Abstract
Abstract. Better characterisation of aerosol processes in pristine, natural environments, such as Antarctica, have recently been shown to lead to the largest reduction in uncertainties in our understanding of radiative forcing. Our understanding of aerosols in the Antarctic region is currently based on measurements that are often limited to boundary layer air masses at spatially sparse coastal and continental research stations, with only a handful of studies in the vast sea-ice region. In this paper, the first observational study of sub-micron aerosols in the East Antarctic sea ice region is presented. Measurements were conducted aboard the icebreaker Aurora Australis in spring 2012 and found that boundary layer condensation nuclei (CN3) concentrations exhibited a five-fold increase moving across the polar front, with mean polar cell concentrations of 1130 cm−3 – higher than any observed elsewhere in the Antarctic and Southern Ocean region. The absence of evidence for aerosol growth suggested that nucleation was unlikely to be local. Air parcel trajectories indicated significant influence from the free troposphere above the Antarctic continent, implicating this as the likely nucleation region for surface aerosol, a similar conclusion to previous Antarctic aerosol studies. The highest aerosol concentrations were found to correlate with low-pressure systems, suggesting that the passage of cyclones provided an accelerated pathway, delivering air masses quickly from the free troposphere to the surface. After descent from the Antarctic free troposphere, trajectories suggest that sea-ice boundary layer air masses travelled equatorward into the low-albedo Southern Ocean region, transporting with them emissions and these aerosol nuclei which, after growth, may potentially impact on the region's radiative balance. The high aerosol concentrations and their transport pathways described here, could help reduce the discrepancy currently present between simulations and observations of cloud and aerosol over the Southern Ocean.
Highlights
Reconciling the radiation budget over the Southern Ocean poses one of the greatest challenges for current climate models (Shindell et al, 2013; Pierce and Adams, 2006)
If local formation was occurring, it is unlikely that once particles grew beyond 3 nm that growth beyond 10 nm would not occur. Where changes in both CN3−10 and CN10 data occur simultaneously, it can be assumed that the measured population is in a steady state – that is, the number of particles growing into the CN3−10 size bin equals the number leaving and entering the CN10 size range
Theoretical calculations of inlet losses suggest that transmission of CN3−10 particles through the inlet could be as low as 30 % (Appendix Fig. A1), which when applied to the data, could lead to an almost 3-fold increase in number concentrations in this size range
Summary
Reconciling the radiation budget over the Southern Ocean poses one of the greatest challenges for current climate models (Shindell et al, 2013; Pierce and Adams, 2006). If local formation was occurring, it is unlikely that once particles grew beyond 3 nm (where they can be measured with the instruments deployed here) that growth beyond 10 nm would not occur Where changes in both CN3−10 and CN10 data occur simultaneously, it can be assumed that the measured population is in a steady state – that is, the number of particles growing into the CN3−10 size bin equals the number leaving and entering the CN10 size range. This steady state is most achieved when this size flux is zero, as is the case when particles are not growing. If there is no delay, the steady state exists, and aerosol populations are not likely to be growing
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