A previously unsuspected chemical process in the Earth's atmosphere is creating sulfuric acid, known to have a significant impact on climate and human health.
An international research team led by the University of Colorado Boulder and the University of Helsinki has discovered a surprising new chemical compound, a type of carbonyl oxide, that is reacting with sulfur dioxide to form sulfuric acid.
The compound is formed from the reaction of ozone with alkenes, a family of hydrocarbons with both natural and human-made sources.
"We have discovered a new and important, atmospherically relevant oxidant," says research associate Roy Mauldin.
"Sulfuric acid plays an essential role in Earth's atmosphere, from the ecological impacts of acid precipitation to the formation of new aerosol particles, which have significant climatic and health effects. Our findings demonstrate a newly observed connection between the biosphere and atmospheric chemistry."
Sulfuric acid is usually formed in the atmosphere throughthe reaction between the hydroxyl radical OH and sulfur dioxide, triggered by sunlight.
But Mauldin and his colleagues started suspecting that there might be other mechanisms too when they began detecting sulfuric acid at night, particularly in forests in Finland.
To find out, he and his colleagues combined ozone - which is ubiquitous in the atmosphere -- with sulfur dioxide and various alkenes in a gas-analyzing instrument known as a mass spectrometer hooked up with a flow tube to add gases.
"Suddenly we saw huge amounts of sulfuric acid being formed," he says.
To make sure the hydroxyl radical OH wasn't reacting with the sulfur dioxide to make sulfuric acid, they added in an OH 'scavenger' compound to remove any traces of it. Later, one of the research team members held up freshly broken tree branches to the flow tube, exposing hydrocarbons known as isoprene and alpha-pinene - commonly found in trees and responsible for the fresh pine tree scent.
"It was such a simple little test," says Mauldin. "But the sulfuric acid levels went through the roof. It was something we knew that nobody had ever seen before."
Mauldin says the newly discovered oxidant may help explain recent studies that have shown large parts of the southeastern United States might have cooled slightly over the past century.
Particulates from sulfuric acid over the forests there may be forming more clouds than normal, cooling the region by reflecting sunlight back to space.