Ice Nucleating Particles: A Mystery Far from Solved

Primary ice formation via heterogeneous ice nucleation in cold clouds (sub 0 °C) can be one of the primary pathways by which hydrometeors can grow to large enough sizes to form precipitation. Ice nucleating particles (INPs), which initiate the phase change to ice have the potential to influence lifetime and radiative properties of clouds. INPs are a rare subset of ambient aerosol particles and can be as low as 1 in 10⁵ – 10⁶ particles (at -30 °C) in the atmosphere, rendering their measurement and characterization challenging. As such, a detailed understanding of these particles in the ambient population is still lacking (Kanji et al., 2017).

It is suggested that certain chemical and physical properties will make specific sites on the INP surfaces candidates for ice nucleation such as cracks and crevices or ability to hydrogen bond with water (Pruppacher and Klett, 1997). Some macroscopic properties of ambient aerosol such as particle size and number have yielded some predictable capability of INP concentrations (DeMott et al., 2010) which may capture the probability of presence of these specific sites. However, neither particle size nor number have proven to be robust for predicting INP concentrations, but are instead applicable in very defined air mass sources. It is not surprising that some particle chemical classes and particle sizes can be used to predict ice nucleation ability sufficiently (due to the presence of H-bonding capacity for example), but incorporating both these parameters to quantify INP populations in the ambient atmosphere is challenging, complex and requires validation measurements with high resource allocation. Given the low abundance of INP and associated challenge in measurements, a better level of understanding can be achieved from long-term ice nucleation measurements to improve the qualitative and quantitative representation of INPs.

Here it is shown that for a large suite of monitored macroscopic properties and environmental parameters (upwards of 10), including physical (size, number, absorption) and chemical properties (particulate matter components, dust and marine influenced aerosol), as well as meteorological parameters (ambient temperature, relative humidity, wind speed and direction), INP concentrations cannot be correlated sufficiently enough to warrant any statistical prediction. This suggests that INP concentrations are decoupled from the influence of such properties, and that some intrinsic property of INPs is still elusive. A broad categorization into chemical classes shows correlation with observed increases in INP concentration. However, depending on the chemical class of INPs, particle size may or may not be able to predict ice nucleation activity sufficiently. For example, smaller polysaccharides can promote ice nucleation whereas larger sizes of such molecules can act as anti-freeze proteins (Dreischmeier et al., 2017). Thus the effect of different INP sources might be crucial to size-inferred parameterizations.

Laboratory studies to understand the fundamental freezing mechanisms for specific particle morphologies and chemical classes are valuable in governing ice nucleation at various temperature regimes. However, should field studies focus more on quantifying the chemical composition of INPs sampled? The need (or lack there off) to include chemistry measurements with particle physical properties to better predict ice nucleation ability of ambient aerosol particles will be addressed.

References:

DeMott et al. (2010). Predicting global atmospheric ice nuclei distributions and their impacts on climate, PNAS, 107(25), 11217-11222, doi:10.1073/pnas.0910818107

Dreischmeier et al. (2017). Boreal pollen contain ice-nucleating as well as ice-binding 'antifreezee' polysaccharides, Sci. Rep., 7, 41890, doi:10.1038/srep41890n activity well enough.

Kanji et al. (2017). Overview of Ice Nucleating Particles, Met. Mon., 58, 1.1-1.33, doi:10.1175/AMSMONOGRAPHS-D-16-0006.1

Pruppacher and Klett (1997). Microphysics of Clouds and Precipitation, 2nd Edition ed., 976 pp., Kluwer, Dordrecht.