Quantification of Coastal New Ultra-Fine Particles Formation from In situ and Chamber Measurements during the BIOFLUX Campaign

Environmental Context. Secondary processes leading to the production of ultra-fine particles by nucleation are still poorly understood. A fraction of new particles formed can grow into radiatively active sizes, where they can directly scatter incoming solar radiation and, if partly water soluble, contribute to the cloud condensation nuclei population. New particle formation events have been frequently observed at the Mace Head Atmospheric Research Station (western Ireland), under low tide and sunny conditions, leading to the hypothesis that new particles are formed from iodo-species emitted from macroalgae.

Abstract. New particle formation processes were studied during the BIOFLUX campaign in September 2003 and June 2004. The goals were to bring new information on the role of I₂ in new particle formation from seaweeds and to quantify the amount of I₂ emitted and new particles formed by a given amount of seaweed. These two goals were achieved by using a simulation chamber filled with selected species of seaweeds from the Mace Head area and flushed with particle-free atmospheric air. It was found that total particle concentrations and particles in the 3–3.4 nm size range produced in the chamber are positively correlated with gaseous I₂ concentrations emitted by the seaweeds, with a typical source rate of 2800 particles cm⁻³ ppt_(I2)⁻¹ in the 3–3.4 nm size range. In fact, I₂ and particle concentrations are also both directly positively correlated with the seaweed mass (64 300 particles cm⁻³ formed per kg of seaweed, and 24 ppt of I₂ per kg of seaweeds) until saturation was reached for a seaweed biomass of 7.5 kg m⁻². From the chamber experiments, the flux of 3–3.4 nm particles was calculated to be 2.5 × 10¹⁰ m⁻² s⁻¹ for a seaweed loading of 2.5 kg m⁻² (representative of a typical seaweed field density), decreasing to 1 × 10¹⁰ m⁻² s⁻¹ for a seaweed loading of 1 kg m⁻². At a seaweed loading of 2.5 kg m⁻², the growth rate of particles produced in the chamber was calculated to be 1.2 nm min⁻¹. The source rates and growth rates determined from the chamber experiments were used in conjunction with seaweed coverage in and around Mace Head to produce local emission inventories for a meso-scale dispersion model. Comparison of the resulting aerosol size distributions from the model simulations and those observed exhibited good agreement suggesting that the chamber fluxes and growth rates are consistent with those associated with the tidal emission areas in and around Mace Head.