Evolution of Aerosol Characteristics during a Marine Diatom (T. weissflogii) Bloom

Marine aerosols represent a complex system in which particles vary by composition, size, and source. In order to determine the relationship between phytoplankton growth and the production of biogenic and sea spray aerosols, a diatom bloom (Thalassiosira weissflogii) was studied over the course of a 17-day mesocosm experiment using a laboratory marine aerosol reference tank (MART). During daily sampling, aerosols were generated for a period of thirty minutes using a plunging jet to simulate oceanic bubble-bursting, and sampled above the surface of the artificial seawater containing the diatom culture. Immediately following aerosol generation, total aerosol concentration, particle size distribution (0.3-20 µm), and cloud condensation nuclei (CCN) activity were measured for a period of two hours.

Particles of diameter ranges 0.06 – 1.0 µm and 1.0-8.0 µm were also collected on PIXE cascade impactor stages and analyzed using Raman microspectroscopy to determine chemical composition.

As the bloom progressed from growth phase to stationary phase, and then to death phase, several changes in aerosol characteristics were noted. Transition to stationary phase was accompanied by a decrease in the concentration of < 2.0 µm particles from 10 cm^-3 to < 1 cm^-3, and an increase in the concentration of 2.0-4.0 µm particles by a similar order of magnitude. A simultaneous increase in the CCN activation efficiency of 200 nm aerosols was observed. By the end of stationary phase, the concentration of 2.0-4.0 µm particles decreased to < 10^-2 cm^-3. Transition to death phase resulted in an increase in the concentration of <1.6 µm particles and a decrease in the concentration of larger particles to < 10^-2 cm^-3. Additional changes in aerosol and CCN properties will be summarized in the context of diatom growth, and atmospheric implications will be discussed.