The amount of ice present in clouds can affect cloud lifetime, precipitation and radiative properties 1,2 . The formation of ice in clouds is facilitated by the presence of airborne ice nucleating particles 1,2 . Sea spray is one of the major global sources of atmospheric particles, but it is unclear to what extent these particles are capable of nucleating ice 3-11 . Sea spray aerosol contains large amounts of organic material that is ejected into the atmosphere during bubble bursting at the organically enriched sea-air interface or sea surface microlayer [12][13][14][15][16][17][18][19] . Here we show that organic material in the sea surface microlayer nucleates ice under conditions relevant for mixed-phase cloud and high-altitude ice cloud formation. The ice nucleating material is likely biogenic and less than ~0.2 μm in size. We find that exudates separated from cells of the marine diatom T. Pseudonana nucleate ice and propose that organic material associated with phytoplankton cell exudates is a likely candidate for the observed ice nucleating ability of the microlayer samples. Global model simulations of marine organic aerosol in combination with our measurements suggest that marine organic material may be an important source of ice nucleating particles in remote marine environments such as the Southern Ocean, North Pacific and North Atlantic.Atmospheric ice nucleating particles (INPs) allow ice to nucleate heterogeneously at higher temperatures or lower relative humidity than is typical for homogeneous ice nucleation. Heterogeneous ice nucleation proceeds via different pathways depending on temperature and humidity. In low altitude mixed-phase clouds, INPs are commonly immersed in supercooled liquid droplets and freezing can occur on them at temperatures between -36 and 0°C 2 . At higher altitudes and lower temperatures (<-36°C) where cirrus clouds form, nucleation occurs below water saturation, proceeding by homogeneous, deposition or immersion-in-solution nucleation 1 . Understanding the sources of atmospheric INPs is important because they affect cloud lifetime, cloud albedo and precipitation 1,2 . Recent modelling work has shown that the ocean is potentially an important source of biogenic atmospheric INPs particularly in remote, high latitude regions 9,10 . However, it has never been directly shown that there is a source of atmospheric INPs associated with organic material found in marine waters or sea-spray aerosol.Organic material makes up a substantial fraction of sub-micron sea-spray aerosol and it is estimated that 10±5 Tg yr -1 of primary organic sub-micron aerosol is emitted from marine sources globally 12 . Rising bubbles scavenge surface active organic material from the water column at their interfaces and this process facilitates the formation of the organic enriched sea-air interface known as the sea surface microlayer (SML). This organic material is ejected into the atmosphere during bubble bursting resulting in sea spray aerosol containing similar organic material to that of the microlaye...

23 This study addresses, through two types of experiments, the potential for the oceans to act as a 24 source of atmospheric ice-nucleating particles (INPs). The INP concentration via deposition 25 mode nucleation was measured in situ at a coastal site in British Columbia in August 2013. The 26 INP concentration at conditions relevant to cirrus clouds (i.e., -40°C and relative humidity with 27 respect to ice, RHice=139%) ranged from 0.2 L -1 to 3.3 L -1 . Correlations of the INP 28 concentrations with levels of anthropogenic tracers (i.e., CO, SO2, NOx, and black carbon) and 29 numbers of fluorescent particles do not indicate a significant influence from anthropogenic 30 sources or submicron bioaerosols, respectively. Additionally, the INPs measured in the 31 deposition mode showed a poor correlation with the concentration of particles with sizes larger 32 than 500 nm, which is in contrast with observations made in the immersion freezing mode. To 33 investigate the nature of particles that could have acted as deposition INP, laboratory 34 experiments with potential marine aerosol particles were conducted under the ice-nucleating 35 conditions used in the field. At -40°C, no deposition activity was observed with salt aerosol 36 particles (sodium chloride and two forms of commercial sea salt: Sigma-Aldrich and Instant 37 Ocean), particles composed of a commercial source of natural organic matter (Suwannee River 38 humic material), or particle mixtures of sea salt and humic material. In contrast, exudates from 39 three phytoplankton (Thalassiosira pseudonana, Nanochloris atomus, and Emiliania huxleyi) 40 and one marine bacterium (Vibrio harveyi) exhibited INP activity at low RHice values, down to 41 below 110%. This suggests that the INPs measured at the field site were of marine biological 42 origins, although we cannot rule out other sources, including mineral dust. 43 44 thank the University of Denver faculty start-up fund and PROF grant for 463 partial financial support. 464 465 References 466 Alpert, P.A., Aller, J.Y., and Knopf, D.A., 2011a. Ice nucleation from aqueous NaCl droplets with and 467 without marine diatoms, Atmos. Chem. Phys, 11, 5539-5555. 469Alpert, P. A., Aller, J. Y., and Knopf, D. A., 2011b. Initiation of the ice phase by marine biogenic surfaces 470 in supersaturated gas and supercooled aqueous phases, Phys.

Article:Aller, JY, Radway, JC, Kilthau, WP et al. ReuseUnless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Takedown

The oceans cover most of the Earth's surface, contain nearly half the total global primary biomass productivity, and are a major source of atmospheric aerosol particles. Here we experimentally investigate links between biological activity in seawater and sea spray aerosol (SSA) flux, a relationship of potential significance for organic aerosol loading and cloud formation over the oceans and thus for climate globally. Bubbles were generated in laboratory mesocosm experiments either by recirculating impinging water jets or glass frits. Experiments were conducted with Atlantic Ocean seawater collected off the eastern end of Long Island, NY, and with artificial seawater containing cultures of bacteria and phytoplankton Thalassiosira pseudonana, Emiliania huxleyi, and Nannochloris atomus. Changes in SSA size distributions occurred during all phases of bacterial and phytoplankton growth, as characterized by cell concentrations, dissolved organic carbon, total particulate carbon, and transparent exopolymer particles (gel-forming polysaccharides representing a major component of biogenic exudate material). Over a 2 week growth period, SSA particle concentrations increased by a factor of less than 2 when only bacteria were present and by a factor of about 3 when bacteria and phytoplankton were present. Production of jet-generated SSA particles of diameter less than 200 nm increased with time, while production of all particle diameters increased with time when frits were used. The implications of a marine biological activity dependent SSA flux are discussed.

Sea spray aerosol (SSA) is a widely recognized important source of ice-nucleating particles (INPs) in the atmosphere. However, composition-specific identification, nucleation processes, and ice nucleation rates of SSA-INPs have not been well constrained. Microspectroscopic characterization of ambient and laboratory-generated SSA confirms that water-borne exudates from planktonic microorganisms composed of a mixture of proteinaceous and polysaccharidic compounds act as ice-nucleating agents (INAs). These data and data from previously published mesocosm and wave channel studies are subsequently used to further develop the stochastic freezing model (SFM) producing ice nucleation rate coefficients for SSA-INPs. The SFM simultaneously predicts immersion freezing and deposition and homogeneous ice nucleation by SSA particles under tropospheric conditions. Predicted INP concentrations agree with ambient and laboratory measurements. In addition, this holistic freezing model is independent of the source and exact composition of the SSA particles, making it well suited for implementation in cloud and climate models.