While in Antofagasta, Chile, studying gases emitted from nutrient-rich waters flowing from Antarctica along South America’s west coast, Rainer Volkamer and his team discovered something they weren’t expecting.
“We found these surprisingly high levels of iodine oxide radicals” in dust layers 1–5 kilometers above the surface, said Volkamer, a chemistry professor at the University of Colorado Boulder. They were puzzled because the ocean supplies most of the atmosphere’s iodine, and iodine levels are typically highest where the ocean meets the air.
Trying to reconcile this contradiction, the research team stumbled upon a potential answer to a question that has vexed atmospheric chemists for years: What eats at ozone in dusty air?
In dust layers floating off the Sahara and other deserts, scientists have seen ozone levels fall below those of less dusty air. Separately, some have also observed dust layers that are rich in iodine. It’s also well known that iodine is a potent destroyer of ozone, a harmful pollutant to humans and crops. But until now, no one had found a likely mechanism that unites ozone, dust, and iodine and explains low ozone levels in dust.
Volkamer and colleagues wanted to know what happens inside the dust layers. They flew between central Chile and southern Peru over 13 days between January and February of 2012 and used high spectral resolution lidar to track dust layers in the free troposphere while spectroscopy instruments recorded levels of ozone and of iodine monoxide.
After the flights, the team looked to meteorological records and traced the source of the dust-laden air masses to the Atacama and Sechura deserts. The field measurements showed that the dust layers had 10 times the iodine oxide concentration of background air at the same altitudes. The researchers then modeled how those iodine concentrations might affect ozone levels in the surrounding air and found that ozone concentrations decreased by 35% locally (over an area of roughly 57,000 square kilometers) and by 9.4% regionally (an area of roughly 5 million square kilometers).
The scientists are uncertain about the precise mechanism but proposed that when wind lofts dust into the air, the dust, which is alkaline, may attract acids. Those acids may activate dust particles to release iodine, leading to the photochemical reactions in which iodine destroys ozone.
“It was a conundrum in the community,” said Volkamer. “There were measurements of dust and low ozone, but there were no simultaneous measurements of dust and ozone with iodine.”
Further lab and field experiments are needed to verify these findings, which were published in Science Advances. “We’ve got a limited data set, and we didn’t measure iodine in the dust [directly],” said Theodore Koenig, a postdoctoral researcher at Peking University and lead author of the study.
But these findings give atmospheric chemists a framework to explain the breakdown of ozone in dust. “It’s definitely credible evidence, but of course, this is not representative for the entire world,” said Jos Lelieveld, director of the Max Planck Institute for Chemistry, who was not involved in the study.
Filling the Gaps
Terrestrial sources of iodine are not as well known as marine sources, and they contribute a much smaller share of the element to the atmosphere. But without accounting for this source, atmospheric models might be missing a small, but impactful, driver of ozone levels. “The iodine provides an explanation that fills the gaps that other techniques have left,” said Volkamer.
What’s more, air quality regulators might want to consider iodine when implementing pollution control measures. The iodine emitted from dust is a sink for ozone, Volkamer explained, which lowers ozone pollution near the surface. But Lelieveld cautioned that more work is needed to understand how iodine chemistry affects air quality.
The findings also have implications for geoengineering proposals to inject dust into the air to cool Earth. Injecting dust without a full understanding of its interactions with ozone could delay recovery of the protective ozone layer in the stratosphere. And although it destroys ozone, iodine chemistry increases the lifetime of other greenhouse gases in the air. “We are cautious about trying to fix one problem and making another worse, and iodine is something that we need to have on the map,” said Volkamer.
—Jackie Rocheleau (@JackieRocheleau), Science Writer