In the terrestrial atmosphere, the presence of iodine, a nutrient element, is important in determining the prevalence of iodine deficiency disorders. Since oceanic iodine emissions decline when the sea surface temperature drops, contributors to atmospheric iodine vary from oceanic sources in summer to other sources in winter. These sources and their respective contributions are of great importance, but have remained unexplored.

Recently, researchers from the Institute of Earth Environment of the Chinese Academy of Sciences (CAS), studied the source terms of iodine in atmospheric fine particles by analyzing daily PM_2.5 samples collected for 1 month from four metropolises of China.

They found that the PM_2.5 samples from Changchun, Beijing and Chengdu had a clear source profile composed of dust, coal and fuel oil, while samples from Hong Kong showed an indistinguishable signature, which was due to the mixing of road dust and oceanic materials. All the profiles of coal and fuel oil correspond to a relatively higher concentration of iodine. 

Coincidently, this pattern was perfectly examined by the analytical data of the atomic ratio of ¹²⁹I to ¹²⁷I, where the ratios decreased with an increasing contribution from coal and fuel oil, and increased when contribution from dust was dominating. This is in line with the isotopic characteristics of iodine that differs greatly between nuclear activity and fossil fuels like coal and petroleum, which formed millions to billions of years ago and thus is depleted in ¹²⁹I.

They also found, for the first time, that in winter inland areas, fuel oil together with coal combustion, was a significant source of atmospheric iodine. This research enhances the comprehension of the human activities’ impact on environmental iodine levels, and is crucial for depicting a complete picture of the iodine geochemistry, to better understand the iodine deficiency-related health issues and the stratospheric ozone depletion.

This work was published as the Supplementary Cover in Environmental Science & Technology on Oct. 31.

Contact: Bai Jie, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China. Email: baijie@ieecas.cn