Scientists identify missing source of atmospheric carbonyl sulfide

Study provides better understanding of plants’ carbon uptake

Tokyo Institute of Technology

IMAGE: By using sulfur isotope distributions as new constraints on the atmospheric carbonyl sulfide budget, the study revealed that anthropogenic sources are likely to be more important than previously thought. view more  Credit: Mindy Takamiya (

Researchers at Tokyo Institute of Technology (Tokyo Tech) report that anthropogenic sources of carbonyl sulfide (OCS), not just oceanic sources, account for much of the missing source of OCS in the atmosphere. Their findings provide better context for estimates of global photosynthesis (taking up CO2) using OCS dynamics.

Carbonyl sulfide (OCS) is the most stable and abundant sulfur-containing gas in the atmosphere. It is derived from both natural and anthropogenic sources and is of key interest to scientists investigating how much carbon dioxide (CO2) plants take out of the atmosphere for photosynthesis. Measuring CO2 alone cannot provide estimates of photosynthesis (taking up CO2) because plants also release CO2 through respiration. In contrast, OCS is taken up like CO2 but is not released by respiration, and can therefore provide valuable information about the rate of global photosynthesis.

Understanding the precise OCS budget (the balance of source and sink) is an ongoing challenge. The most critical point of uncertainty related to the OCS budget is its missing source. Lack of observational evidence has so far led to debate about whether the missing OCS source is oceanic or anthropogenic emission.

In a new study published in Proceedings of the National Academy of Sciences of the United states of America (PNAS), researchers from Tokyo Tech’s School of Materials and Chemical Technology and Earth-Life Science Institute (ELSI) have used a unique method of measuring sulfur isotope ratios (minor 34S isotope abundance relative to major isotope 32S, 34S/32S) of OCS that enabled them to distinguish oceanic and anthropogenic OCS sources.

“It’s very exciting that we were able to separate anthropogenic and oceanic signals for OCS sources based on sulfur isotope ratios,” says Shohei Hattori, an assistant professor at Tokyo Tech and lead author of the study. “These measurements required at least 200 liters of air for each sample measurement. We overcame this challenge by developing a new sampling system, and eventually succeeded in measuring sulfur isotope ratios of the atmospheric OCS.”

The team found a north–south latitudinal gradient in the 34S isotope abundance corresponding to OCS concentrations during wintertime in eastern Asia. Their results provide evidence of the importance of anthropogenic OCS emissions from China. Also, by using the sulfur isotope level of OCS as a new constraint, they found that anthropogenic OCS sources, and not only oceanic sources, are likely to be major constituents of the missing source of atmospheric OCS.

“The higher relevance of anthropogenic OCS at mid-to-low latitudes has implications for understanding climate change and stratospheric chemistry in both past and future contexts,” says co-author Kazuki Kamezaki.

Given that the historical estimation of how much CO2 is taken up by plants is sensitive to the estimate of the anthropogenic OCS inventory, a more detailed picture of the OCS budget revealed by sulfur isotopic approach will enable more precise estimation of its interactions with global change. The research team will continue to undertake more observations to make detailed quantitative estimates and predictions of the global photosynthesis rate.

“Our sulfur isotopic approach for measuring atmospheric OCS is an important step, but more observations, together with analysis using a chemical transport model, will enable detailed quantitative conclusions,” Hattori says.

From EurekAlert!

34 thoughts on “Scientists identify missing source of atmospheric carbonyl sulfide

  1. You mean they used actual data and not a model? Amazing.

    But I thought the science was settled. Hmmm…

    • They know too little for a model. Oops .. wrong, you never know too little for a model.

  2. I’ve long been amused by the acronym for that fine institution.

    At one time there was a bombed out wind tunnel just inside its gates.

    • From what I’ve been able to find, it looks like the absorption bands are in the high UV range.

    • Wolf, I know what you mean. The main thing thing I could not understand from the article is why the source of this trace gas matters.

      There seems to be an underlying implication that it is a wildly active “greenhouse” gas – like Sulphur Hexafluoride – and so if it is predominately man-made then we need to do something about it NOW ! (and start by giving us boys at T.I.T. a huge grant so we can do some more research).

