Scientists find how 'atmosphere's detergent' is formed

Scientists have identified an important intermediate molecule and tracked its transformation to hydroxyl radicals - highly reactive molecules that are known as the atmosphere's 'detergent.

Earth's atmosphere is a complicated dance of molecules. The chemical output of plants, animals and human industry rise into the air and pair off in sequences of chemical reactions.

Such processes help maintain the atmosphere's chemical balance; for example, some break down pollutants emitted from the burning of fossil fuels.

Understanding exactly how these reactions proceed is critical for predicting how the atmosphere will respond to environmental changes, but some of the steps of this dance are so quick that all of the molecules involved haven't been measured in the wild.

A University of Pennsylvania team has now observed one of these rapid atmospheric reactions in the lab. They identify an important intermediate molecule and track its transformation to hydroxyl radicals, also demonstrating the amount of energy necessary for the reaction to take place.

Their findings help explain how the atmosphere maintains its reserves of hydroxyl radicals, highly reactive molecules that are called the "atmosphere's detergent."

Hydroxyl radicals are composed of a hydrogen atom bonded to an oxygen atom.
They are highly reactive, readily stealing hydrogen atoms from other molecules to form water. Many common pollutants and greenhouse gases, such as methane and more complex hydrocarbons, are initially broken down by these radicals.

"Hydroxyl radicals are called the atmosphere's detergent because most pollutants that go into the air are broken down by them," Marsha Lester, professor of chemistry in Penn's School of Arts & Sciences said.

"Since they're so reactive, the question is then. 'How is it that there is so much of it in the atmosphere?' They're reacting away all of the time, so they must be constantly replenished," said Lester, who led the study.

The Sun is responsible for most of the hydroxyl radicals in the air in the daytime. Sunlight has sufficient energy to breakdown ozone, releasing oxygen atoms that react with water vapour to produce hydroxyl radicals.

However, another important source of these molecules does not require sunlight.
This less-well-understood process, investigated by the team, is the dominant source of hydroxyl radicals at night, and plays a large role in the atmosphere during the winter.
The study was published in the journal Science.