Iodine-laden desert dust eats away at ozone pollution
While in Antofagasta, Chile, studying gases emitted by nutrient-rich waters flowing from Antarctica along the west coast of South America, Rainer Volkamer and his team discovered something something they did not expect.
“We found these surprisingly high levels of iodine oxide radicals” in layers of dust 1 to 5 kilometers above the surface, said Volkamer, professor of chemistry at the University of Colorado at Boulder. They were puzzled because the ocean provides most of the atmosphere’s iodine, and iodine levels are generally highest where the ocean meets the air.
In trying to reconcile this contradiction, the research team stumbled upon a potential answer to a question that has plagued atmospheric chemists for years: what eats away at ozone in dusty air?
In layers of dust floating off the Sahara and other deserts, scientists have seen ozone levels drop below those in less dusty air. Separately, some have also observed layers of iodine-rich dust. It is also well known that iodine is a powerful destroyer of ozone, a pollutant harmful to humans and crops. But until now, no one had found a likely mechanism that unites ozone, dust and iodine and explains the low levels of ozone in dust.
Volkamer and his colleagues wanted to know what is going on inside the layers of dust. They flew between central Chile and southern Peru for 13 days between January and February 2012 and used high spectral resolution lidar to track dust layers in the free troposphere while spectroscopy instruments recorded dust levels. ozone and iodine monoxide.
After the flights, the team pored over weather records and traced the source of the dust-laden air masses back to the Atacama and Sechura deserts. 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 these iodine concentrations might affect ozone levels in ambient air and found that ozone concentrations decreased by 35% locally (over an area of approximately 57,000 square kilometers) and 9.4% at the regional level (an area of approximately 5 million square kilometres). ).
Scientists are unsure of the precise mechanism, but have proposed that when the wind blows dust into the air, the dust, which is alkaline, can attract acids. These acids can activate dust particles to release iodine, resulting in photochemical reactions in which iodine destroys ozone.
“It was an enigma in the community,” Volkamer said. “There were measurements of dust and weak ozone, but there were no simultaneous measurements of dust and ozone with iodine.”
Further laboratory and field experiments are needed to verify these results, which have been published in Scientists progress. “We have a limited data set, and we haven’t measured iodine in dust. [directly],” noted Theodore Koenigpostdoctoral researcher at Peking University and lead author of the study.
But these findings give atmospheric chemists a framework to explain the degradation of ozone in dust. “It’s certainly credible evidence, but of course it’s not representative for the whole world,” said Jos Lelievelddirector of the Max Planck Institute for Chemistry, who was not involved in the study.
Land-based 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 could miss a small, but impactful, factor in ozone levels. “Iodine provides an explanation that fills in the gaps left by other techniques,” Volkamer said.
Additionally, air quality regulators might want to consider iodine when implementing pollution control measures. The iodine emitted from the dust is a sink for ozone, Volkamer explained, which reduces ozone pollution near the surface. But Lelieveld warned 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 the 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, the chemistry of iodine increases the lifespan of other greenhouse gases in the air. “We’re careful before we try to solve one problem and make another worse, and iodine is something we need to have on the map,” Volkamer said.
—Jackie Rocheleau (@JackieRocheleau), science writer