Algal exudate in seawater and its influence on particle hygroscopic growth and activation

We want to present results of a study, where we examined the influence of algal exudate, i.e. of biogenic organics, in seawater samples on hygroscopic growth and droplet activation behaviour of particles produced from the samples. The algal exudates were obtained from culturing four different phytoplankton species (Chaetoceros sp., Emiliana huxleyi, Phaeocystis, and Thalassiosira rotula), which are all major contributors to primary production of organic matter in the ocean. Mixtures of these exudates were added to seawater, yielding seawater proxies with concentrations of dissolved organic carbon on the order of those expected in regions of high biological activity. These thus prepared samples were examined, together with a sample of artificial seawater (that contained no organics) and with algae medium, which was the initial natural seawater sample with added nutrients employed for growing the algal cultures. The hygroscopic growth of samples containing algal exudate was measured twice, once after 3 and once after 10 seconds of residence time at humidified conditions. No change in the measured hygroscopic growth was detected, i.e. no kinetic effects could be observed in this time range. Hygroscopic growth and particle activation of the algal exudate samples and of the algae medium were found to be reduced by less than 10%, compared to artificial seawater, and they were clearly above that of ammonium sulphate. For the determination of the hygroscopicity parameter of the samples, the surface tension of water was used. Concentration dependent non-ideal behaviour was observed for the artificial seawater and for the algae medium sample, following that described for NaCl. This concentration dependence was not observed for the samples containing algal exudate. Therefore it is possible to use a single parameter description of the hygroscopicity of the particles generated from the algal exudate samples, independent of the concentration of inorganic and organic solutes in the particle or droplet solution, with a single hygroscopicity parameter as e.g. ρ_ion or κ. Overall the examined algal exudates in the samples did not complicate the description of the hygroscopic behaviour of particles generated from these samples but rather made it more simple. It should be stressed, however, that marine particles can contain additional matter acquired from the gas phase, where marine sources can add inorganic matter (e.g. non-sea-salt sulphates from Dimethylsulfide) and organic matter, and to which also anthropogenic sources and the terrestrial biosphere can contribute.