Atmospheric aerosols spring a chemical surprise

Unexpected chemical processes on the surface of aerosol particles might bring new understanding of the chemistry of the atmosphere, including the reactions of pollutants, and lead to new possibilities for industry and research.

Aerosols are composed of tiny particles of solid or liquid suspended in a gas, such as air. Researchers at the University of Gothenburg in Sweden, the Paul Scherrer Institute in Switzerland and Qatar Environment and Energy Research Institute/Hamad Bin Khalifa University in Qatar, discovered the new chemistry occurs when ammonium sulfate aerosol particles begin to absorb water and dissolve.

Ammonium sulfate can form naturally, for example during volcanic emissions, but it is mainly found in the atmosphere due to human activities, including fuel combustion and fertiliser use.

The researchers found that the aerosols could spontaneously promote a reaction called sulfate-reducing ammonia oxidation. This converts ammonium ions (NH₄⁺) and sulfate ions (SO₄^2-) into sulfur and nitrogen.

“It was a surprise to see this,” says Xiangrui Kong, lead author of the study. He explains that an energy barrier normally prevents the reaction from proceeding spontaneously. “This has led us to rethink the catalytic effects of surfaces,” Kong adds, referring to the ability of surfaces to facilitate mechanisms that reduce the energy barriers for specific reactions.

The investigation was assisted by theoretical and molecular simulation work conducted by Ivan Gladich at Hamad Bin Khalifa University in Qatar. This used new high-performance computational facilities at Qatar Environment and Energy Research Institute and Texas A&M University at Qatar.

The discovery could improve understanding of atmospheric chemistry, including both natural aspects of the sulfur cycle and the interactions of chemical pollutants. It might also lead to industrial applications including sewage treatment.

The research team is now conducting further exploration of the catalytic powers of aerosol surfaces. “We have already observed more unexpected surface-promoted redox reactions that cannot be explained by classic chemistry principles,” says Kong.

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