A team of atmospheric scientists researching pollution in China say they've cracked a 60-year-old mystery — with research that explains not only the haze over Beijing, but also the remarkably toxic Great Smog of London from 1952.

By examining conditions in China and experimenting in a lab, the scientists suggest that a combination of weather patterns and chemistry could have caused London's fog to turn into a haze of concentrated sulfuric acid.

In the middle of the last century, London was no stranger to oppressive pea-soup fogs laced with smoke from coal fires. But the smog that blanketed the city in December 1952 was unusually vicious: It lasted for five days and killed thousands of people. Official estimates put the death toll at 4,000, but the BBC reports research suggests the fatal fog might have killed as many as 12,000 people.

(The Great Smog was the topic of an episode in the new Netflix show The Crown. You can also hear the stories of survivors on PRI or read some of the contemporary coverage from The Telegraph.)

Research published in the Proceedings of the National Academy of Sciences this month argues that the deadly smog might have been the result of a chemical process that's also happening in China — albeit in a different way.

The scientists weren't setting out to explain the potency of the London fog, explains Renyi Zhang, a professor of atmospheric sciences at Texas A&M University and one of the lead authors of the paper.

They were trying to solve a different mystery: why some severe haze around Beijing has a higher concentration of sulfates than they could explain based on their normal models.

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Somehow, sulfur dioxide, a common byproduct of coal-burning, was getting turned into sulfate, very quickly.

"I was doing some work in my lab to see if I can reproduce some of the things my colleagues see in China," Zhang explains. "What we found out is that the traditional ways that people make sulfate didn't work," or worked too slowly.

They had another way to try to recreate the haze — combining nitrogen dioxide and sulfur dioxide in a humid environment, which could produce sulfates. Both chemicals are produced by burning coal, so it seemed like "a very natural candidate" to explain what was happening in China.

But the result was extremely acidic. And as soon as the particles got too acidic, the reaction stopped: "The product would basically drive the reaction backward," Zhang explains.

Here, there were two separate breakthroughs. One explained the situation in China: The team found that ammonia, present in the atmosphere because of agricultural activity, could neutralize the acidity without stopping the creation of sulfates.

"Now we understand what's going on [in China]," Zhang said. "So then we said, wait a minute: The London fog in 1952 ..."

They couldn't go back in time and take air samples, but they knew there was nitrogen dioxide and sulfur dioxide — because London then, like China today, was burning a lot of coal. And measurements at the time suggested the fog was highly acidic.

There's no indication there was any ammonia, like there is in China. So why didn't the acidity stop the reaction?

The researchers say that while Chinese haze is made up of tiny particles, the London smog was based on natural fog — water droplets that are, comparatively speaking, enormous.

That means the water would have diluted the acid enough to keep it from inhibiting the reaction. A little bit of acid could have quickly formed in each water droplet.

And then the sun rose, and it was time for the fog to burn off, like it did every morning.

"Now you have a mixture of sulfuric acid and water. All you evaporate is the water," Zhang said. "You make very concentrated sulfuric acid droplets."

In fact, that's a common industrial process for making concentrated sulfuric acid, he says.

As The New York Times reported in 2003, "a combination of factors may have made the smog so deadly." But Zhang's experiments suggest it's possible that one of those factors was a high degree of acidity.

Both in China today and in London then, the process would only happen under the right atmospheric conditions, Zhang says: "It's a combination of meteorology and chemistry."

But it could be happening more often than we think — and might be a contributing factor to acid rain, in addition to the widely understood pathways by which sulfur dioxide creates sulfuric acid.

And unfortunately, there might be another big difference between the haze in China and the smog in London.

"The 1952 London Fog led to the 1956 Clean Air Act in the U.K., and it literally solved the problem right away," Zhang said. (The BBC reports that by broad consensus, the act did have a transformative impact on public health.)

But you probably shouldn't hold your breath waiting for China to do the same thing.

"London fog only had to do with coal burning — it is relatively easy to solve the problem," Zhang says. "In Beijing or any other cities in China, you have coal burning, traffic emissions, agriculture. It's very complicated ... it's very, very difficult [to solve]."

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

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