Modelling Iodine Particle Formation and Growth from Seaweed in a Chamber

Abstract

Environmental Context.Iodine is an important trace species in the marine atmosphere. It contributes to ozone depletion and new particle formation. In recent years, its importance has been realised, however, there is still a gap in our knowledge, from a theoretical framework, of the dominant mechanisms leading to new particle formation and previous theoretical frameworks have not been adequately developed or well understood. This paper presents a state-of-the-art theoretical framework for evaluating the prediction of iodine oxide nucleation and subsequent aerosol growth. Abstract. A sectional atmospheric chemistry and aerosol dynamics box model (AEROFOR) was further developed and used to simulate ultra-fine particle formation and growth from seaweed in a chamber flushed with particle-free atmospheric air. In the model, thermodynamically stable clusters were formed by dimer nucleation of OIO vapour, whose precursor was assumed to be molecular I2 emitted by seaweed. Fractal geometry of particles was taken into account. For the I2 fluxes of (0.5–1.5) × 109 cm−3 s−1 the model predicted strong particle bursts, the steady state concentrations of I2 vapour and particles larger than 3 nm were as high as 4 × 109–1.2 × 1010 cm−3 and 5.0 × 106–9.2 × 106 cm−3 respectively. The steady state was reached in less than 150 s and the predicted growth rates of 3–6 nm particles varied in the range of 1.2–3.6 nm min−1. Sensitivity of the size distribution against I2O3 cluster formation, an extra condensable vapour, the photolysis rate of the OIO vapour as well as against the density of (OIO)n-clusters was discussed. The modelled results were in good agreement with the chamber measurements performed during the BIOFLUX campaign in September, 2003, in Mace Head, Ireland, confirming that I2 emissions and nucleation of iodine oxides can largely explain the coastal nucleation phenomenon.