Size and compositional dependent effects of marine aerosol on cloud condensation nuclei

Abstract

This work investigates marine aerosol physico-chemical properties (e.g. size and chemistry) and its Cloud Condensation Nuclei (CCN) properties under natural background conditions. Black carbon (BC), a tracer for anthropogenic pollution, was used to classify Southern Ocean air mass cleanliness, where the study focussed on anthropogenic influences and was compared to the North East Atlantic, which is closer to pollution sources. Despite this, the lowest prevailing BC mass concentration levels were similar for either ocean (~0.1 ng m-3) with extreme pollution levels above 80 ng m-3 for about 0.3 % of the time over both observation periods. In order to elucidate the relative contribution of ‘primary’ wind-produced sea spray and ‘secondary’ gas-to-particle aerosols to marine cloud droplet formation, a novel detailed analysis of droplet activation critical supersaturation versus critical diameter was conducted in remote environmental marine air (i.e. maritime polar and modified continental Antarctic air masses) in parallel to modelled chemically-homogenous aerosols. The analysis revealed that, for realistic marine boundary layer cloud supersaturations, primary CCN contributed 8–51 % to the estimated cloud droplet concentration (as determined by the Hoppel intermodal-minimum) at wind speeds < 16 m s−1. At higher wind speeds, primary marine aerosol could contribute up to 100 % of estimated cloud droplet concentration. It was observed that within air masses enriched with sea spray CCN, the contribution of secondary (mainly non-sea-salt-sulphate) particles to cloud droplet concentration was significantly reduced despite a higher availability of sulphate CCN. Further analysis revealed a highly correlated inverse linear trend between activated sea spray particles and the percentage of activated sulphate particles. In practice, the addition of sea-salt CCN appeared to suppress the activation of sulphate CCN. An ensemble of three 1-D microphysical droplet growth and activation parcel models corroborated this suppression effect and found that under favourable conditions, as much as a ~100 % enhancement in cloud droplet concentration were predicted as the availability of sea-salt nuclei decreased and vertical updraft increased.