Are iodic acid measurements by chemical ionization unambiguous?

Field observations of IO³⁻ and HIO³⁻-containing cluster anions by chemical ionization–atmospheric pressure interface–time-of-flight mass spectrometry (CI-API-ToF-MS) have been reported1. These observations, which employ nitrate (NO³⁻) reagent ions for reaction with the analytes, have been interpreted as resulting from atmospheric gas-phase iodic acid (HOIO₂) and molecular cluster formation via HOIO₂ addition steps. CI-API-ToF-MS chamber measurements with alternative ionization schemes have also reported signals that could be attributed to gas-phase HOIO and HOIO₂. However, well-established chemical kinetics and thermochemistry do not indicate any straightforward route to gas-phase iodine oxyacids and HOIO2 particle formation in the atmosphere. This does not only hinder the ability of chemical models for linking iodine emissions and particle formation, but also calls into question the interpretation of these CI-API-ToF-MS measurements. It has been proposed that water plays an important role in generating gas phase and HOIO₂-containing molecular clusters, but recent flow tube experiments have established extremely low upper limits to the rate constants of possible reactions between iodine oxides (IO_x and I_xO_y) and water. In this presentation, we discuss experimental and theoretical kinetics and thermochemistry of proposed routes to gas-phase HOIO and HOIO₂ in the atmosphere as well as potential ion-molecule reactions turning iodine oxides into IO^3- ions in the CI-API-ToF-MS inlet. We show that there is an important ambiguity in the interpretation of IO^3- and other signals observed with CI instruments as a result of barrierless reactions between I_xO_y and the reagent ions. Experiments for solving this ambiguity and reconciling conflicting results are proposed.