December 5, 1952 – residents of London, England woke to cloudless skies characteristic of a prolonged pattern of unusually chilly weather. Shivering citizens fed coal burning fireplaces with earnest, soot belched from thousands of chimneys. Within a few hours thick fog settled over the city, by afternoon fog began to turn “sickly yellow” in hue.
Unaware of temperature inversion caused by a stalled high pressure weather system, Londoners had no way of knowing warm temperatures 1,000 feet above ground blocked noxious soot’s escape. Reeking of rotten eggs, poisonous sulfur rich smog halted air, train, boat and surface transportation. Those who ventured outside reported streets thick with sticky goo and blackened faces of coal miners. Over 5 days an estimated 12,000 succumbed to the killer fog. A government investigation resulted in the Clean Air Act of 1956, restricting burning of coal in urban areas and grants to convert coal heat to gas, oil or electric.
Knowing coal emissions trapped in fog are lethal isn’t the same as understanding chemical interactions at play. It took a trip to China (a coal burning nation, home to 16 of the 20 most polluted cities in the world) by Texas A&M researcher Renyi Zhang to publish findings Oct. 9 2016 in Proceeding of The National Academy of Sciences. Research indicating the same phenomenon takes place today.
“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.
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.”