude of this contribution, this source is often not included in estimates of net emissions.  The following table summarizes the current annual emissions in TG/year of ethane by source:

  Emissions (TG/year)
Fossil Fuels 8.0-9.2
Biomass burning 2.4-2.8
Total 13

It is important to note that ethane emissions occur predominantly in the northern hemisphere because this is the location of most fossil fuel emissions. Ethane emissions in the southern hemisphere may be primarily attributed to biomass burning.

Sinks of atmospheric ethane:
The primary sink of ethane is its destruction via reacting with the hydroxide radical.  Through this reaction, it is one of many precursors to tropospheric (as opposed to stratospheric) ozone.  However, the magnitude of this sink has a seasonal and latitudinal variation because atmospheric concentrations of the hydroxide radical rise during summer months and in the tropics.  The detailed kinetics of these reactions are discussed more in the 'Atmospheric Kinetics - Photochemistry' section.  Briefly, the creation of the hydroxide radical requires the presence of incident radiation, oxygen, and water vapor.  As radiation and water vapor both increase in the tropics and during summer months, this leads to an increased concentration of hydroxide radical, and thus an increased rate of ethane removal.

Relationship Between Atmospheric Ethane and Methane:

Ethane and methane share a common primary source: the venting and flaring of natural gas.  Natural gas and oil appear together in oil fuels.  In the past, natural gas was typically considered less valuable than oil, and was simply vented to the atmosphere.  Today, that process is less common due to rising oil prices; however in areas that lack the infrastructure to capture natural gas it is still a common practice.  The magnitude of venting and flaring at a global scale may be estimated.  Although the ratio of methane and ethane varies by source these values are known and thus the relationship between released ethane and released methane may be determined.  Some sources of these materials vary; for example, ethane is also released by biomass burning and biofuels, which is not a significant source of methane.

Ethane and methane also share a common sink.  Both are destroyed by their reaction with the hydroxide radical.  However, the kinetics of the reaction with ethane are significantly more rapid thane with methane (ethane has an average lifespan of 2 months in the atmosphere, while methane has an average lifespan of about 9 years).  The result is that atmospheric concentrations of methane are significantly higher than those of ethane.

If you wish to learn about the composition of gases in the atmosphere, and specifically about the role of ethane in the atmosphere, here are a few additional resources you may want to consult:

Composition of the atmosphere, by gas: (more resources are linked to from here)

An older study into atmospheric ethane, especially examining seasonal variations: