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London’s 1952 killer fog mystery solved


A new study finally revealed the exact cause of the 1952 killer fog which hovered over London for several days. The fog contained large amounts of pollutants which caused widespread breathing problems and killed thousands of city residents.

The fog covered London for five days in December 1952. Although the residents didn't pay much attention to it at first, as it was similar to the usual natural fogs which Great Britain experienced for thousands of years, the skies completely darkened as the situation developed.

The witnesses reported visibility reduced to only 0.9 meters (3 feet) in parts of the city, forcing all traffic to shut down, and causing breathing difficulties for tens of thousands of affected residents. As the fog finally disappeared on December 9, 4 000 people were reported dead while 150 000 were hospitalized. Thousands of animals were also killed.

According to the more recent investigation, the death toll was even greater, and it appears over 12 000 people died from the pollutants impact. Such an effect was probably caused by emissions resulting from coal burning, although the exact chemical process that produced a deadly mixture wasn't known until now.

The research team has finally come up with a satisfying answer by engaging in laboratory experiments and atmospheric measurements in China.

According to results, as the sulfur dioxide was released by coal burning, sulfuric acid particles were released from sulfur dioxide.

"But how sulfur dioxide was turned into sulfuric acid was unclear. 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 micrometers in size, and the acid formed was sufficiently diluted. Evaporation of those fog particles then left smaller acidic haze particles that covered the city," explained Renyi Zhang, a Texas A&M researcher, University Distinguished Professor and the Harold J. Haynes Chair of Atmospheric Sciences and Professor of Chemistry.

The new research suggests a similar situation is quite frequent in China, where severe air pollution is often significantly above the air standards acceptable by the US Environmental Protection Agency. However, the haze in China results from much smaller nanoparticles, and the sulfate formation can only occur with ammonia, which neutralizes the particles.

"In China, sulfur dioxide is mainly emitted by power plants, nitrogen dioxide is from power plants and automobiles, and ammonia comes from fertilizer use and automobiles. Again, the right chemical processes have to interplay for the deadly haze to occur in China. Interestingly, while the London fog was highly acidic, contemporary Chinese haze is basically neutral," said Zhang.

Frequent air pollution often forces China residents to hide behind the breathing masks, although the experts have been working to minimize the pollution over the last decade. The scientists are confident that a better understanding of air chemistry is the key to developing effective regulatory actions.

"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 for nitrogen oxides and ammonia is likely effective in disrupting this sulfate formation process."

The killer fog event of 1952 prompted the Clean Air Act which was passed by the British Parliament in 1956. The pollution is considered the worst air pollution recorded in the history of Europe.

China has been battling similar problems with air pollution for decades, following its strong industrial growth. 16 out of 20 most polluted cities on the globe are in located in China.


  • "Persistent sulfate formation from London Fog to Chinese haze" –  Gehui Wang, Renyi Zhang, Mario E. Gomez et al. – Proceedings of the National Academy of Sciences of the United States of America (2016) –  DOI: 10.1073/pnas.1616540113 

Featured image: Hyde Park London, 1952. Image credit: Leonard Bentley (Flickr-CC)

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