An evaluation of the heat test for the ice-nucleating ability of minerals and biological material
Authors: Daily Martin I. , Tarn Mark D. , Whale Thomas F. , Murray Benjamin J. 
Published: May 2022

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.