Resolving the controls over the production and emission of ice-nucleating particles in sea spray

Abstract

The role of marine ice-nucleating particles (INPs) in modifying clouds and radiation balance over oceans is uncertain. While recent studies have advanced our understanding of the abundance of marine INPs, characterizing their sources and composition remains a challenge. INP concentrations above oceans are typically low, sometimes extraordinarily so, but there is evidence of elevated levels associated with phytoplankton blooms. Mesocosm experiments have shown that ice-nucleating entities (INEs, which include discrete particles as well as ice-nucleating monolayers) are produced, and INP emissions raised, in the decay phase following bloom collapse. To test if INE production depends upon phytoplankton type, we added dead particulate biomass of a green alga (Nannochloris atomus), a diatom (Skeletonema marinoi) and a cyanobacterium (Prochlorococcus marinus) to a miniature Marine Aerosol Reference Tank filled with seawater. As decomposition progressed, heterotrophic bacteria initially increased and plateaued, then declined, coinciding with an increase in heterotrophic nanoflagellates (HNF) and viruses. Enzyme activities typically increased over several days before plateauing or decreasing, while humic-like substances (HULIS) steadily accumulated. INEs in the seawater peaked 3–5 days after each detritus addition, increasing ∼10- to ∼20-fold. INE concentration was closely correlated with HNF counts, viruses and the concentration of HULIS, but not with bacteria or enzyme activities. Newly-fabricated INEs were organic, primarily heat stable (95 °C), and varied in size. INP concentrations in sea spray aerosol (SSA) tended to peak shortly before the peak of INEs in the seawater, at 4-, 35- and 15-fold higher than at the start in the N. atomus, S. marinoi, and P. marinus incubations, respectively. Using data from the P. marinus incubation, we were able to provide the first estimate of INP enrichment in SSA (over its concentration in the water): it was initially ∼200× for the fresh seawater and increased further after the addition of the P. marinus inoculum. We also tested if a simple nutrient mix (bovine serum albumin (BSA) and three monosaccharides) could stimulate INP production: INEs in the seawater changed little, but INP emissions fell abruptly immediately upon BSA addition due to it forming a monolayer which displaced the sea surface microlayer (SML). These experiments revealed that INE production in the decay phase of a phytoplankton bloom requires the addition of a natural, complex substrate to initiate a realistic succession of decomposers, and that INP emissions are further controlled by their concentration in the SML and, indirectly, by the impact of SML composition upon jet and film drop production.