Characterization of iodine particles with Volatilization-Humidification Tandem Differential Mobility Analyser (VH-TDMA), Raman and SEM techniques

Abstract. Particles formed upon photo-oxidation of CH₂I₂ and particles of I₂O₅ and HIO₃ have been studied using a Volatilisation and Humidification Tandem Differential Mobility Analyser (VH-TDMA) system. Volatilization and hygroscopic behaviour have been investigated as function of temperature (from 25 to 400°C), humidity (RH from 80 to 98%), initial aerosol sizes (from 27 to 100 nm mobility diameter) and in nitrogen or air as the sheath gasses. The volatility behaviour of particles formed upon photo-oxidation of CH₂I₂ is more similar to that of HIO₃ particles in a filtered sheath air than in nitrogen, with the particle shrinkage occurring at 190°C and accompanied by hygroscopic growth. Despite its high solubility, HIO₃ was found not to be hygroscopic at room temperature with no significant growth displayed until the thermodenuder temperature reached 200°C or above when the particles have transformed into I₂O₅. Diiodopentaoxide (I₂O₅) particles exhibit relatively low hygroscopic growth factors of 1.2–2 in the humidity range investigated. Scanning Electron Microscopy (SEM) of particles formed upon photo-oxidation of CH₂I₂ shows that their primary elemental components were iodine and oxygen in a stoichiometric ratio of approximately 1:2 with 10% error. Both Raman spectra and SEM show poor crystallinity for all the aerosols produced.