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 (INPSW) collected in natural seawater of the Western Tropical South Pacific and in sea spray aerosol (INPSSA) 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 INPSW were on average 80 % heat labile, strongly suggesting a biological origin. INPSW 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 INPSW concentrations and pigments (notably with bacteriochlorophyll-a and carotene), bacterial abundance and POC. The heat stable fraction of INPSW exhibited correlations with Dissolved Organic Carbon (DOC) concentrations and were not as variable as the heat labile INPSW. In the nascent sea spray, INPSSA were also mostly heat labile in coherence with the INPSW. INPSSA were predominantly (60 %), submicron in size (presumed originating from film drops), but the supermicron INPSSA constituted 40 % of the INPSSA and were all heat labile (presumably originating from jet drops). Supermicron INPSSA were between 60 to 80 % heat stable with a high variability between samples, indicating different nature of the two fractions of INPs. Supermicron INPSSA were generally more abundant in the Lau basin, while submicron INPSSA 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.LSW and 3.3 m-2.LSW 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.