Emissions of carbon dioxide (CO2) during the extraction and combustion of fossil fuels has contributed to major changes in the Earth’s atmosphere over the centuries since man realized its value as an energy source. Often included CO2 are benign gases such as helium (He) that can be used to track such emissions.
Researchers have long speculated that the amount 4He — an isotope of helium — in the atmosphere increases because it is found in the same reservoirs as natural gas and other hydrocarbons. But the measurements have so far been contradictory and imprecise. Now researchers have developed a new way of measuring the noble gas, which sheds light on the decades-old mystery.
“With our measurements, we can show it for the first time [the theory is] in fact, it is true that helium concentrations in the atmosphere are increasing, ”he said Benjamin Birneran atmospheric chemist and postdoctoral fellow at the Scripps Institution of Oceanography.
The new discovery may lead researchers to better identify sources of CO2 in the atmosphere, which could guide policies to reduce emissions. The increase in 4He also asks questions about its isotopic companions, 3He and a potential undiscovered reservoir of gas – a critical resource in some research and commercial industries.
Helium pairs with fossil fuels
Some minerals contain natural uranium and thorium. These radioactive elements decay to stable for millions of years and are released 4He in the process. Because 4He is a noble gas, it does not bind easily to other elements and slowly leaks out of its host crystal over time. Rogue helium in the earth’s crust seeps against the surface before escaping into the atmosphere.
In some cases, the rising gas gets stuck under an impenetrable rock. Natural gas, which escapes from buried source rocks, also rises through the lower surface and gets stuck together with helium. “If you have a geological environment that is suitable to contain [natural] gas, it is probably also appropriate to capture the helium, says Birner.
When humans come and extract the gas from these reservoirs, 4He is also liberated. With the increasing use of fossil fuels since the beginning of the industrial era, 4He should flood the atmosphere. And researchers have been looking for it. Unfortunately, conflicting data to date have confused all evidence of a long-term increase in atmospheric helium – some studies measured an increase, while others showed little or no change.
An exact 4He Measurement
Birner and colleagues developed a new way of counting 4He who prides himself on a precision that is higher than that achieved by any previous studies.
First they got samples. Due to the leaky nature of helium, air samples are difficult to store, and researchers have had to break creative sources into stale air. An earlier study extracted air from inside carburetors and sealed metal boule game balls. “[Helium] does not diffuse through metals. So you had to find some good metal boxes, ”he said Bernard Marty, a geochemist at the University of Lorraine who was not involved in the study. Birner and colleagues used gas stored in metal tanks sporadically collected by researchers for other experiments since the 1970s.
Then the group measured the change in the relationship between 4He and nitrogen (N2) through time. Nitrogen levels in the atmosphere remain relatively constant over the years; therefore, each change in the ratio of samples indicates a change in the amount of 4He. The researchers discovered a significant increase in 4He in air samples dating back to 1974 – two orders of magnitude more than expected from the earth’s natural processes, according to the study. The increase is also greater than the small amount released by commercial and research applications.
Because 4He can now be measured accurately and increases demonstrably, scientists can trace the origin of associated greenhouse gases such as carbon dioxide. 4He concentrations are highest in natural gas compared to other fossil fuels such as coal and petroleum. By measuring the amount of both 4He and coal in an air test, researchers hope to be able to determine how much of the total emissions come from natural gas combustion as opposed to cars or a coal-fired power plant, Birner said.
Surprisingly, scientists still have a lot to learn about the earth’s natural carbon dioxide emissions. Having an accurate way to track carbon with helium can help them determine how much is being pumped into the atmosphere by nature, Marty said.
“I think we’ll learn a lot more about how the world works from helium,” Birner said.
A 3He mystery
The new information determines the long-running debate about 4He in the atmosphere. “They are amazing measurements,” Marty said. But, he added, they pose an interesting problem.
Previous studies, including some by Marty and colleagues, examined the relationship between 3He to 4He in air samples to get to 4He concentrates on the atmosphere. 3He is a naturally occurring, stable isotope of helium. The most accurate 3He/4The available measurements have shown that the ratio is unchanged in the atmosphere over time. The fact that the researchers in this study independently observed an increase in 4He means it 3He must also increase.
3He is rare on earth; it is mainly released from a mantle reservoir that remains from the formation of our planet. It is also produced from the bombardment of cosmic radiation, solar wind and interstellar gases and in the manufacture of nuclear weapons. But none of these sources can account for the amount that enters the atmosphere. “The signal is about 10 times the geological flows, and we do not know how to explain the source of this further. 3Han, sa Birner.
3He is used in applications such as cryogenics, nuclear fuel and medical imaging. In recent decades, as the demand for world supply has increased, it has become a scarce resource. The view to an undiscovered source 3He is therefore exciting. “People have been thinking about flying to the moon to mine 3He there. That resource is so important, ”said Birner. “It will be even more important in the future because nuclear fusion reactors are theorized to run on 3He, he added.
—Jennifer Schmidt (@DrJenGEO), Science Writer
Quote: Schmidt, J. (2022), Fossil fuels drive the increase in atmospheric helium, Eos, 103, https://doi.org/10.1029/2022EO220270. Published June 9, 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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