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

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, 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.

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 ice-nucleating activity [Christner et al, 2008;Garcia et al, 2012;O'Sullivan et al, 2018]. In contrast, it is assumed that the ice-nucleating activity of 210 the most active component of mineral dusts, K-feldspar, is not heat sensitive, consistent with recent tests [Daily et al, 2021].…”

“…CC BY 4.0 License.To determine whether biological ice-nucleating material contributes to the INP population, we heated sample suspensions to 280 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, 2021;…”

Ice Nucleating Particle Connections to Regional Argentinian Land Surface Emissions and Weather During the Cloud, Aerosol, and Complex Terrain Interactions Experiment

“…Negative impacts of heat on certain mineral INP activities have been noted by Daily et al (2021), although this is primarily a potential influence only at the highest temperatures assessed in this study, primarily for much longer immersion periods, and primarily for mineral types that are not highlighted later in this study as being present in our air samples. The balance of prior work (e.g., Hill et al, 2016;Suski et al, 2018;Tobo et al, 2014) demonstrating equivalent impacts (or lack thereof) of heated peroxide digestion on the INP activity of bulk arable soils (minerals), dry thermal treatment of bulk soils at 300°C, and dry thermal treatment of free-flowing single particles suggests that this is a reasonable assumption, and that the changes seen in INP spectra after treatments are due to impacts on organic INP components.…”

Export of ice-nucleating particles from watersheds: results from the Amazon and Tocantins river plumes

“…However, observations of fluorescent biological aerosol particles above and below a rainforest canopy in Borneo have suggested only a weak coupling of larger aerosol particles between these layers [ 52 ]. Known heat-tolerant INPs that could be washed from litter and soil into the river are mineral particles [ 53 ], lignin [ 54 ] and other heat-stable organics [ 9 , 36 ], such as macromolecules of pollen [ 55 ]. A much stronger source of such heat-tolerant INPs below the canopy in the Amazon compared with the Tocantins watershed would explain (i) the greater abundance of INPs in the Amazon, (ii) the larger fraction of heat-tolerant INPs in the Amazon compared with the Tocantins River plume, and (iii) the difference in heat-tolerant fraction between INPs in the river plume and INPs in the air above the Amazon rainforest.…”