(160b) Chemical and Physical Investigation of Fractal-Like Iodine Oxide Particle Formation

Particulate matters in the atmosphere as well as in engineering processes are often non-spherical. Non-sphericity poses technical challenges especially in the system where chemical reactions continuously occur and thus particles’ chemical composition, size, number, morphology, and mass change accordingly.

Atmospheric iodine oxide particle (IOP) formation is an example of systems with evolving fractal-like particles. Atmospheric chemistry of iodine influences the oxidizing capacity of the atmosphere (e.g., catalytic loss of tropospheric ozone). Furthermore, atmospheric iodine could be a significant precursor of newly formed aerosol in coastal areas. Previous experimental studies investigated IOP formation typically without NOx; however, recent studies point out IOP formation occurs in polluted environment with elevated NOx concentrations as well.

This study advanced the understanding of the chemical and physical processes leading to atmospheric IOP formation through the application of on-line particle effective density analysis (with approximately one-minute time resolution) to take into account the evolution of particle shape. IOP was produced via CH3I photolysis under varied O3 and NOx concentrations. An on-line chemical analysis showed minor contributions from nitrogen containing species to IOP, suggesting that nitrogen containing iodine species act as gas-phase reservoir of iodine. Real-time particle density measurement allowed determination of mass-based yield of IOP. Yield was nearly one (meaning the mass of IOP was equal to the mass of reacted CH3I) when there is sufficiently high ozone concentration for the formation of I2Ox species regardless of NOx concentration; on the other hand, particle yield approached zero when ozone formation was suppressed by the presence of NO.

Finally, a predictive model for IOP formation is developed by extending a gas-phase chemical reaction mechanism, SAPRC-07, to include iodine chemistry. IOP formation and O3 decay were reasonably predicted in the absence of NOx. However, when NOx was present, IOP was underpredicted due to build-up of reservoir species (e.g., I2 and IONO2) within the model, which suggests that further improvements are needed in describing reactions involving NOx in the iodine mechanism.