The ice-nucleating activity of Arctic sea surface microlayer samples and marine algal cultures

Abstract

Abstract. In recent years, sea spray as well as the biological material it contains has received increased attention as a source of ice-nucleating particles (INPs). Such INPs may play a role in remote marine regions, where other sources of INPs are scarce or absent. In the Arctic, these INPs can influence water–ice partitioning in low-level clouds and thereby the cloud lifetime, with consequences for the surface energy budget, sea ice formation and melt, and climate. Marine aerosol is of a diverse nature, so identifying sources of INPs is challenging. One fraction of marine bioaerosol (phytoplankton and their exudates) has been a particular focus of marine INP research. In our study we attempt to address three main questions. Firstly, we compare the ice-nucleating ability of two common phytoplankton species with Arctic seawater microlayer samples using the same instrumentation to see if these phytoplankton species produce ice-nucleating material with sufficient activity to account for the ice nucleation observed in Arctic microlayer samples. We present the first measurements of the ice-nucleating ability of two predominant phytoplankton species: Melosira arctica, a common Arctic diatom species, and Skeletonema marinoi, a ubiquitous diatom species across oceans worldwide. To determine the potential effect of nutrient conditions and characteristics of the algal culture, such as the amount of organic carbon associated with algal cells, on the ice nucleation activity, Skeletonema marinoi was grown under different nutrient regimes. From comparison of the ice nucleation data of the algal cultures to those obtained from a range of sea surface microlayer (SML) samples obtained during three different field expeditions to the Arctic (ACCACIA, NETCARE, and ASCOS), we found that they were not as ice active as the investigated microlayer samples, although these diatoms do produce ice-nucleating material. Secondly, to improve our understanding of local Arctic marine sources as atmospheric INPs we applied two aerosolization techniques to analyse the ice-nucleating ability of aerosolized microlayer and algal samples. The aerosols were generated either by direct nebulization of the undiluted bulk solutions or by the addition of the samples to a sea spray simulation chamber filled with artificial seawater. The latter method generates aerosol particles using a plunging jet to mimic the process of oceanic wave breaking. We observed that the aerosols produced using this approach can be ice active, indicating that the ice-nucleating material in seawater can indeed transfer to the aerosol phase. Thirdly, we attempted to measure ice nucleation activity across the entire temperature range relevant for mixed-phase clouds using a suite of ice nucleation measurement techniques – an expansion cloud chamber, a continuous-flow diffusion chamber, and a cold stage. In order to compare the measurements made using the different instruments, we have normalized the data in relation to the mass of salt present in the nascent sea spray aerosol. At temperatures above 248 K some of the SML samples were very effective at nucleating ice, but there was substantial variability between the different samples. In contrast, there was much less variability between samples below 248 K. We discuss our results in the context of aerosol–cloud interactions in the Arctic with a focus on furthering our understanding of which INP types may be important in the Arctic atmosphere.

Influence of Arctic Microlayers and Algal Cultures on Sea Spray Hygroscopicity and the Possible Implications for Mixed‐Phase Clouds

“…The experiments described herein took place as part of a larger laboratory campaign (Ickes et al, 2020), which was conducted at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) aerosol and cloud chamber (Möhler et al, 2003(Möhler et al, , 2008Wagner et al, 2006Wagner et al, , 2012. The same set of samples used for the ice nucleation studies described by Ickes et al (2020) were used for this work on CCN properties.…”

“…For 50 nm (diameter) particles generated by spiked seawater (SM100 and SM10) in AEGOR a slight (∼7%) increase in CCN activity compared to sea salt is observed, while for the atomized MA100 culture a slight (∼6%) decrease in CCN activity is observed. The fact that the atomized MA100 increased the SS c is likely related to the fact that the salinity of this sample was only 6 PSU (Ickes et al, 2020), compared to ∼35 PSU when spiked in SigmaSS. Overall the agreement with the CCN activity (SS c ) of sea salt is always within 10%.…”

“…Two types of organic containing sample were studied: laboratory grown algal cultures and ambient SML samples. For more explicit details on the samples and sample treatment we refer the reader to the campaign overview paper and references therein (Ickes et al, 2020). A short description is given below.…”

“…The atomizer generates higher number concentrations of aerosols than AEGOR from the same solution. Such high number concentrations were needed in some experiments (Ickes et al, 2020), where an additional suite of instruments were used to probe IN properties of the generated aerosols. Figure 1a shows the AEGOR configuration.…”

A link between the ice nucleation activity and the biogeochemistry of seawater

“…DeMott et al (2016) investigated the ice nucleation activity of seawater from several remote locations, including the Caribbean, the oligotrophic Pacific, and the Bering Sea. Several other studies have focused on high-latitude oceans, including the North Atlantic (Wilbourn et al, 2020;Wilson et al, 2015), Arctic (Ickes et al, 2020;Irish et al, 2017), and Southern oceans (McCluskey et al, 2018b). Gong et al (2020) found that INPs were both enriched and depleted in the sea surface microlayer relative to subsurface water near the islands of Cabo Verde, indicating the effects of both transient biological activity as well as physical parameters such as ocean mixing.…”