Temporal variation in ${}^{129}$I and ${}^{127}$I in aerosols from Xi'an, China. Influence of East Asian monsoon and heavy haze events

Aerosol iodine isotopes are pivotal links in atmospheric circulation of iodine in both atmospheric and nuclear sciences, while their sources, temporal change and transport mechanism are still not well understood. This work presents the day-resolution temporal variation in iodine-129 (${}^{129}$I) and iodine-127 (${}^{127}$I) concentrations in aerosols from Xi'an, north-west China, during 2017/18. Both iodine isotopes have significant fluctuations with time, showing the highest levels in winter, approximately 2–3 times higher than in other seasons, but the correlation between ${}^{129}$I and ${}^{127}$I concentrations reflects that they have different sources. Aerosol ${}^{127}$I concentrations are found to be noticeably positively correlated with air quality index and five air pollutants. Enhanced fossil fuel combustion and inverse weather conditions can explain the increased concentrations and peaks of ${}^{127}$I in winter. The change in ${}^{129}$I concentrations confirms that the source and level of ${}^{129}$I in the monsoonal region were alternatively dominated by the ${}^{129}$I-enriched East Asian winter monsoon and the ${}^{129}$I-poor East Asian summer monsoon. The mean ${}^{129}$I/${}^{127}$I number ratio of (92.7±124)×10${}^{-10}$ provides an atmospheric background level for the purpose of nuclear environmental safety monitoring. This study suggests that locally discharged stable ${}^{127}$I and externally input ${}^{129}$I are likely involved in fine particles formation in urban air, which provides insights into the long-range transport of air pollutants and iodine's role in particulate formation in urban atmosphere.