Chemical insights into the ice nucleating ability of macromolecules in immersion freezing

Cloud glaciation is an atmospheric process with important implications for climate and weather. Indeed, clouds made of liquid water and of ice crystals impact the global radiative balance of the atmosphere by reflecting incoming solar radiation and by absorbing outgoing terrestrial radiation. The relevance of ice nucleating particles (INPs) to the atmosphere depends on three main factors, namely on (1) their atmospheric concentration, (2) their freezing temperature and relative humidity, and (3) their freezing mechanism (Cziczo et al., 2013). Research on characterizing ice nucleating organic matter often takes a “top-down” approach where a whole sample of a complex mixture of organic, often biological, macromolecules is subjected to separation techniques and heat treatments to identify IN active sub-components. Studies have used this approach for characterizing bulk soil organic matter, volcanic ash and biological macromolecules from pollen, fungi, and bacteria.

 

We and others have recently found that dissolved organic matter collected from rivers and swamps surprisingly contain active INP (Borduas-Dedekind et al., 2019; Knackstedt et al., 2018; Moffett et al., 2018). Yet, all three studies state that it is unclear which sub-component of the dissolved organic matter is responsible for the ice nucleating ability. There are clear challenges in attributing the ice nucleating ability when starting with a complex mixture of organic and/or biological material, including matrix effects, impurities accumulated through the separation and/or heating process and lack of molecule identity.

 

We present here a “bottom-up” approach to compliment the top-down approach for atmospheric ice nucleation research of macromolecules. Using our home-built drop Freezing Ice Nuclei Counter (FINC) with automated imaging, a range of macromolecules were investigated. Indeed, we have analysed a wide range of dissolved organic matter subcomponents including proteins and fulvic acids. We find a range of ice nucleating ability. We find that lignin, the second most abundant biopolymer in plants, is ice active with 50% frozen fraction temperatures (T₅₀) at –18 °C at a concentration of 100 mg C/L. Furthermore, we have investigated the ice nucleation ability of common diatom exudates and found that at atmospherically relevant concentration they are likely not ice active in immersion freezing within the detection of our FINC instrument. We are currently investigating the effect of atmospheric processing on these macromolecules with the goal of understanding how macromolecules’ ice activity evolves over their one-week lifetime in the atmosphere.