U. of Colorado researchers propose answer to basic atmospheric chemistry question

Scientists from the University of Colorado at Boulder have proposed a long-sought answer to how atmospheric sulfate aerosols are formed in the stratosphere.

Conducted by researchers at the Cooperative Institute for Research in Environmental Sciences, or CIRES, the research shows how a fundamental molecular process driven by sunlight may play a significant role in determining the planet’s energy budget.

The research was a collaboration between Veronica Vaida, chair of the CU-Boulder chemistry and biochemistry department and a CIRES fellow, CIRES visiting fellows H. G. Kjaergaard from the University of Otago in New Zealand and D. J. Donaldson from the University of Toronto, and CIRES doctoral candidate P. E. Hintze.

A paper on the subject will appear in the March 7 issue of Science magazine. CIRES is a joint institute of CU-Boulder and the National Oceanic and Atmospheric Administration headquartered on campus.

Atmospheric sulfates gather in a stratospheric region called the Junge layer that surrounds Earth’s surface at altitudes between nine and 21 miles, said Vaida. The Junge layer reflects sunlight back into space and radiation to Earth, affecting the planet’s energy budget.

The Junge layer is thought to be composed primarily of sulfuric acid and water molecules, she said. Because sulfates have important chemical and climate effects, scientists have wanted to understand how atmospheric sulfuric acid breaks down, releasing sulfur oxides in the upper stratosphere where concentrations have been measured.

When high-altitude air descends in the cold polar vortex each spring, the gases recombine and form the Junge layer, Vaida said. Sunlight can be absorbed by sulfuric acid molecules and in some instances decompose them.

“It was thought that solar radiation could break the bonds of sulfuric acid molecules at very high energies in the ultraviolet spectrum,” Vaida said. But high-energy radiation is present only at the top of and above the atmosphere because the atmosphere effectively absorbs ultraviolet radiation.

“We ruled out the standard hypothesis that had been proposed but never observed,” Vaida said. Instead, she said, the CIRES team sought ways that the sulfates could be breaking down within the visible range of light.

In order to explain the measured and modeled concentrations of sulfates found in the upper stratosphere and mesosphere, “The mechanism we proposed was really the only game in town,” she said.

Using spectroscopy, the team investigated the effect of visible light on sulfuric acid molecules to prove that molecular rearrangements could be induced to explain the observed sulfate layer. “We found visible radiation at much lower energies than previously thought could accomplish the molecular breakdown,” Vaida said.

“Understanding the fundamental properties of sulfuric acid, we now know what affects formation of the sulfate layer, and can predict its formation by looking at the altitude, temperature and solar flux,” she said. “The work allows us to model chemical properties of the Earth’s atmosphere.”

Support for the research was provided by the National Science Foundation, the Marsden Fund administered by the Royal Society of New Zealand and NSERC of Canada. Vaida credits the success for the team’s discovery to the CU-NOAA partnership at CIRES that unites university academic departments with eight NOAA laboratories. The collaboration produces an increased flow of ideas and additional access to specialized expertise.

“We had a lot of help from NOAA people uniquely qualified in the areas that we needed – the connection fostered by CIRES was key,” she said. “We could bring together fundamental chemistry with atmospheric science in a way that can’t be done anywhere else – it was rather magical.”

The next step is “to quantify the yield with which sulfur oxides are going to be released and refining our knowledge of related processes,” Vaida said.

Contact: Veronica Vaida, (303) 492-8605
vaida@spot.colorado.edu
Annette Varani, (303) 492-5952
Nicolle Wahl, (416) 978-6974
Nicolle.wahl@utoronto.ca