Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratorygenerated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0°C, averaging an order of magnitude increase per 5°C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using "dry" geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.marine aerosols | ice nucleation | clouds

Abstract. Soil organic matter (SOM) may be a significant source of atmospheric ice nucleating particles (INPs), especially of those active > −15 • C. However, due to both a lack of investigations and the complexity of the SOM itself, the identities of these INPs remain unknown. To more comprehensively characterize organic INPs we tested locally representative soils in Wyoming and Colorado for total organic By contrast, lysozyme, which digests bacterial cell walls, only reduced INPs active at ≥ −7.5 or ≥ −6 • C, depending on the soil. The known IN bacteria were not detected in any soil, using PCR for the ina gene that codes for the active protein. We directly isolated and photographed two INPs from soil, using repeated cycles of freeze testing and subdivision of droplets of dilute soil suspensions; they were complex and apparently organic entities. Ice nucleation activity was not affected by digestion of Proteinase K-susceptible proteins or the removal of entities composed of fulvic and humic acids, sterols, or aliphatic alcohol monolayers. Organic INPs active colder than −10 to −12 • C were resistant to all investigations other than heat, oxidation with H 2 O 2 , and, for some, digestion with papain. They may originate from decomposing plant material, microbial biomass, and/or the humin component of the SOM. In the case of the latter then they are most likely to be a carbohydrate. Reflecting the diversity of the SOM itself, soil INPs have a range of sources which occur with differing relative abundances.

Abstract. Biological residues in soil dust are a potentially strong source of atmospheric ice nuclei (IN). So far, however, the abundance, diversity, sources, seasonality, and role of biological – in particular, fungal – IN in soil dust have not been characterized. By analysis of the culturable fungi in topsoils, from a range of different land use and ecosystem types in southeast Wyoming, we found ice-nucleation-active (INA) fungi to be both widespread and abundant, particularly in soils with recent inputs of decomposable organic matter. Across all investigated soils, 8% of fungal isolates were INA. All INA isolates initiated freezing at −5 to −6 °C, and belonged to a single zygomycotic species, Mortierella alpina (Mortierellales, Mortierellomycotina). To our knowledge this is the first report of ice nucleation activity in a zygomycotic fungi because the few known INA fungi all belong to the phyla Ascomycota and Basidiomycota. M. alpina is known to be saprobic and widespread in soil, and Mortierella spores are present in air and rain. Sequencing of the ITS region and the gene for γ-linolenic elongase revealed four distinct clades, affiliated to different soil types. The IN produced by M. alpina seem to be proteinaceous, < 300 kDa in size, and can be easily washed off the mycelium. Ice nucleating fungal mycelium will ramify topsoils and probably also release cell-free IN into it. If these IN survive decomposition or are adsorbed onto mineral surfaces, their contribution might accumulate over time, perhaps to be transported with soil dust and influencing its ice nucleating properties.

We have known for around 4 decades that soil, decaying vegetation, and plant surfaces harbor organic ice-nucleating particles (INP) and that their abundance and high temperature of activity suggest they are significant sources of atmospheric ice nuclei (1-8). The focus of more recent research has been to elucidate their identities, reservoirs, and emissions to and role, if any (9), in the troposphere (10-22).DeMott and Prenni (23) identified key outstanding questions regarding the role of biogenic INP in cloud and precipitation processes. These included whether they are the sole source of natural atmospheric ice nucleators warmer than about Ϫ15°C, whether they are sufficiently abundant to trigger precipitation directly and/or via secondary ice multiplication, which operates principally in the range of Ϫ3 to Ϫ8°C (24,25,26), whether there are large seasonal and regional variations in their emissions, and whether their numbers can be defined and their sources identified. This work reports progress toward answering the first and last priorities through the development and application of a quantitative PCR (qPCR) test for the ice nucleation-active (INA) bacteria, the abundant and highly active source of biological INP (12).To date, around a dozen species of INA bacteria have been identified, isolated mainly from plant surfaces. They are spread among three orders of the Gammaproteobacteria and include Pseudomonas syringae, Ps. fluorescens, Pantoea agglomerans, and

Abstract. Heterogeneous ice nucleation is a crucial process for forming ice-containing clouds and subsequent iceinduced precipitation. The importance for ice nucleation by airborne desert soil dusts composed predominantly of minerals is widely acknowledged. However, the potential influence of agricultural soil dusts on ice nucleation has been poorly recognized, despite recent estimates that they may account for up to 20-25 % of the global atmospheric dust load. We have conducted freezing experiments with various dusts, including agricultural soil dusts derived from the largest dustsource region in North America. Here we show evidence for the significant role of soil organic matter (SOM) in particles acting as ice nuclei (IN) under mixed-phase cloud conditions. We find that the ice-nucleating ability of the agricultural soil dusts is similar to that of desert soil dusts, but is clearly reduced after either H 2 O 2 digestion or dry heating to 300 • C. In addition, based on chemical composition analysis, we demonstrate that organic-rich particles are more important than mineral particles for the ice-nucleating ability of the agricultural soil dusts at temperatures warmer than about −36 • C. Finally, we suggest that such organic-rich particles of agricultural origin (namely, SOM particles) may contribute significantly to the ubiquity of organic-rich IN in the global atmosphere.