CCN in the marine boundary layer over the Atlantic Ocean

The concentration of cloud condensation nuclei (CCN) influences the optical and physical properties of clouds. Significant uncertainties are related to the roles of and climate forcing of aerosol cloud interactions in the climate system. Global climate is in particular sensitive to aerosol-cloud interactions over the oceans due to the large surface of the oceans and the significant difference in albedo between clouds and the open ocean. It is very challenging to incorporate marine boundary layer (MBL) clouds correctly in climate models (Bony and Dufresne, 2005). Measurements of CCN in marine environments in general are scarce. The present study will focus on measurements of CCN activity during different seasons in the MBL covering a large region of the Atlantic Ocean.

Measurements were carried out on three ship cruises with the German research vessel FS Polarstern between Cape Town, South Africa and Bremerhaven, Germany in April/May and in October/November 2011, as well as between Punta Arenas, Chile and Bremerhaven in April/May 2012. Land based measurements were carried out during June/July 2013 at Ragged Point (13°N, 59°W) located on the east coast of Barbados. The CCN counter (Droplet Measurement Technologies, USA) was operated with supersaturations in the range from 0.1% to 0.4% on the cruises. On the cruises a differential mobility analyser (DMA) was operated in front of the CCN counter, and the CCN activity was thus inferred from size segregated measurements of CCN. The hygroscopicity parameter κ (Petters and Kreidenweis, 2007) was derived. A high variability in κ was observed which could be explained by different origin of the studied air masses. On Barbados the CCN counter was operated with supersaturations in the range from 0.1% to 0.7% and the total CCN concentration was measured. The CCN activities and κ values were derived by comparing the CCN concentration to the size distributions measured with an aerodynamic particle sizer (APS) and a scanning mobility particle sizer (SMPS).

During the cruises the chemical composition of non-refractory PM1 was characterised with an aerosol mass spectrometer (AMS, Aerodyne Research Inc., USA). Together with backtrajectories the studied airmasses could thus be characterized as (a) background marine, (b) influenced by continental anthropogenic emissions or (c) dominated by Saharan dust. κ values at around 0.2 to 0.3 were inferred when the airmasses were expected to be dominated by continental emissions, which corresponds well to the modelled κ value of 0.2-0.3 when marine environments are dominated by outflow from the Saharan desert and the κ=0.36±0.16 modelled for continental European aerosol (Pringle et al., 2010). The majority of the studied airmasses were expected to represent background marine conditions. A median κ value about 0.5-0.6 was inferred for all the measurements, which is found to be very close to model results of κ values of 0.59±0.18 for the North Atlantic and in the lower range of a modelled κ value of 0.72±0.24 for clean marine environments as reported by Pringle et al. (2010).

The hygroscopicity parameter κ was modelled based on the chemical composition of the aerosol particles measured with the AMS as done by e.g. Jurányi et al., (2010). A very good agreement between κ inferred from CCN measurements and κ inferred from AMS measurements was observed for large parts of the dataset.

On Barbados total particle number concentrations were typically around 300 cm-3, and total CCN concentrations were thus relatively low. The studied airmasses were found to be influenced by Saharan dust during most of the campaign, which influenced the measured CCN activities.

In general our observations compares very well to the model results presented by Pringle et al. (2010), and it is evident that continental emissions and long range transport may influence the CCN activity in marine environments significantly.