Reply on RC2

Martin Daily

doi:10.5194/amt-2021-208-ac2

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.

Highlights

30 In the absence of nucleation sites, cloud water droplets can supercool to temperatures below around −35 °C before freezing via homogeneous nucleation (Ickes et al, 2015; Herbert et al, 2015)
300 Sanidine, which exhibits much lower icenucleating ability (INA) compared to the other samples due to a lack of cracks related to exsolution microtexture
305 In general, the INA of these samples did not respond substantially to wet heating for 30 mins with no significant reductions of T50 in four out of five of the samples of K-feldspar, An exception was Amazonite Microcline which showed a ∆T50wet of −1.5 °C, which was significantly greater than the experimental uncertainty, as defined above

Summary

Introduction

30 In the absence of nucleation sites, cloud water droplets can supercool to temperatures below around −35 °C before freezing via homogeneous nucleation (Ickes et al, 2015; Herbert et al, 2015). A rare subset of atmospheric aerosol particles known as ice-nucleating particles (INPs) can elevate the temperature of ice formation (Murray et al, 2012; Hoose and Möhler, 2012; Kanji et al, 2017). INPs are important because newly formed ice crystals can grow at the expense of supercooled liquid droplets. This is a process that strongly modulates the radiative properties of shallow mixed-phase clouds (i.e., their albedo) 35 (Storelvmo and Tan, 2015; Murray et al, 2021), can initiate precipitation by enhancing collision and coalescence processes (Vergara-Temprado et al, 2018; Rosenfeld et al, 2011) and can influence anvil cirrus properties in deep convective systems (Hawker et al, 2021). Two important general categories of INPs are mineral dust (Hoose et al, 2010; DeMott et al, 2003;Vergara-Temprado et al, 2017) and biogenic materials (Vergara-Temprado et al, 2017; Creamean et al, 2013)

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