Aerosol modulation of small ion growth in coastal air

Molecular cluster ions are produced throughout the troposphere by radon isotopes and cosmic rays, causing the slight electrical conductivity (σ) of atmospheric air. In a field campaign at the Weybourne Atmospheric Observatory on the Norfolk coast of the UK (52°57′23′′N 1°7′40′′E), aerosol size spectra were obtained simultaneously with σ measurements and background radioactivity, from 11th to 17th May 2004. σ varied with wind direction, from the modulation of the total aerosol number concentration Z rather than changes in ion production from radioactivity. σ was at its greatest when the air originated over the sea. Cluster ion properties were deduced using the σ data and the ion balance equation. The ion mobility μ decreased as Z decreased, corresponding to an increase in average cluster ion size. Using the attachment timescale of ions to aerosol as the typical time during which the ions can grow, negative ion growth occurred at ∼3 nm hr−1. Aerosol concentration changes modulate the time in which the ions can grow and the availability of condensable vapour.