Aerosolization, Chemical Characterization, Hygroscopicity and Ice Formation of Marine Biogenic Particles

The oceans cover the majority of the earth's surface, host nearly half the total global primary productivity and are a major source of atmospheric aerosol particles. However, effects of biological activity on sea spray generation and composition, and subsequent cloud formation are not well understood. Our goal is to elucidate these effects which will be particularly important over nutrient rich seas, where microorganisms can reach concentrations of 10^9 per mL and along with transparent exopolymer particles (TEP) can become aerosolized. Here we report the results of mesocosm experiments in which bubbles were generated by two methods, either recirculating impinging water jets or glass frits, in natural or artificial seawater containing bacteria and unialgal cultures of three representative phytoplankton species, Thalassiosira pseudonana, Emiliania huxleyi, and Nannochloris atomus. Over time we followed the size distribution of aerosolized particles as well as their hygroscopicity, heterogeneous ice nucleation potential, and individual physical-chemical characteristics. Numbers of cells and the mass of dissolved and particulate organic carbon (DOC, POC), TEP (which includes polysaccharide-containing microgels and nanogels >0.4 μm in diameter) were determined in the bulk water, the surface microlayer, and aerosolized material. Aerosolized particles were also impacted onto substrates for ice nucleation and water uptake experiments, elemental analysis using computer controlled scanning electron microscopy and energy dispersive analysis of X-rays (CCSEM/EDX), and determination of carbon bonding with scanning transmission X-ray microscopy and near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). Regardless of bubble generation method, the overall concentration of aerosol particles, TEP, POC and DOC increased as concentrations of bacterial and phytoplankton cells increased, stabilized, and subsequently declined. Particles <100 nm generated by means of jets were enhanced with time compared with larger sizes. In contrast, all particle sizes were equally enhanced when frits were used. Aerosolized particles were hygroscopic, a finding with significance for warm cloud formation and potential liquid-to-ice phase transformations. Aqueous and dry aerosolized particles from biologically active mesocosm water were found to efficiently nucleate ice exposed to supersaturated water vapor. The majority of particles, including those nucleating ice, consisted of a sea salt core coated with organic material dominated by the carboxyl functional group, and corresponded to a particle type commonly found in marine air. Our results provide improved estimates of marine aerosol production, chemical composition, and hygroscopicity, as well as an accurate physical and chemical representation of ice nucleation by marine biogenic aerosol particles for use in cloud and climate models.