An evaluation of the heat test for the ice-nucleating ability of minerals and biological material

Abstract

Abstract. 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. 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 some 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 up 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 biogenic INP proxy materials and found that cellulose and pollen washings were relatively resistant to wet heat. In contrast, bacterially and fungally derived ice-nucleating samples were highly sensitive to wet heat as expected, although their activity remained non-negligible after wet heating. Dry heating at 250 ∘C leads to deactivation of all biogenic INPs. However, the use of dry heat at 250 ∘C for the detection of biological INPs is limited since K-feldspar's activity is also reduced under these conditions. Future work should focus on finding a set of dry heat conditions where all biological material is deactivated, but key mineral types are not. We conclude that, while wet INP heat tests at (>90 ∘C) 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 very heat-sensitive biogenic INPs in ambient samples if the mineral-based INA is controlled by K-feldspar.

The ice-nucleating activity of African mineral dust in the Caribbean boundary layer

“…A subset of samples underwent heat testing to evaluate the presence of protein-based biological INPs, since ice-active proteins associated with some classes of biological INPs can be denatured with heat, causing a decrease in their icenucleating activity (O' Sullivan et al, 2018;Christner et al, 2008;Garcia et al, 2012). In contrast, it is assumed that the ice-nucleating activity of the most active component of mineral dusts, K-feldspar, is not heat-sensitive, consistent with recent tests (Daily et al, 2022). A selection of suspensions were heated to 100 • C for 1 h by immersing a suspension in a sealed polypropylene falcon tube within boiling water.…”

“…To determine whether biological ice-nucleating material contributes to the INP population, we heated sample suspensions to about 100 • C. Protein-based biological INPs are denatured by heat, and hence if they are present, we expect to see a decrease in activity, whereas the activity of K-feldspar is affected only marginally (a deactivation of around 1 • C) (Daily et al, 2022;O'Sullivan et al, 2018). On heating a selection of aerosol suspensions, we observed that there was no substantial decrease in ice-nucleating activity and therefore conclude that proteinaceous biological INPs were not the dominant INP type (Figs.…”

“…This assumption is supported by the SEM-EDS analysis, which showed S1. We also show the heat response of a 0.05 wt % Snowmax ® (lyophilized Pseudomonas syringae), a protein-based ice-nucleating material that is highly sensitive to heat from Daily et al (2022). These heat tests are wet heat tests, where particles were suspended in water and heated in a bath of boiling water.…”

The Urmia playa as a source of airborne dust and ice-nucleating particles – Part 2: Unraveling the relationship between soil dust composition and ice nucleation activity

“…Finally, the samples were dried in the oven at 60 • C and stored in 20 mL glass vials for further analysis. Based on the study by Daily et al (2022), samples needed to be heated above 90 • C to damage the IN activity of the minerals. Therefore, we do not expect a negative effect on the IN activity of the mineral components due to the heating required to remove organic matter; yet, we cannot exclude it.…”

A highly active mineral-based ice nucleating agent supports in situ cell cryopreservation in a high throughput format

“…While the physical reasons for their hyperactivity remain unclear, their active sites are thought to be concentrated on microscopic steps, cracks and pores [ 43 , 44 ]. These sites are unlikely to be proteinaceous ice nucleating contaminants due to their resistance to heat treatments [ 42 , 45 ].…”