New Pathway for Atmospheric Formation of Sulfuric Acid Discovered

Researchers from two universities discovered a new sunlight-independent pathway for the formation of sulfuric acid in the atmosphere. Sulfuric acid is known to have significant environmental and health impacts.

An international team of scientists headed by the University of Colorado Boulder and the University of Helsinki made the discovery. A paper on the discovery was published in the August 9 issue of Nature.

The new pathway involves the reaction of ozone and alkenes to form a carbonyl oxide type of compound. This compound oxidizes sulfur dioxide to form sulfuric acid.

Roy Mauldin III, research associate in CU-Boulder’s atmospheric and oceanic sciences department and lead author, says they have discovered an important atmospheric oxidant. Understanding the role of this new oxidant in atmospheric chemistry will be crucial to assess impacts of air pollutants and to mitigate climate change.

Sulfuric acid is usually formed from the reaction of hydroxyl radical and sulfur dioxide. The catalyst for the reaction is sunlight, which acts as a trigger to ignite the chemical process.

The researchers had suspicions other sunlight-independent pathways are at work when they began detecting high concentrations of sulfuric acid at night, particularly over forests in Finland.

Using a mass spectrometer instrument connected to a flow tube for adding gases, the researchers combined gases ubiquitous to the atmosphere. These gases were ozone, various alkenes and sulfur dioxide. Maudlin reported that the instrument was able to detect rapid formation of sulfuric acid in huge amounts.

In order to be certain that no hydroxyl radical was present in the gas mixture, they added a scavenger compound to remove any hydroxyl impurity. The same results were obtained.

In one experiment, one of the researchers held up freshly broken tree branches to the flow tube and the sulfuric acid level went up the roof. The tree branch contained isoprene and alpha-pinene; these are alkenes commonly found in trees and responsible for the fresh pine tree scent.

Mauldin says the new pathway will drastically increase atmospheric levels of sulfuric acid. The problem is aggravated by increasing emissions of sulfur dioxide from fossil fuel combustion in power plants and other facilities.

The environmental effect of sulfuric acid is primarily formation of acid rain, which is harmful to both terrestrial and aquatic life. Airborne sulfuric acid particles in the atmosphere are largely responsible for the formation of clouds, while particles near the earth’s surface cause respiratory problems in humans.

Maudlin concludes that the new pathway could help explain recent studies showing large parts of the southeastern United States might have cooled slightly in the past century. Heavy cloud formation due to sulfuric acid particulates over the forests could have a cooling effect by reflecting sunlight back to space.



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