Recent analyses of atmospheric aerosols from different regions have demonstrated the ubiquitous presence of strong surfactants and evidenced surface tension values, σ, below 40 mN m(-1), suspected to enhance the cloud-forming potential of these aerosols. In this work, this approach was further improved and combined with absolute concentration measurements of aerosol surfactants by colorimetric titration. This analysis was applied to PM2.5 aerosols collected at the Baltic station of Askö, Sweden, from July to October 2010. Strong surfactants were found in all the sampled aerosols, with σ = (32-40) ± 1 mN m(-1) and concentrations of at least 27 ± 6 mM or 104 ± 21 pmol m(-3). The absolute surface tension curves and critical micelle concentrations (CMC) determined for these aerosol surfactants show that (1) surfactants are concentrated enough in atmospheric particles to strongly depress the surface tension until activation, and (2) the surface tension does not follow the Szyszkowski equation during activation but is nearly constant and minimal, which provides new insights on cloud droplet activation. In addition, both the CMCs determined and the correlation (R(2) ∼ 0.7) between aerosol surfactant concentrations and chlorophyll-a seawater concentrations suggest a marine and biological origin for these compounds.