Modelling iodine particle formation and growth from seaweed in a chamber

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 I-2 emitted by seaweed. Fractal geometry of particles was taken into account. For the I-2 fluxes of (0.5-1.5) x 10(9) cm(-3) s(-1) the model predicted strong particle bursts, the steady state concentrations of I-2 vapour and particles larger than 3 nm were as high as 4 x 10(9)-1.2 x 10(10) cm(-3) and 5.0 x 10(6)-9.2 x 10(6) 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 I-2 emissions and nucleation of iodine oxides can largely explain the coastal nucleation phenomenon.