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

Abstract

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.

Determination of 129I in aerosols using pyrolysis and AgI–AgCl coprecipitation separation and accelerator mass spectrometry measurements

“…Under the optimal pyrolysis temperature in this work (800°C), iodate can be decomposed to iodide and oxygen, then iodide can be oxidized to gaseous I2 that will be released and trapped in a trapping solution. In addition to iodide and iodate, iodine occurs in aerosols as organic iodine (such as those associated to humic-like substances) and metal oxides combined iodine [6,13]. For organic iodine species, it is easy to be decomposed under high temperature in the optimal conditions.…”

“…Even though analytical methods for determination of 129 I in aerosols have been reported, large sample size and time-consuming analytical procedure are the challenge for determination of low-level 129 I concentrations in aerosols. 129 I concentrations in the atmosphere range from 10 4 atoms/m 3 in Asia to 10 6 atoms/m 3 in northern Europe [12,13]. The conventional methods generally requires a few hundreds to thousands cubic meters of air for determination of 129 I in aerosols.…”

Distribution of Anthropogenic 129I in the Western South China Sea and Its Application for Tracing the Sources and Movement of Pollution

“…Meanwhile, more than 6500 kg of 129 I has been discharged to the seas from the NFRPs at Sellafield and La Hague since the 1990s, and the annual discharge of 129 I from these plants to the seas still remains at about 250 kg/y. Re-emission of the NFRP-derived 129 I to the atmosphere from the contaminated seawater is estimated to be about 13.8 kg/y since 2000, making it the main source of 129 I in the present air …”

On the Speciation of Iodine in Marine Aerosol

“…The fraction of TSI in TI is also consistent across latitude and longitude (∼65%–80%). There is only one campaign at low latitudes (C12 [Gómez Martín et al., 2021 ]), reporting almost all TI being soluble, while another campaign (S35 [Zhang et al., 2016 ]) reports an extremely high nonsoluble fraction (82%), with complete absence of iodate and SOI. Insoluble aerosol iodine does not seem to be linked to coastal new particle formation as previously speculated (Baker, 2004 ), since it appears to exist ubiquitously, and iodine‐driven new particle formation leads to iodic acid particles (Gómez Martín et al., 2020 ) and ultimately to iodate.…”