Abstract
Abstract. We present a framework for estimating concentrations of episodically elevated high-temperature marine ice nucleating particles (INPs) in the sea surface microlayer and their subsequent emission into the atmospheric boundary layer. These episodic INPs have been observed in multiple ship-based and coastal field campaigns, but the processes controlling their ocean concentrations and transfer to the atmosphere are not yet fully understood. We use a combination of empirical constraints and simulation outputs from an Earth system model to explore different hypotheses for explaining the variability of INP concentrations, and the occurrence of episodic INPs, in the marine atmosphere. In our calculations, we examine the following two proposed oceanic sources of high-temperature INPs: heterotrophic bacteria and marine biopolymer aggregates (MBPAs). Furthermore, we assume that the emission of these INPs is determined by the production of supermicron sea spray aerosol formed from jet drops, with an entrainment probability that is described by Poisson statistics. The concentration of jet drops is derived from the number concentration of supermicron sea spray aerosol calculated from model runs. We then derive the resulting number concentrations of marine high-temperature INPs (at 253 K) in the atmospheric boundary layer and compare their variability to atmospheric observations of INP variability. Specifically, we compare against concentrations of episodically occurring high-temperature INPs observed during field campaigns in the Southern Ocean, the Equatorial Pacific, and the North Atlantic. In this case study, we evaluate our framework at 253 K because reliable observational data at this temperature are available across three different ocean regions, but suitable data are sparse at higher temperatures. We find that heterotrophic bacteria and MBPAs acting as INPs provide only a partial explanation for the observed high INP concentrations. We note, however, that there are still substantial knowledge gaps, particularly concerning the identity of the oceanic INPs contributing most frequently to episodic high-temperature INPs, their specific ice nucleation activity, and the enrichment of their concentrations during the sea–air transfer process. Therefore, targeted measurements investigating the composition of these marine INPs and drivers for their emissions are needed, ideally in combination with modeling studies focused on the potential cloud impacts of these high-temperature INPs.
Highlights
Clouds are important components and drivers of the climate system (Boucher et al, 2013), with mixed-phase cloud processes being a major factor in determining cloud radiative properties (Cesana and Storelvmo, 2017; Vergara-Temprado et al, 2018), the strength of the cloud–climate feedback (McCoy et al, 2016), and the equilibrium climate sensitivity (Tan et al, 2016)
We focus on marine bacteria and marine biopolymer aggregates (MBPAs) as potential sources of episodically emitted marine biogenic ice nucleating particles (INPs)
We show INP concentrations observed at 253 K, with episodic events characterized by concentrations above the threshold derived from the combined INP data set (Fig. 1)
Summary
Clouds are important components and drivers of the climate system (Boucher et al, 2013), with mixed-phase cloud processes being a major factor in determining cloud radiative properties (Cesana and Storelvmo, 2017; Vergara-Temprado et al, 2018), the strength of the cloud–climate feedback (McCoy et al, 2016), and the equilibrium climate sensitivity (Tan et al, 2016). While modeling studies have demonstrated that INPs from surfaceactive macromolecules contained in submicron sea spray aerosol (SSA) can be predicted skillfully (Huang et al, 2018; McCluskey et al, 2019; Zhao et al, 2021), there are still major challenges in representing episodic contributions from marine, biogenic particulate INPs larger than 200 nm (Creamean et al, 2019; Trueblood et al, 2021) These INPs are not explicitly described by currently used parameterizations which depend solely on the observed aerosol surface area or the organic carbon content (Wilson et al, 2015; DeMott et al, 2016; McCluskey et al, 2018a). We evaluate to what extent the observed number concentrations and variability of high-temperature INPs can potentially be explained by emissions of sea spray containing either bacteria or marine gel particles and highlight gaps in our current understanding
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