Ice Nucleating Activity and Residual Particle Morphology of Bulk Seawater and Sea Surface Microlayer

Ice nucleating particles (INPs) in sea spray aerosols (SSAs) are critical in estimating cloud radiative forcing and precipitation with implications in climate change. Laboratory experiments simulating the ocean-atmosphere environment are becoming increasingly popular for studying the nature of INPs in SSAs. Understanding the ice nucleating characteristics of bulk seawater and the sea surface microlayer (SSML) can provide valuable information about the emitted SSAs. Samples for this study were collected from a waveflume during a phytoplankton bloom and analyzed with complementary methods. The primary method used is a polydimethylsiloxane (PDMS)-based microfluidic static well array, designed and fabricated to trap droplets and measure the ice nucleation (IN) spectra of nanoliter-scale droplets of bulk seawater and SSML. Droplets were subsequently dehydrated in situ until efflorescence, and the dried residual particle morphology was correlated to the droplet IN temperature. Four distinct morphologies were found in the effloresced droplets, among which the aggregate and amorphous morphologies were present in larger amounts in the SSML compared to bulk. These particles also had a different IN spectra, nucleating ice at warmer temperatures than the single and fractal crystal morphologies. The microfluidic studies were complemented by micro-Raman spectroscopy and immersion freezing measurements of larger droplets assessing IN sensitivity to heat and organic carbon removal in an ice spectrometer (IS). This study highlights the compositional diversity of marine samples and paves the way for novel multiplexed microfluidic approaches to study the chemical and biological complexity behind the IN activity of aerosol liquid samples in an integrated platform.