Overview of aerosol observations from the Marion Dufresne Atmospheric Program – Indian Ocean (MAP-IO) program

The study of marine aerosols size distribution and cloud condensation nuclei (CCN) properties is of major interest as they influence clouds life and clouds radiative properties, particularly in the remote ocean which remains poorly documented. Several short campaigns focusing on specific regions as phytoplankton bloom regions, pristine regions or remote areas influenced by continental air masses took place to address this issue. However, long sampling periods targeting different in-situ conditions had not been realized.

In this context, the MAP-IO program was launched with the aim of providing a large new set of marine aerosol observations (size distribution from 10 nm to 10 µm and CCN properties) on different sea state and meteorological conditions. Thus, the Marion Dufresne vessel has been equipped with a set of various instruments described in Tulet et al. (in preparation) or on the website www.mapio.re. Two years after the launch of the program, we now have aerosol observations (about 200 days) over an area covering 50 ° of latitudes and extending from the Tropics to the upper Southern Ocean. 

These measurements were first used to investigate the size distribution and CCN variability of marine aerosols according to local conditions (sea states and wind speed).The results highlight that at the lowest latitudes (south of 50 °S) the minimum concentration of CCN tends to increase when the wind speed exceeds approximately 12 m s-1, which is consistent with the literature as sea salt  emissions are mechanically driven by local conditions and tend to be predominant from 10 m s-1. .When analyzing the size distributions of aerosols according to the wind speed during a 5-day storm that occurred in the Southern Ocean, we found that: (1) the number of particles with a diameter less than 500 nm is predominant and stable over the full range of wind speeds (4 to 33 m s-1), (2) the number of aerosols with diameter greater than 500 nm remains low under 10 m s-1 and increases from 10 m s-1 to 33 m s-1 to finally reach the concentration of the particles with diameter less than 500 nm at 33 m s-1. 

Taking this first analysis into account, further work will focus on average size distributions per region, season, origin of air masses (from simulated FLEXPART back trajectories) and wind speed conditions. Analysis of these distributions is unique due to the size of the database, the variability of regions encountered and knowing that the measurements were carried out with the same experimental device. 

 Finally, to deepen the study, the activation diameter of marine aerosols will be determined and the hygroscopicity parameter Kappa-Köhler will be calculated (Petters and Kreidenweis, 2007) in this case to distinguish sea salts (Kappa~1.2) from organic matter (0.01<Kappa<0.5).