Preprints
https://doi.org/10.5194/amt-2021-208
https://doi.org/10.5194/amt-2021-208

  29 Jul 2021

29 Jul 2021

Review status: this preprint is currently under review for the journal AMT.

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

Martin Ian Daily1, Mark Duncan Tarn1, Thomas Francis Whale1,a, and Benjamin John Murray1 Martin Ian Daily et al.
  • 1Institute of Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
  • acurrent address: Department of Chemistry, University of Warwick, Gibbet Hill, Coventry, CV4 7AL, UK

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.

Martin Ian Daily et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'RC: Reviewer comment on Daily et al. AMTD 2021', Anonymous Referee #1, 28 Aug 2021
  • RC2: 'Comment on amt-2021-208', Anonymous Referee #2, 11 Sep 2021

Martin Ian Daily et al.

Martin Ian Daily et al.

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Short summary
Mineral dust and particles of biological origin are important types of ice-nucleating particles (INP) that can trigger ice formation of supercooled cloud droplets. Heat treatments are used to “detect” the presence of biological INP in samples collected from the environment as the activity of mineral INP is assumed unchanged, yet this has not been fully assessed. We show that the ice-nucleating ability of some minerals can change after heating and discuss how INP heat tests should be interpreted.