      Yeah, unless and until they spell out WHY it’s important to know anything about the source of this gas, then we should simply say “how interesting” and turn the page.

  3. It looks like to me that they have jumped to their conclusions that what they observed was “anthropogenic and coming from China” on very limited data. What is the natural latitudinal variability of the isotope coming from the oceans? There is a lot more biological activity in the oceans than there is on land and the level of that activity varies with latitude.

    • I question the reasoning of blaming a signatory of the Paris Accords that isn’t required to do anything to be fulfilled.

  4. China again? No more Chinese food for me. By the way, the atmospheric content of carbonyl sulfide is 0.05 ppb (that’s 5 parts in every 100 billion). Utilizing sulfur isotopes is more complicated than suggested by the article, as only the S 34/32 ratio is measured, and any of several differing sources of the two sulfur isotopes can influence the results. The reference ratio for S 34/32, set as 0 per mil, is the Canyon Diablo meteorite, which was found in Arizona.

  5. Carbonyl sulphide is broken up in the presence of humidity and basic pH into H2S and CO2 (oceans are basic). In the stratosphere (presumably from volcanoes) it breaks down into sulphuric acid and CO2. Likely the H2S of the first reaction also ultimately oxidizes to sulphuric as well.

    At half a ppb or less, I wonder how they sort all this out in an air sample of only 1 litre which contains other oxygen and carbon and sulphur phases? Also, chemically, disperse phases are more reactive. Maybe the “missing” carbonyl sulphide has reacted itself out of existence. Possibly this factor could account for the “gradient” they discovered.

    • Gary, I was just going to mention this. COS is not stable. In the presence of water it moves towards CO2 and H2S. It also goes the other way (chemistry is funny that way). We were conducting an experiment for an oil company and the mixture included CO2 and H2S and water; as many reservoirs do. A new peak started to show up on the GC. It grew bigger every day. The boss chuckled and showed us the reaction. We were making COS.

  6. The 0.5ppb might even be the equilibrium amount from the dynamics of emissions and chemical breakdown.

    • Sulphur is abundantly taken up by the roots of plants, too. It forms a variety of compounds and is categorized as a fertilizer by agronomists (it is adjacent to Phosphorus in the periodic table of elements and is diagonally below Nitrogen, another major fertilizer element). Sulphur is also metabolized by plants.

      The details of plant sulphur uptake and metabolization is not fully understood as yet.

      “Nevertheless, the relationship with photosynthesis has never been tested experimentally and thus it is presently uncertain as to whether S assimilation can be impacted by environmental conditions that affect photosynthetic metabolism”

      I judge this paper to be overly confident about its subject, more or less in the same way that climate science glosses over its universe of unknowns.

      • Garlic and other members of the lily family are known to be heavy sulphur feeders.
        I wonder why?

      • Simple-minded regulators made coal-fired power stations in the USA remove the SO2 from the emissions, and it was no longer being deposited downwind. As a result, farmers now have to buy fertilizer with S in it to compensate.

    • Again, reread the article Gary. You are out by a factor of 100 with your OCS concentration in the atmosphere.

  7. researchers from Tokyo Tech’s School of Materials and Chemical Technology and Earth-Life Science Institute (ELSI) have used a unique method of measuring sulfur isotope ratios (minor 34S isotope abundance relative to major isotope 32S, 34S/32S) of OCS that enabled them to distinguish oceanic and anthropogenic OCS sources.

    Ok, so would it be too much to ask to actually tell us what that unique method is?

    • It is in the paper not the press release. They have a novel method for concentrating OCS and then
      use standard mass spectrometery. They also describe it in detail in a previous paper.

  8. From wiki

    Carbonyl sulfide is the most abundant sulfur compound naturally present in the atmosphere, at 0.5 ± 0.05 ppb, because it is emitted from oceans, volcanoes and deep sea vents. As such, it is a significant compound in the global sulfur cycle.

    [my bold]

    Volcanoes. Humm, maybe an unreliable source of OCS but what is the isotope mix?

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