Hexbyte Glen Cove Before geoengineering to mitigate climate change, researchers must consider some fundamental chemistry

Hexbyte Glen Cove

Some scientists have proposed planetary-scale solutions to address climate change, such as geoengineering using sulfur compounds to create a sunshield in the upper atmosphere. New research suggests there’s a good deal more chemistry to understand before proceeding. Credit: Francisco laboratory

It’s a tempting thought: With climate change so difficult to manage and nations unwilling to take decisive action, what if we could mitigate its effects by setting up a kind of chemical umbrella—a layer of sulfuric acid in the upper atmosphere that could reflect the sun’s radiation and cool the Earth?

According to a new study in the Journal of the American Chemical Society, a collaboration among Penn scientists and two groups in Spain, in the stratosphere pose a challenge to generating sulfuric acid, making its production less efficient than might have previously been expected. Thus more groundwork exploring the of how sulfuric acid and its building blocks will react in the is required in order to confidently move forward with this climate geoengineering strategy, the researchers say.

“These fundamental insights highlight the importance of understanding the photochemistry involved in geoengineering,” says Joseph S. Francisco, an atmospheric chemist in Penn’s School of Arts & Sciences and a co-corresponding author on the study. “That’s critically important and it’s something that’s been ignored.”

Using sulfuric acid to blunt the sun’s rays as a means of curbing impacts is based on a natural phenomenon: When volcanoes erupt, the sulfur they emit creates localized—or sometimes even far-reaching—cooling clouds that filter the sun. But those clouds emerge in the troposphere, which ranges from the Earth’s surface to about 10 kilometers up. Geoengineering using sulfuric acid would happen a good deal higher, in the stratosphere, from about 10 to 20 kilometers above the planet.

Conditions change as the altitude increases. Notably, the air becomes drier, and the energy of the sun’s rays becomes stronger. In the new work, Francisco, his postdoc Tarek Trabelsi, and colleagues from Spain’s Rocasolano Institute of Physical Chemistry and the University of València partnered to explore how these variables affected the involved in making sulfuric acid.

The major inputs are (SO2), which reacts with hydroxyl radicals (OH), a kind of atmospheric “detergent,” to create HOSO2. HOSO2 reacts with oxygen to create sulfur trioxide (SO3), which then reacts with water vapor to create sulfuric acid. Aerosols formed from the sulfuric acid have the ability to reflect sunlight.

These reactions are well characterized; together, they are responsible for creating rain in the troposphere. But whether that chemistry would work in the stratosphere and achieve the same efficiency was unknown.

To find out, the team used —an approach that considers the ground, transition, and excited states of atoms and molecules—to consider how HOSO2 and SO3 would behave in the stratosphere’s conditions of high light and low humidity. Though geoengineering approaches factor in the ability of these two molecules to reflect sunlight, the researchers found that when HOSO2 is produced in the stratosphere, solar radiation causes the molecule to quickly photolyze, essentially breaking apart into its component parts, including sulfur dioxide, which is harmful to humans in high concentrations.

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