Influence of Biogenic Constituents on the Heterogeneous Ice Nucleation Ability of Sea Salt Aerosol Particles at Cirrus Temperatures

In situ measurements of the composition of the residuals of heterogeneously formed cirrus crystals have indicated a substantial abundance of sea salt in sampling regions above the ocean. Independent laboratory measurements have confirmed that inorganic sea salt aerosol (SSA) particles are capable of acting as ice-nucleating particles (INPs) at cirrus temperatures. Most recently, we have used the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber of the Karlsruhe Institute of Technology to obtain a comprehensive data set for the ice nucleation efficiencies of particles generated from two commercially available sea salt proxies (Instant Ocean sea salt and Sigma Aldrich sea salt) and from a bulk Atlantic water sample in the temperature range from 235 to 200 K. The mixed-phase (solid/liquid) SSA particles were found to act as INPs in the immersion freezing mode at temperatures below about 220 K, with freezing onsets in terms of the ice saturation ratio, S_ice, between 1.24 and 1.42 for temperatures between 219 and 202 K. Above 220 K, the SSA particles fully deliquesced and nucleated ice homogeneously.

In the present contribution, we explore how the ice nucleation ability of the SSA particles at cirrus temperatures changes if particles enriched in organic matter or biological material are present. It is well known that during phytoplankton blooms, SSA particles enriched in biogenic material can be generated at the air-sea interface via the bubble-bursting process. Numerous studies have shown that organic matter in the sea surface microlayer is capable of nucleating ice via immersion freezing in low-altitude mixed-phase clouds. In contrast, the number of investigations that have addressed the impact of organic matter on the efficiency of the heterogeneous freezing mode of SSA particles at cirrus temperatures is still limited. In a new series of expansion cooling experiments at the AIDA facility, we have therefore probed the low-temperature ice nucleation behavior of a marine diatom species (Skeletonema marinoi) and a variety of sea surface microlayer samples collected during several Arctic Ocean research cruises. Results from this campaign along with the potential implications are presented.