Inorganic aerosol formation and growth in the Earth's lower and upper atmosphere

Abstract
This chapter describes the photo-chemical production of aerosol particles in two very different regions of the atmosphere: iodine oxide particles in the marine boundary layer (MBL), and meteoric smoke particles that form in the upper mesosphere from the ablation of interplanetary dust. These two systems are surprisingly analogous - the source of the condensable inorganic vapours is external to the atmosphere, being injected into the atmosphere from the ocean or from space - and the particles are formed by homogeneous nucleation. The purpose of the chapter is to describe a laboratory and modelling study to understand at a fundamental level how the nucleation and growth of the particles occurs. Iodine oxide particles were produced from the photo-oxidation of gaseous I₂ with O₃, which is most likely the primary photo-chemical route to produce the bursts of new particles observed in the MBL at seaweed-rich coastal locations. The captured particles were observed to be fractal-like (i.e., with open or non-compact structures), and to be composed of the stable oxide I₂O₅. Meteoric smoke analogues of iron oxide, silicon oxide, and iron silicate composition were similarly formed from the photo-oxidation of iron- and silicon-containing gas-phase precursors in the presence of O₃. Imaging of the iron-containing particles showed them to be extended, fractal aggregates. For each system, models were developed to elucidate the growth kinetics of the particles and to characterise them in terms of standard fractal parameters. I₂O₅ particles were found to have a fractal dimension (D_f) value of 2.5 at long growth times, consistent with a particle-cluster diffusion-limited aggregation (DLA) mechanism, whereas smoke analogues had lower D_f values (1.75) which appear to result from a magnetic aggregation process.