Postfrontal nanoparticles at Cape Grim: impact on cloud nuclei concentrations

Abstract

Environmental context. Accurate prediction of climate change requires good knowledge of all the contributing processes; those processes controlling clouds and cloud properties are of particular importance. In this study the growth of bursts of nanometre-sized particles observed following cold fronts over the Southern Ocean was modelled to assess their importance as a source of cloud droplet nuclei. This showed that these post-frontal events were responsible for ~8% of the cloud nucleus population in winter but much less in summer. Abstract. Aerosol removal and growth rates were determined for the Cape Grim marine boundary layer (MBL) using local observations. Background particle growth rates, estimated using replacement of condensable sulfur species lost to particle removal are 0.04 nm h–1 (winter) and 0.17 nm h–1 (summer) and for post-frontal nucleation-events growth rates determined using evolution of the concentration ratio of particles with diameter >3 nm and 11 nm are ~0.3–0.4 nm h–1, consistent with reported high-latitude events. A box model using region-specific loss and growth rates predicts free-troposphere/MBL N3 ratios of 1.3–2.1 and 2.4–2.5 for background and event growth rates, compared with observations in the range of 0.7–1.5. Post-frontal nucleation events were found to contribute from <1 to ~8% of the CCN population depending on season and growth rate. However, these events help maintain the MBL Aitken population, contributing up to ~30%.