Temporal variation of 129 I and 127 I in aerosols from Xi'an, China: influence of East Asian monsoon and heavy haze events

Abstract. Aerosol iodine isotopes are pivotal links in atmospheric circulation of iodine in both atmospheric and nuclear sciences, while their sources, temporal change and transport are still not well understood. This work presents the day-resolution temporal variation of iodine-129 (129I) and iodine-127 (127I) in aerosols from Xi'an, northwest China during 2017/2018. Both iodine isotopes have significant fluctuations with time, showing highest levels in winter, approximately two to three times higher than in other seasons, but the correlation between 129I and 127I reflects they have different sources. Aerosol 127I is 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 127I in winter. The change of 129I confirms that source and level of 129I in the monsoonal region were alternatively dominated by the 129I-enriched East Asian winter monsoon and 129I-poor East Asian summer monsoon. The mean 129I/127I of (101 ± 124) × 10−10 provides an atmospheric background level for the purpose of nuclear environmental safety monitoring. This study suggests that locally discharged stable 127I and externally input 129I are 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.