Surface tension and anionic, cationic, and non-ionic surfactant concentration in aerosols from contrasting regions: filling the gaps in the ecosystems-aerosol-cloud relationship.

SSCI-VIDE+ATARI+VGE:BNO:CFE:LFIInternational audienceClouds are essential components of the Earths hydrological and climate systems but predicting their formation is still a challenge because of the limited knowledge on the physical-chemical processes involved. In particular, the role surfactants in transforming atmospheric aerosols into cloud droplets has been largely considered as negligible until now. But many recent studies, both in laboratory and in the atmosphere, have brought evidence of the contrary. Some of them consisted in the development of a specific extraction method for these compounds, and evidenced the presence of strong surfactants in aerosols from many different regions. In this work, this method has been further improved and combined with absolute concentration measurements for anionic, cationic and non-ionic surfactants, in order to quantify their importance for cloud activation and investigate their sources. These concentrations were measured by colorimetric methods, where the double extraction approach was found to eliminate all interferring species (inorganic salts, organic acids), thus ensuring the reliability of the results. Applying these analyses to PM2.5 aerosol fractions from the Baltic station of Askö, Sweden, collected from July to October 2010 showed the presence of strong surfactants (smin 30 - 40 mN/m) in all samples, with a total concentration between 27 and 143 mM or 100 and 800 pmol m-3, dominated by anionic and non-ionic surfactants, while cationic ones were minor. The absolute surface tension curves obtained also showed with certainty and, to our knowledge for the first time, that the surfactants in these aerosols were concentrated enough to strongly lower the surface tension (s < 40 mN/M) during the entire activation process into cloud droplets. The CMC values determined for these aerosol surfactants (47 to 280 µM) were in the typical range for microbial ones, and correlations (R2 = 0.65 - 0.75) between their concentrations and seawater chlorophyll a concentrations near the site obtained from MODIS aqua satellite data, provide compelling evidence for a mainly marine and biogenic origin for these compounds