The pristineness of the Southern Ocean aerosol system: a model view

Abstract:

The remote southern ocean regions are often described as being pristine. Here we use global aerosol model simulations to define what we mean by pristine in terms of aerosol concentrations and behaviour. The model results show that the processes and emissions controlling cloud condensation nuclei (CCN) over the southern oceans vary substantially with season and with location. In the true Southern Ocean (south of 60 degrees) much of CCN derives from sulfate aerosol entrained into the boundary layer from the free troposphere, but sea spray emissions dominate further north. The Southern Ocean is the part of the atmosphere where CCN concentrations show the highest sensitivity to variations in DMS emissions, although only part of the seasonal variation can be attributed to changes in biological activity, with the rest coming from variations in atmospheric circulation and particle formation. We define the pristineness of the aerosol system in terms of how closely it resembles conditions in the unperturbed pre-industrial period. By this definition, large parts of the southern oceans are pristine in the southern hemisphere summer, but the aerosol system may be significantly perturbed by anthropogenic emissions in winter. Using a multi-parameter sensitivity analysis we explore to what extent these variations in pristineness affect the aerosol processes, and whether the remote southern ocean is a good analogue for pre-industrial marine aerosols in other parts of the world.