It may seem odd for scientists to be researching an atmospheric event from 64 years ago when there are plenty of modern issues to grapple with, but sometimes lessons from the past can assist with our current crop of problems. That’s the case with new research from Texas A&M University: not only have they gotten to the bottom of a decades-old historical problem, but the results could give valuable insights in tackling pollution in the world’s most heavily polluted towns and cities.

The event in question is the “killer fog” that hit London for several days in 1952. First appearing on 5 December, it lifted four days later with an estimated death toll of at least 12,000 people, with around 150,000 hospitalisations and thousands of undocumented animal deaths. Although the Clean Air Act was passed partly in response four years later, the actual causes of the incident have gone unknown until now.

Through atmospheric measurements of fog in China, and laboratory experiments, Texas A&M researcher Dr Renyi Zhang and his team came up with their answer: nitrogen dioxide converting comparatively benign sulfur dioxide into lethal sulfuric acid.

“People have known that sulfate was a big contributor to the fog, and sulfuric acid particles were formed from sulfur dioxide released by coal burning for residential use and power plants, and other means,” explained Zhang. “But how sulfur dioxide was turned into sulfuric acid was unclear.”

READ NEXT: What smart cities of the future can learn from London in 1854

“Our results showed that this process was facilitated by nitrogen dioxide, another co-product of coal burning, and occurred initially on natural fog. Another key aspect in the conversion of sulfur dioxide to sulfate is that it produces acidic particles, which subsequently inhibits this process. Natural fog contained larger particles of several tens of micrometres in size, and the acid formed was sufficiently diluted. Evaporation of those fog particles then left smaller acidic haze particles that covered the city.”

Similar things are happening in China’s most polluted cities right now. The reason there hasn’t been an incident like London’s “killer fog” is down to chemical happenstance: China uses a lot of fertiliser and that, combined with the heavy road traffic, results in high levels of ammonia, which neutralises the particles. “The right chemical processes have to interplay for the deadly haze to occur in China,” explained Zhang. “While the London fog was highly acidic, contemporary Chinese haze is basically neutral.”

Which isn’t to say it’s exactly healthy, but compared to those four days in London in 1952, it’s certainly preferable.

Zhang hopes that understanding what caused the killer fog in London can help other cities get to grips with their air-pollution problems in the long run. “The government has pledged to do all it can to reduce emissions going forward, but it will take time. We think we have helped solve the 1952 London fog mystery and also have given China some ideas of how to improve its air quality. Reduction in emissions of nitrogen oxide and ammonia is likely effective in disrupting this sulfate formation process.”

Image: Wikipedia and Wikimedia