Iodine-129 is a long-lived anthropogenic radioisotope of iodine, which is a fission product of nuclear fuel and mainly originates from human nuclear activities. Due to its volatile properties such as high radiation risk of short-lived radioiodine, and long half-life, iodine-129 is a key radionuclide for nuclear environmental safety monitoring and an ideal tracer for investigation of environmental process such as dispersion, transport pathway of air pollution.

The baseline, the present level, and the source of iodine-129 in the atmosphere in China are not well understood.

In a recent study published in Atmospheric Chemistry and Physics, a group led by Prof. HOU Xiaolin at the Institute of Earth Environment of the Chinese Academy of Sciences analyzed day-resolution aerosols samples collected from Xi’an, China, during 2017 and 2018. Using ultra-sensitive accelerator mass spectrometry, the researcher revealed the first temporal variation of iodine-129 in the atmosphere in this region.

In 2017 and 2018, the researchers observed significant fluctuations of iodine-127 and iodine-129 in the atmosphere with different sources. Aerosol iodine-127 concentration in the atmosphere showed a positively correlation with air quality index and the level of five air pollutants, indicating its artificial sources. In winter, enhanced fossil fuel combustion and relatively stagnant weather conditions contribute to the increased iodine-127 level. The variation of the anthropogenic iodine-129 in atmosphere showed a pattern with low level in summer and high level in winter, with some exceptional high iodine-129 signals in summer.

Besides, the researchers found that the European nuclear fuel reprocessing plant located at the westerly belt was the dominant contributor of iodine-129 in the environment as the released iodine-129 was transport to Asia by the westerly. Interacting with the iodine-129-contaminated westerly, the East Asian winter monsoon was labeled with high level of 129I, blew from northwest to southeast, and reach Xi’an. While, the East Asian summer monsoon from the west Pacific with low content of iodine-129 level of global fallout blew to north and reach Xi’an in summer.

The temporal variation of iodine-129 in the atmosphere in Xi’an suggested that the level of iodine-129 in Chinese monsoonal regions was alternatively dominated by the iodine-129-enriched East Asian winter monsoon or the iodine-129-poor East Asian summer monsoon. The high iodine-129 signals occasionally occurred in summertime might be the break of the East Asian summer monsoon.

This study presented the first dataset of aerosol iodine-129 baseline of (92.7±124) ×10^-10 (expressed as the ratio of iodine-129 to iodine-127) in China, and showed that the temporal variation of the atmospheric baseline was related to metrological conditions. It suggested that locally released stable iodine-127 and externally input iodine-129 were likely involved into fine particles formation in urban air, shedding insights into long-range transport of air pollutants and iodine’s role in particulate formation in urban atmosphere.