A Two-Component Parameterization of Marine Ice Nucleating Particles Based on Seawater Biology and Sea Spray Aerosol Measurements in the Mediterranean Sea.

Ice nucleating particles (INP) have a large impact on the climate-relevant properties of clouds over the oceans. Studies have shown that sea spray aerosols (SSA), produced upon bursting of bubbles at the ocean surface, can be an important source of marine INP, particularly during periods of enhanced biological productivity. Recent mesocosm experiments using natural seawater spiked with nutrients have revealed that marine INP are derived from two separate classes of organic matter in SSA. Despite this finding, existing parameterizations for marine INP abundance are based solely on single variables such as total organic carbon (TOC) or SSA surface area, which may mask specific trends in the separate classes of INPs. The goal of this paper is to improve the understanding of the connection between ocean biology and marine INP abundance by reporting results from a field study and proposing a new parameterization of marine INP that accounts for the two associated classes of organic matter. The PEACETIME cruise took place from May 10 to June 10, 2017 in the Mediterranean Sea. Throughout the cruise, INP concentrations in the surface microlayer (SML) and in SSA produced using a plunging aquarium apparatus were continuously monitored while surface seawater (SSW) and SML biological properties were measured in parallel. The organic content of artificially generated SSA was also evaluated. A dust wet deposition event that occurred during the cruise increased the INP concentrations measured in the SML by an order of magnitude, in line with increases of iron in the SML and bacterial abundances. Increases of INPs in marine SSA (INP[sub SSA]) were not observed before a delay of three days compared to increases in the SML, and are likely a result of a strong influence of bulk SSW INP for the temperatures investigated (T=-18°C for SSA, T=-16°C for SSW). Results confirmed that INP_SSA are divided into two classes depending on their associated organic matter. Here we find that warm (T ≥ -22C) INP_SSA concentrations are correlated with water soluble organic matter in the SSA, but also to SSW parameters (POCSSW INP_SSW,-16C) while cold INP_SSA (T < -22C) are correlated with SSA water-insoluble organic carbon (WIOC) and SML dissolved organic carbon (DOC) concentration. A relationship was also found between cold INP_SSA and SSW microphytoplankton cell abundances, indicating that these species might be at the origin of water insoluble organic matter with surfactant properties and specific IN properties. Using these results, we propose a two-component parameterization for the abundance of INP in marine SSA and compare it with previous single-component models based on SSA surface area and TOC content. This new, two-component parameterization should improve attempts to incorporate marine INP emissions into numerical models. Future studies will be conducted to confirm if our parameterization can be extended to regions of higher biological productivity, such as the Southern Ocean. [ABSTRACT FROM AUTHOR]