An ozone destroyer

Researchers have detected the presence of a pollutant-destroying compound, iodine monoxide (IO), in surprisingly high levels high above the tropical ocean, according to a new study led by the Cooperative Institute for Research in Environmental Sciences (CIRES), a joint institute of the University of Colorado, United States, and the National Oceanic and Atmospheric Administration (NOAA). The results of the study were published recently in the Proceedings of the National Academy of Sciences.

“The high concentrations in air that has not recently been in contact with the ocean surface point to the intriguing possibility of a recycling mechanism whereby instead of IO decaying away as previously thought, it’s released back to the atmosphere by heterogeneous chemistry on aerosol particles,” said Rainer Volkamer, who led the study.

IO is an important chemical because it destroys ozone, a greenhouse gas that warms the planet and also indirectly lowers methane levels. Additionally, IO can form aerosols —tiny particles suspended in the atmosphere—that can initiate the production of clouds, which help cool the climate. If IO is recycled in the atmosphere, as the research findings suggest, “it means IO has a longer effective lifetime and… can destroy much more ozone”, Volkamer said. The team’s analysis indicates that IO accounts for up to 20 per cent of the overall ozone loss rate in the upper troposphere. This ozone sink is currently not included in most atmospheric models.

The origin of IO is thought to be iodine emitted by microalgae or inorganic reactions at the ocean surface. Because IO occurs in relatively very small concentrations, one in 1,013 molecules, it had been difficult to quantify the amount in the upper atmosphere. The problem was solved by attaching the University of Colorado Airborne Multi-Axis Differential Optical Absorption Spectroscopy instrument to a research plane and flying it over the tropical Pacific. Analysing air samples collected from about 100 metres up to 1,100 m, the researchers created a vertical profile of the atmosphere’s composition. The efforts marked the first aircraft measurements of IO. “Based on current understanding, iodine oxide shouldn’t be hanging around for more than one hour,” Volkamer said. “But these measurements reveal a surprising persistence of IO in air masses disconnected from the ground. We don’t see that the IO decays away.”

The persistence of IO suggests that it is not irreversibly lost to aerosol. The aerosol “returns” the IO to the atmosphere. Such a recycling mechanism would be novel because iodine is a very heavy atom. “It tends to form polymers and stick onto particles. But a portion seems to be returning into the gas phase,” he said. The findings will help improve climate models’ predicative capability about how atmosphere behaves and how the atmosphere cleanses itself of pollutants and greenhouse gases.

Stories compiled by R. Ramachandran