The sensitivity of the ice-nucleating ability of minerals to heat and the implications for the heat test for biological ice nucleators

Ice-nucleating particles (INPs) are atmospheric aerosol particles that can strongly influence the radiative properties and precipitation onset in mixed-phase clouds by triggering ice formation in supercooled cloud water droplets. The ability to distinguish between INPs of mineral and biological origin in samples collected from the environment is needed to better understand their distribution and sources, but this is challenging. A common method for assessing the relative contributions of mineral and biogenic INPs in samples collected from the environment (e.g., aerosol, rainwater, soil) is to determine the ice-nucleating ability (INA) before and after heating, where heat is expected to denature proteins associated with biological ice nucleants. The key assumption is that the ice nucleation sites of biological origin are denatured by heat, while those associated with mineral surfaces remain unaffected; we test this assumption here. We exposed atmospherically relevant mineral samples to wet heat (INP suspensions warmed to above 90 °C) or dry heat (dry samples heated to 250 °C) and assessed the effects on their immersion mode INA using a droplet freezing assay. K-feldspar, thought to be the dominant mineral-based atmospheric INP type where present, was not significantly affected by wet heating, while quartz, plagioclase feldspars and Arizona test Dust (ATD) lost INA when heated in this mode. We argue that these reductions in INA in the aqueous phase result from direct alteration of the mineral particle surfaces by heat treatment rather than from biological or organic contamination. We hypothesise that degradation of active sites by dissolution of mineral surfaces is the mechanism in all cases due to the correlation between mineral INA deactivation magnitudes and their dissolution rates. Dry heating produced minor but repeatable deactivations in K-feldspar particles but was generally less likely to deactivate minerals compared to wet heating. We also heat tested proteinaceous and non-proteinaceous biogenic INP proxy materials and found that non-proteinaceous samples (cellulose and pollen) were relatively heat resistant. In contrast, the proteinaceous ice-nucleating samples were highly sensitive to wet and dry heat, as expected, although their activity remained non-negligible after heating. We conclude that, while INP heat tests have the potential to produce false positives, i.e., deactivation of a mineral INA that could be misconstrued as the presence of biogenic INPs, they are still a valid method for qualitatively detecting proteinaceous biogenic INP in ambient samples, so long as the mineral-based INA is controlled by K-feldspar.

Understanding Bacterial Ice Nucleation

<p>Bacterial ice-nucleating proteins (INPs) promote heterogeneous ice nucleation better than any known material. On the molecular scale, bacterial INPs are believed to function by organizing water into ice‑like patterns to enable the formation of embryonic crystals. However, the details of their working mechanism remains largely elusive. Here, we report the results of comprehensive evaluations of environmentally relevant effects such as changes in pH, the presence of ions and temperature on the activity, three-dimensional structure and hydration shell of bacterial ice nucleators using ice affinity purification, high-throughput ice nucleation assays and surface-specific sum-frequency generation spectroscopy.</p><p> </p><p>[1] Lukas, Max, et al. "Electrostatic Interactions Control the Functionality of Bacterial Ice Nucleators." Journal of the American Chemical Society 142.15 (2020): 6842-6846.</p><p>[2] Lukas, Max, et al. "Interfacial Water Ordering Is Insufficient to Explain Ice-Nucleating Protein Activity." The Journal of Physical Chemistry Letters 12 (2020): 218-223.</p>

The Surface of Tree Tissues as Source of Extractable Ice Nucleating Macromolecules during Rainfall Events

<p>Several biological particles are able to trigger heterogeneous ice nucleation at subzero temperatures above -38°C. Many plants species such as winter rye [1], certain berries [2], pines and birches [3, 4] are known to contain biological ice-nucleating particles (BINPs) or rather ice-nucleating macromolecules (INMs). However, the influence of these BINPs on atmospheric processes including cloud glaciation and precipitation formation, as well as transport mechanisms of BINPs from the land surface into the atmosphere remain uncertain. If those INMs are easily available on the surfaces of a plant, they could be washed down by heavy rain events and could add an important new source for BINPs in the atmosphere, which has not received enough attention in the past.</p><p>In this study, we have focused on alpine trees, which form INMs extractable from their surfaces. We examined ice nucleation activity of samples from different birches (Betula pendula) and pines (Pinus sylvestris) growing in the Alps in Austria, Europe. Filtered aqueous extracts of leaves, needles, bark and wood were analyzed in the laboratory in terms of heterogeneous ice nucleation using VODCA (Vienna Optical Droplet Crystallization Analyzer), a cryo-microscope  for  emulsion  samples.  All plant tissues contained INMs in the submicron size range. Furthermore, we conducted a field experiment, in which we investigated the possibility of INMs to be released from the surface of the trees into the atmosphere during rain showers.</p><p>[1] Brush, R.A., M. Griffith, and A. Mlynarz, Characterization and Quantification of Intrinsic Ice Nucleators in Winter Rye (Secale cereale) Leaves. Plant Physiol, 1994. <strong>104</strong>(2): p. 725-735.</p><p>[2] Felgitsch, L., et al., Heterogeneous Freezing of Liquid Suspensions Including Juices and Extracts from Berries and Leaves from Perennial Plants. Atmosphere, 2019. <strong>10</strong>(1): p. 1-22.</p><p> [3] Pomeroy, M.K., D. Siminovitch, and F. Wightman, Seasonal biochemical changes in the living bark and needles of red pine (Pinus resinosa) in relation to adaptation to freezing. Canadian Journal of Botany, 1970. <strong>48</strong>(5): p. 953-967.</p><p>[4] Felgitsch, L., et al., Birch leaves and branches as a source of ice-nucleating macromolecules. Atmospheric Chemistry and Physics, 2018. <strong>18</strong>(21): p. 16063-16079.</p><p>[5] Pummer, B.G., et al., Ice nucleation by water-soluble macromolecules. Atmospheric Chemistry and Physics, 2015. <strong>15</strong>(8): p. 4077-4091.</p>

Cloud history changes water–ice–surface interactions of oxide mineral aerosols (e.g. Silica)

Mineral aerosol particles can act as ice nucleators, and many insights have been obtained on water freezing as a function of mineral surface properties such as the charge or morphology. Previous studies have mainly focused on pristine samples, despite the fact that under natural atmospheric conditions, aerosol particles age. For example, an aerosol-containing cloud droplet can go through different freeze-melt cycles, so that not only the aerosol surface structure may change, but also the ionic strength and pH of the cloud droplet. The potential variation of the surface properties of an ice nucleating particle during its residence in the atmosphere has been largely overlooked. Here, we use an environmental cell in conjunction with nonlinear spectroscopy (second-harmonic generation) to study the effect of freeze-melt processes on aqueous chemistry at silica surface at low pH. We found that the successive freeze-melt cycles disrupt the dissolution equilibrium, substantially changing the surface properties, giving rise to marked variations in the interfacial water structure and the ice nucleation ability of the surface. The degree-of-order of water molecules, next to the surface at a specific temperature, decreases and then increases again with sample aging. The water ordering–cooling dependence and ice nucleation ability improve continuously.