Hydrothermal activity indirectly influences ice nuclei particles in seawater and nascent sea spray of the Subtropical Pacific Ocean

Abstract. Particles of marine origin may act as ice nuclei when clouds form and therefore influence cloud properties and lifetime. Here we investigate the abundance of Ice Nuclei Particles in bulk seawater (INP_SW) collected in natural seawater of the Western Tropical South Pacific and in sea spray aerosol (INP_SSA) artificially generated from the surface seawater. The study area was separated into two oligotrophic zones (the Melanesian Basin and the Western South Pacific Gyre), and a mesotrophic one (the Lau basin), characterized by high plankton biomass due iron fertilization by underwater hydrothermal activity of the Tonga volcanic arc. Our results show that INP_SW were on average 80 % heat labile, strongly suggesting a biological origin. INP_SW concentrations were two-fold higher in the Lau basin as compared to both oligotrophic areas at all freezing temperatures. This trend is consistent with a higher abundance of planktonic microorganisms, pigments and particulate organic carbon (POC) concentrations in the Lau basin. Over the whole cruise transect, medium to strong correlations were found between INP_SW concentrations and pigments (notably with bacteriochlorophyll-a and carotene), bacterial abundance and POC. The heat stable fraction of INP_SW exhibited correlations with Dissolved Organic Carbon (DOC) concentrations and were not as variable as the heat labile INP_SW. In the nascent sea spray, INP_SSA were also mostly heat labile in coherence with the INP_SW. INP_SSA were predominantly (60 %), submicron in size (presumed originating from film drops), but the supermicron INP_SSA constituted 40 % of the INP_SSA and were all heat labile (presumably originating from jet drops). Supermicron INP_SSA were between 60 to 80 % heat stable with a high variability between samples, indicating different nature of the two fractions of INPs. Supermicron INP_SSA were generally more abundant in the Lau basin, while submicron INP_SSA did not exhibit any significant difference between the three regions. We report a transfer function of seawater INPs to SSA INPs of 1.70 m^-2.L_SW and 3.3 m^-2.L_SW for heat stable INPs, hinting that heat stable INPs were more efficiently transferred to the SSA. Our results suggest that hydrothermal activity indirectly enhances the INP concentration of surface waters, through boosting the biological activity, which results in increases of the ice forming ability of supermicron sea spray particle. Given the extent of hydrothermal activity throughout the global Ocean, its impact on cloud properties should be considered in future ocean-atmosphere interaction studies.