Abstract
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 mechanism are still not well understood. This work presents the day-resolution temporal variation in iodine-129 (129I) and iodine-127 (127I) 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 129I and 127I concentrations reflects that they have different sources. Aerosol 127I 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 127I in winter. The change in 129I concentrations confirms that the source and level of 129I in the monsoonal region were alternatively dominated by the 129I-enriched East Asian winter monsoon and the 129I-poor East Asian summer monsoon. The mean 129I∕127I 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 stable127I and externally input 129I 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.
Highlights
Iodine is one of the active halogen elements and is involved in plenty of atmospheric chemical reactions, drawing increasing attention in atmospheric science and environmental fields in recent years (Saiz-Lopez et al, 2012)
Accelerator mass spectrometry (AMS; 3MV, High Voltage Engineering Europa B.V., the Netherlands) and inductively coupled plasma mass spectrometry (ICPMS; Agilent 8800, USA) were applied for a determination of 129I/127I number ratios and 127I concentrations, respectively. 129I/127I number ratio of iodine carrier is less than 2×10−13, and the analytical precisions are within 5 % for all the aerosol samples
The concentrations of 127I and 129I and the 129I/127I number ratio fell within 1.21–21.4 ng m−3, (0.13– 7.53) ×105 atoms m−3 and (10.6–743) ×10−10, respectively
Summary
Iodine is one of the active halogen elements and is involved in plenty of atmospheric chemical reactions (i.e. ozone depletion and new particle formation from condensable iodinecontaining vapours), drawing increasing attention in atmospheric science and environmental fields in recent years (Saiz-Lopez et al, 2012). A number of studies on atmospheric iodine just focus on the processes and mechanisms in marine boundary layer since over 99.8 % of iodine is derived from ocean (McFiggans et al, 2000). Other sources of iodine in air are comprised of volatile iodine and resuspended particles from soil, as well as combustion of fossil fuel (Fuge and Johnson, 1986). Whitehead (1984) estimated annual release of iodine from fossil fuel combustion is about 400 t, accounting for only 0.1 % of total iodine in air. A few studies have shown high iodine concentrations in air and particles in China (Gao et al, 2010; Xu et al, 2010). Marine atmospheric iodine has been proven to form fine particles, little is known about terrestrial atmospheric iodine, in urban sites with severe air pollution
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