Articles | Volume 16, issue 4
Research article
23 Feb 2016
Research article |  | 23 Feb 2016

Speciation of 127I and 129I in atmospheric aerosols at Risø, Denmark: insight into sources of iodine isotopes and their species transformations

Luyuan Zhang, Xiaolin Hou, and Sheng Xu

Abstract. Speciation analysis of iodine in aerosols is a very useful approach for understanding geochemical cycling of iodine in the atmosphere. In this study, overall iodine species, including water-soluble iodine species (iodide, iodate and water-soluble organic iodine), NaOH-soluble iodine, and insoluble iodine have been determined for 129I and 127I in the aerosols collected at Risø, Denmark, during March and May 2011 (shortly after the Fukushima nuclear accident) and in December 2014. The measured concentrations of total iodine are in the range of 1.04–2.48 ng m−3 for 127I and (11.3–97.0)  ×  105 atoms m−3 for 129I, corresponding to 129I ∕ 127I atomic ratios of (17.8–86.8)  ×  10−8. The contribution of Fukushima-derived 129I (peak value of 6.3  ×  104 atoms m−3) is estimated to be negligible (less than 6 %) compared to the total 129I concentration in northern Europe. The concentrations and species of 129I and 127I in the aerosols are found to be strongly related to their sources and atmospheric pathways. Aerosols that were transported over the contaminated seas contained higher concentrations of 129I than aerosols transported over the European continent. The high 129I concentrations of the marine aerosols are attributed to secondary emission of marine discharged 129I in the contaminated seawater in the North Sea, North Atlantic Ocean, English Channel, Kattegat, etc., rather than direct gaseous release from the European nuclear reprocessing plants (NRPs). Water-soluble iodine was found to be a minor fraction to the total iodine for both 127I (7.8–13.7 %) and 129I (6.5–14.1 %) in ocean-derived aerosols, but accounted for 20.2–30.3 % for 127I and 25.6–29.5 % for 129I in land-derived aerosols. Iodide was the predominant form of water-soluble iodine, accounting for more than 97 % of the water-soluble iodine. NaOH-soluble iodine seems to be independent of the sources of aerosols. The significant proportion of 129I and 127I found in NaOH-soluble fractions is likely bound with organic substances. In contrast to water-soluble iodine, the sources of air masses exerted distinct influences on insoluble iodine for both 129I and 127I, with higher values for marine air masses and lower values for terrestrial air masses.

Short summary
Speciation analysis of long-lived anthropogenic iodine isotopes (129I) in time series Danish aerosols shows that secondary emission from heavily 129I-contaminated seawater is a major source of 129I in the Atmosphere, at least in North Europe. Iodide is the major form of water-soluble iodine, while NaOH-soluble iodine is the dominant species of iodine in aerosol, which is likely bound with organic substances. The contribution of Fukushima-derived 129I is estimated to be negligible in Europe.
Final-revised paper