We examined the effect of organic matter released by four different algal species on the hygroscopic growth and droplet activation behaviour of laboratory-generated marine aerosol particles. Hygroscopic growth factors and dry diameters for activation were reduced by less than 10%, compared to that of sodium chloride or of artificial seawater that was devoid of marine surfactants. Concentration-dependent nonideal behaviour was observed for the artificial seawater. But within measurement uncertainty, the measured hygroscopic growth and droplet activation behaviour for the samples that contained organic matter were consistent with a hygroscopicity parameter that was constant between the sub- and supersaturated measurement points. Also, the hygroscopic growth measured for hydrated particles after 3 and after 10 seconds was similar, which implies that in this time range no kinetic effects were detected. 1. Introduction Approximately 71% of the Earth's surface is covered by oceans, which provide a constant source of marine aerosol particles. Atmospheric marine aerosols consist of particulate matter with both, primary and secondary origin. Marine primary particles are produced on the ocean surface by bubble-bursting and tearing from breaking waves, that is, by processes depending on the wind speed. Secondary particulate matter originates from gas-to-particle conversion processes, such as nucleation and condensation [1], and, in the marine case, comprises substances as non-sea-salt sulphate and organic species. Regarding the Earth's radiation budget, marine particles affect both the aerosol direct and indirect effects. Pilinis et al. [2] stated that the single most important parameter in determining direct aerosol forcing is the relative humidity (RH), and the most important process is the increase of the aerosol mass as a result of water uptake. Compared to continental aerosol, marine aerosol particles generally are more hygroscopic (see e.g., [3], where the particle hygroscopicity parameter [4] of the more hygroscopic particle fraction was determined to be around 0.3 for continental and 0.45 for marine aerosol, and with a of 0.95 for an additional (small) sea-salt mode for the marine case). This makes marine aerosol particles particularly susceptible to changes in relative humidity. Likewise, they easily can be activated to cloud droplets at atmospheric relevant supersaturations. The largest sea-salt particles may behave as giant Cloud Condensation Nuclei (CCN), [5] and their role in the initiation of precipitation in warm shallow clouds is still under
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