Abstract
Abstract. Utilising the Leipzig Aerosol Cloud Interaction Simulator (LACIS), the immersion freezing behaviour of droplet ensembles containing monodisperse particles, generated from a Snomax™ solution/suspension, was investigated. Thereto ice fractions were measured in the temperature range between −5 °C to −38 °C. Snomax™ is an industrial product applied for artificial snow production and contains Pseudomonas syringae} bacteria which have long been used as model organism for atmospheric relevant ice nucleation active (INA) bacteria. The ice nucleation activity of such bacteria is controlled by INA protein complexes in their outer membrane. In our experiments, ice fractions increased steeply in the temperature range from about −6 °C to about −10 °C and then levelled off at ice fractions smaller than one. The plateau implies that not all examined droplets contained an INA protein complex. Assuming the INA protein complexes to be Poisson distributed over the investigated droplet populations, we developed the CHESS model (stoCHastic modEl of similar and poiSSon distributed ice nuclei) which allows for the calculation of ice fractions as function of temperature and time for a given nucleation rate. Matching calculated and measured ice fractions, we determined and parameterised the nucleation rate of INA protein complexes exhibiting class III ice nucleation behaviour. Utilising the CHESS model, together with the determined nucleation rate, we compared predictions from the model to experimental data from the literature and found good agreement. We found that (a) the heterogeneous ice nucleation rate expression quantifying the ice nucleation behaviour of the INA protein complex is capable of describing the ice nucleation behaviour observed in various experiments for both, Snomax™ and P. syringae bacteria, (b) the ice nucleation rate, and its temperature dependence, seem to be very similar regardless of whether the INA protein complexes inducing ice nucleation are attached to the outer membrane of intact bacteria or membrane fragments, (c) the temperature range in which heterogeneous droplet freezing occurs, and the fraction of droplets being able to freeze, both depend on the actual number of INA protein complexes present in the droplet ensemble, and (d) possible artifacts suspected to occur in connection with the drop freezing method, i.e., the method frequently used by biologist for quantifying ice nucleation behaviour, are of minor importance, at least for substances such as P. syringae, which induce freezing at comparably high temperatures. The last statement implies that for single ice nucleation entities such as INA protein complexes, it is the number of entities present in the droplet population, and the entities' nucleation rate, which control the freezing behaviour of the droplet population. Quantities such as ice active surface site density are not suitable in this context. The results obtained in this study allow a different perspective on the quantification of the immersion freezing behaviour of bacterial ice nucleation.
Highlights
Ocean Science dictions from the model to experimental data from the literature and found good agreement.We found that (a) the heterogeneous ice nucleation rate expression quantifying the ice nucleation behaviour of the ice nucleation active (INA) protein complex is capable of describing the ice nucleation behaviour observed in various experiments for both, Ice formation in clouds influences the formation of precipitation and cloud radiative properties and impacts both weather and climate (DeMott et al, 2003; Lohmann, 2006).Ice formation in clouds occurSs oeilthider Ethraourtghh homogeneous or heterogeneous ice nucleation
Our results imply that only 20 % of the 650 nm and 40 % of the 800 nm particles generated from SnomaxTM solution/suspension contain one or more INA protein complexes, and that the unfrozen droplets do not even contain a single INA protein complex
At Leipzig Aerosol Cloud Interaction Simulator (LACIS), the immersion freezing behaviour of monodisperse particles generated from a SnomaxTM solution/suspension was investigated by measuring ice fractions as a function of temperature in the range between −6.6 ◦C and −38 ◦C
Summary
Ocean Science dictions from the model to experimental data from the literature and found good agreement. It is concluded that oligomers consisting of two up to a few single proteins could correspond to class III IN, i.e., initiate freezing in the temperature range from about −7 ◦C to −10 ◦C (Govindarajan and Lindow, 1988; Garnham et al, 2011), where the majority of the INA bacteria can be ice nucleation active. We take a different perspective in quantifying the bacteria’s ice nucleation behaviour, i.e., we are not looking at the number of bacteria, but the number of ice nucleation active protein complexes present in the investigated droplets and we use ice nucleation rates instead of onset temperatures and frozen fractions
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