Ice nucleation by water-soluble macromolecules

Abstract

Cloud glaciation is critically important for the global radiation budget (albedo) and for initiation of precipitation. But the freezing of pure water droplets requires cooling to temperatures as low as 235 K. Freezing at higher temperatures requires the presence of an ice nucleator, which serves as a template for arranging water molecules in an ice-like manner. It is often assumed that these ice nucleators have to be insoluble particles. We point out that also free macromolecules which are dissolved in water can efficiently induce ice nucleation: the size of such ice nucleating macromolecules (INMs) is in the range of nanometers, corresponding to the size of the critical ice embryo. As the latter is temperature-dependent, we see a correlation between the size of INMs and the ice nucleation temperature as predicted by classical nucleation theory. Different types of INMs have been found in a wide range of biological species and comprise a variety of chemical structures including proteins, saccharides, and lipids. Our investigation of the fungal species Acremonium implicatum, Isaria farinosa, and Mortierella alpina shows that their ice nucleation activity is caused by proteinaceous water-soluble INMs. We combine these new results and literature data on INMs from fungi, bacteria, and pollen with theoretical calculations to develop a chemical in-terpretation of ice nucleation and water-soluble INMs. This has atmospheric implications since many of these INMs can be released by fragmentation of the carrier cell and subsequently may be distributed independently. Up to now, this process has not been accounted for in atmospheric models

Characterization of free amino acids, bacteria and fungi in size-segregated atmospheric aerosols in boreal forest: seasonal patterns, abundances and size distributions

“…Elevated PBAP abundances occur during the pollen season (Manninen et al, 2014), and based on our estimation even up to ∼ 77 % of total PM may be of biological origin (see Supplement for details). Even though our estimation is highly uncertain, the magnitude of biological cloud-active particles during this period may be atmospherically relevant (Diehl et al, 2002(Diehl et al, , 2001Pummer et al, 2012Pummer et al, , 2015. The importance of rainfall was observed in this study as well as to some extent in previous studies conducted at the same site (Manninen et al, 2014;Schumacher et al, 2013).…”

Refreeze experiments with water droplets containing different types of ice nuclei interpreted by classical nucleation theory

“…If we compare this area with the range of a calculated critical site area of 20-50 nm 2 , a considerable part of the macromolecules' surfaces should be involved in ice nucleation. Pummer et al (2015), who consider the macromolecules to be polysaccharides, attribute the ice nucleation ability to a hydration shell around the polysaccharides. This hydration shell might form an ice template that does not randomly dissociate like ice embryos in homogeneous ice nucleation.…”

Potential of polarization lidar to provide profiles of CCN- and INP-relevant aerosol parameters

“…In the case of heterogeneous ice nucleation it is found that mineral dust particles are favorable INP at temperatures below about −20 • C Murray et al, 2012;Augustin-Bauditz et al, 2014) that marine particles seem to be comparably inefficient INPs at temperatures > −25 • C, whereas continental aerosols (mixtures of anthropogenic haze, biomass burning smoke, soil and road dust, and organic and biogenic particles from soils and plants) seem to contain always a significant amount of efficient INPs, already leading to ice nucleation at temperatures as high as −5 to −15 • C (Seifert et al, 2010;Zhang et al, 2010;Kamphus et al, 2010;Ebert et al, 2011;Augustin et al, 2013;Hartmann et al, 2013;Bühl et al, 2013;Pummer et al, 2015;Umo et al, 2015).…”