Ultra-Sensitive Determination of Particulate, Gaseous Inorganic and Organic Iodine-129 and Iodine-127 in Ambient Air.

Atmospheric iodine cycling is of significance in climate change and environmental and health impacts. To better explore speciation transformation of atmospheric stable and radioactive iodine, an ultra-sensitive analytical method was established for determination of 129 I and 127 I in particulate, gaseous inorganic, and gaseous organic species, which was conducted with a self-designed cascade sampling apparatus, followed by their separation with a pyrolysis system and accelerator mass spectrometry and ICP-MS measurements. Combustion protocols for three sampling matrices and NaOH concentration for iodine trapping were optimized to achieve a safe analytical procedure with a high chemical yield of iodine. Based on the lowest concentrations of 129 I and 127 I, a suitable activated carbon product for adsorption of gaseous organic iodine was carefully selected. The detection limits of the three species were 0.30-2.21 ng m -3 for 127 I and 0.05-0.22 × 10 5 atoms m -3 for 129 I. This newly established method was successfully applied to analyze the levels and species of 129 I and 127 I in ambinet air from Xi'an, China, from May to August, 2020. Gaseous organic iodine was found to be the dominant species of 127 I and 129 I, accounting for about half of total iodine, and gaseous inorganic iodine and particulate iodine accounted for one-quarter each during the whole sampling period. Speciation variation of 129 I and 127 I indicates that speciation transformation apparently occurred at the turn of spring and summer, mainly between particulate and gaseous organic iodine. This study has implications on delicate tracing of the atmospheric behavior of iodine with long-lived anthropogenic 129 I.