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

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.