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...

Predicting the formation of ice in the atmosphere presents one of the great challenges in 14 physical sciences with important implications for the chemistry and composition of the Earth's atmos-15 phere, the hydrological cycle, and climate. Among atmospheric ice formation processes, heterogeneous 16 ice nucleation proceeds on aerosol particles ranging from a few nanometers to micrometers in size, 17 commonly referred to as ice nucleating particles (INPs). Research over the last two decades has demon-18 strated that organic matter (OM) is ubiquitous in the atmosphere, present as organic aerosol (OA) parti-19 cles or as coatings on other particle types. The physicochemical properties of OM make predicting how 20 OM can contribute to the INP population challenging. This review focuses on the role of OM in INPs, 21

[1] Secondary organic aerosol (SOA) generated from the oxidation of organic gases are ubiquitous in the atmosphere, but their interaction with water vapor and their ice cloud formation potential at low temperatures remains highly uncertain. We report on onset conditions of water uptake and ice nucleation by amorphous SOA particles generated from the oxidation of naphthalene with OH radicals. Water uptake above 230 K was governed by the oxidation level of the SOA particles expressed as oxygen-to-carbon (O/C) ratio, followed by moisture-induced phase transitions and immersion freezing. For temperatures from 200 to 230 K, SOA particles nucleated ice via deposition mode from supersaturated water vapor independent of O/C ratio at relative humidity with respect to ice (RH ice ) $10-15% below homogeneous ice nucleation limits. The glass transition temperature (T g ) for the amorphous SOA particles was derived as a function of two parameters: (1) relative humidity (RH) with respect to water and (2) oxidation level of the SOA. The data show that particle phase and viscosity govern the particles' response to temperature and RH and provide a straightforward interpretation for the observed different heterogeneous ice nucleation pathways and water uptake by the laboratory-generated SOA and field-collected particles. Since SOA particles undergo glass transitions, these observations suggest that atmospheric SOA are potentially important for ice cloud formation and climate.

An important mechanism for ice cloud formation in the Earth's atmosphere is homogeneous nucleation of ice in aqueous droplets, and this process is generally assumed to produce hexagonal ice. However, there are some reports that the metastable crystalline phase of ice, cubic ice, may form in the Earth's atmosphere. Here we present laboratory experiments demonstrating that cubic ice forms when micrometre-sized droplets of pure water and aqueous solutions freeze homogeneously at cooling rates approaching those found in the atmosphere. We find that the formation of cubic ice is dominant when droplets freeze at temperatures below 190 K, which is in the temperature range relevant for polar stratospheric clouds and clouds in the tropical tropopause region. These results, together with heat transfer calculations, suggest that cubic ice will form in the Earth's atmosphere. If there were a significant fraction of cubic ice in some cold clouds this could increase their water vapour pressure, and modify their microphysics and ice particle size distributions. Under specific conditions this may lead to enhanced dehydration of the tropopause region.

[1] Field studies have shown that mineral dust particles can act as ice nuclei in cirrus clouds. Here, we present a laboratory investigation of heterogeneous ice nucleation on surrogates of mineral dust particles, in particular pure Arizona test dust (ATD) particles, and ATD particles coated with sulfuric acid. The experiments have been performed using a new apparatus in which ice formation on the particles is determined by optical microscopy at temperatures between 197 and 260 K and relative humidities up to water saturation. The experiments reveal that pure and sulfuric acid coated ATD particles nucleate ice at considerably lower relative humidities than required for homogeneous ice nucleation in liquid aerosols. Nucleation occurred over a broad relative humidity range indicating that the different minerals contained in ATD have different ice nucleation thresholds. No significant difference in the ice nucleation ability of pure and H 2 SO 4 coated ATD particles was observed. Below 240 K, ice nucleated on ATD particles apparently by deposition nucleation. Preactivation of ATD particles, that is, a reduction in supersaturation, required for heterogeneous ice nucleation after a previous ice nucleation event on the same particle, has been observed for temperatures as low as 200 K. Differences of 10-30% in the onset RH ice values were obtained for particles with or without preactivation. The results indicate that pure and sulfuric acid coated mineral dust particles may act as efficient ice nuclei in the atmosphere. Preactivation of the particles should be considered when modeling long-range transport of mineral dust particles and their impact on cloud formation.