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. Sea spray aerosol particles are a recognised type ofice-nucleating particles under mixed-phase cloud conditions. Entities thatare responsible for the heterogeneous ice nucleation ability include intactor fragmented cells of marine microorganisms as well as organic matterreleased by cell exudation. Only a small fraction of sea spray aerosol istransported to the upper troposphere, but there are indications frommass-spectrometric analyses of the residuals of sublimated cirrus particlesthat sea salt could also contribute to heterogeneous ice nucleation undercirrus conditions. Experimental studies on the heterogeneous ice nucleationability of sea spray aerosol particles and their proxies at temperaturesbelow 235 K are still scarce. In our article, we summarise previousmeasurements and present a new set of ice nucleation experiments at cirrustemperatures with particles generated from sea surface microlayer andsurface seawater samples collected in three different regions of the Arcticand from a laboratory-grown diatom culture (Skeletonema marinoi). The particles were suspended in the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber and ice formation was induced by expansion cooling. We confirmed that under cirrus conditions, apart from theice-nucleating entities mentioned above, also crystalline inorganic saltconstituents can contribute to heterogeneous ice formation. This takes placeat temperatures below 220 K, where we observed in all experiments a strongimmersion freezing mode due to the only partially deliquesced inorganicsalts. The inferred ice nucleation active surface site densities for thisnucleation mode reached a maximum of about 5×1010 m−2at an ice saturation ratio of 1.3. Much smaller densities in the range of108–109 m−2 were observed at temperatures between 220 and235 K, where the inorganic salts fully deliquesced and only the organicmatter and/or algal cells and cell debris could contribute to heterogeneousice formation. These values are 2 orders of magnitude smaller than those previously reported for particles generated from microlayer suspensionscollected in temperate and subtropical zones. While this difference mightsimply underline the strong variability of the number of ice-nucleating entities in the sea surface microlayer across different geographical regions, we also discuss how instrumental parameters like the aerosolisationmethod and the ice nucleation measurement technique might affect the comparability of the results amongst different studies.

atmospheric chemistry and physics

Heterogeneous ice nucleation ability of aerosol particles generated from Arctic sea surface microlayer and surface seawater samples at cirrus temperatures