Sulfur dioxide pollution can come from a variety of sources, both natural and industrial, including volcanoes, oil refineries and the burning of fossil fuels. Although it has a relatively short lifespan in the atmosphere — a few hours to a few days — it’s important for scientists to keep track of its presence to help inform air quality and climate models and create pollution-cutting policies.

Until now, scientists have mostly relied on emissions inventories drawing on national reports to identify the world’s sulfur dioxide sources and the amount of pollution they’re putting out. Satellite information has been able to help scientists further quantify sulfur dioxide emissions — but this method has mostly been useful when the scientists already know where the emissions are coming from. That’s because winds can help obscure sulfur dioxide hotspots, making it difficult to pick them out if their location isn’t already known.

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But in the new study, researchers from Canada’s environment and climate change department and other institutions in the United States and Canada have described a new method that allows them to identify and map sulfur dioxide sources all over the world — including sources that may not have been previously identified or reported. And they’ve found that anywhere from 7 to 14 million metric tons of sulfur dioxide may be missing from global inventories each year.

The new method combines satellite data with wind information to more accurately pinpoint pollution sources.

“For each satellite measurement, we would know wind speed and direction and thus, using both pieces of information together, we can actually detect the location of where the sulfur dioxide is coming from,” said Chris McLinden, a research scientist with Environment and Climate Change Canada and the new study’s lead author. “And so we were able to essentially do a global search for these sources.”

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The researchers then compared their results with three major global emissions inventories. Overall, they identified nearly 500 major sources of sulfur dioxide emissions, 75 of which were volcanic, and therefore natural, sources. Out of the remaining, human-caused sources, 39 could not be matched to any of the sources already identified in the inventories.

Of these 39 “missing” sources, there was a significant cluster in the Middle East. Fourteen were located there, and 12 of those corresponded with oil and gas operations, such as oil refineries. The rest of the missing locations were spread across the world, including Asia, Africa, Europe and the Americas, and corresponded largely with power plants.

“Generally, these previously unreported sources tended to pop up in more developing types of nations where perhaps their legal requirements for reporting are not as rigorous as what we might be used to in the U.S and Canada, for example,” McLinden said.

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Altogether, emissions from these 39 missing sources came to about 7 million metric tons annually, or approximately 6 percent of the total amount of sulfur dioxide produced by humans. However, the researchers noted that their method was only able to capture about half of the known anthropogenic sulfur dioxide sources in the world, owing to limitations in satellite technology — meaning that there may actually be twice as many missing sources out there as the method detected this time around.

“[The satellites] have a relatively coarse spatial resolution — it’s not like these instruments that can picture the head of a dime from space,” McLinden said. “It measures maybe 10 kilometers at a time, 20 kilometers at a time. So you can imagine a very small source that doesn’t emit that much sulfur dioxide would go unnoticed.”

Extrapolating from the 50 percent of known sources that the method was able to capture, then, the researchers suggested that there may be up to 14 million metric tons of missing sulfur dioxide emissions in the world.

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In addition to pinpointing sulfur dioxide sources, the authors suggest that the new method may also have the potential to be used for identifying other types of emissions, such as nitrogen oxides. “I wouldn’t go so far to say it’s useful for all pollutions, but I would say the ones that have a fairly short lifetime or residence time in the atmosphere, it should work quite well,” McLinden said. And he added that one of the method’s most useful features is that it’s “independent of political boundaries and source types.”

Identifying these missing pollution sources — both sulfur dioxide and other types of emissions — is especially important for informing global air quality models, McLinden said, which can be used for everything from issuing health advisories to writing pollution and climate policies. The study, then, presents the alarming idea that the models to date have not been informed with the best and most accurate information — and highlights the need both for better monitoring and reporting of pollution sources and the continued improvement of the kind of satellite technology that might aid in those endeavors.

“When you have a large missing source somewhere, then you’re not accounting for that pollution and you’re providing a biased view of what the pollution’s going to look like in the future,” McLinden said.

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Correction: A previous version of this article stated that sulfur dioxide emissions may exacerbate global warming. In fact, they typically lead to a temporary reduction in the effects of global warming.

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