Marine Polymer-Gels' Relevance in the Atmosphere as Aerosols and CCN

Abstract

Marine polymer gels play a critical role in regulating ocean basin scale biogeochemical dynamics. This brief review introduces the crucial role of marine gels as a source of aerosol particles and cloud condensation nuclei (CCN) in cloud formation processes, emphasizing Arctic marine microgels. We review the gel's composition and relation to aerosols, their emergent properties, and physico-chemical processes that explain their change in size spectra, specifically in relation to aerosols and CCN. Understanding organic aerosols and CCN in this context provides clear benefits to quantifying the role of marine nanogel/microgel in microphysical processes leading to cloud formation. This review emphasizes the DOC-marine gel/aerosolized gel-cloud link, critical to developing accurate climate models.

Keywords: CCN; DOC; SML; aerosols; central Arctic Ocean; marine gels.

Figure 1

Conceptual figure indicating dynamic processes affecting gels as aerosols, CCN, and free polymers. (A) At the ocean–air interface in the surface microlayer, dissolved organic carbon (DOC) polymers assemble in a reversible process into microgels stabilized by entanglements and Ca⁺² bonds and/or hydrophobic moieties. Microgels are then available for air–sea exchange as organic aerosols by diverse processes (bubble bursting/wind). Aerosolized microgels can fragment into smaller size nanometer-size gels by UV exposure, and/or dispersion, or other processes. If the nanometer size gels are activated, they can nucleate into forming CCN. Furthermore, nanogels can further disperse into DOC free polymers, (adapted from Verdugo, 2012). (B) Microgels can also attain nanometer sizes by undergoing volume phase transition induced by environmental conditions such as changes temperature (T), pH, light, H₂SO₄, DMSP, and DMS. These nanometer size gels may also become CCN, however, this route has not been proven yet. (C) TEM pictures of aerosol particles collected over the central Arctic Ocean north of 80°N. Examples of the changing nature of the high Arctic particles in different modal diameters: (a–c) sub-Aitken mode, (a) penta-hexagonal structure, crystalline and hydrophobic in nature assumed to be a colloidal building block of a polymer gel, (b) small polymer gel-aggregate forming a “pearl necklace” morphology possibly indicating hydrophobicity, according to Saiani et al. (2009), slightly covered with hydrophilic viscous but not gelling polymeric material “mucus,” (c) another particle example similar to b, (d–f) Aitken to small accumulation mode, (d) particle with a high sulfuric acid content and with a gel-aggregate inclusion embedded in a viscous non-gelling film of high organic content, (e) gel-aggregate, and a particle resembling a bacterium with a small aggregate attached to it, possibly detached from the larger one. The “bubble-like shaped particles” may indicate a possible recent injection to the atmosphere at the air–sea interface, (f) particle containing mainly ammonium sulfate and methane sulfonate, (g–i) large accumulation mode, (g) a bacterium, (h) sea-salt with an organic content only present at the rare occasion of high winds > 12 m s⁻¹, (h) sea salt and a bacterium coated with an organic film and by the concentric rings typical of droplets of sulfuric acid.