Ice nucleation ability of mineral dust particles mixed with biological substances

It is known that ice in clouds plays an important role for both, climate and weather as it influences precipitation initiation and radiative forcing. Ice formation in clouds occurs either through homogeneous or heterogeneous ice nucleation. For the latter case an insoluble particle called ice nucleus (IN) causes freezing at temperatures higher than observed for homogeneous ice nucleation. The most abundant IN in the atmosphere is mineral dust (e.g. Pratt et al., 2008). Several laboratory studies showed that mineral dust particles are ice active at temperatures lower than -20 °C (e.g. Murray et al., 2012). In contrast, Lidar measurements observed ice nucleation in the atmosphere at temperatures higher than -20 °C (e.g. Kanitz et al., 2011). A possible explanation for this discrepancy could be the presents of biological particles such as bacteria or pollen which are known to nucleate ice at temperatures similar to those observed in the atmosphere.

It is known that submicron ice nucleating active (INA) entities (e.g. INA proteins in the case of bacteria, (e.g. Wolber et al., 1986) and INA macromolecules in the case of pollen (Pummer et al., 2012)) are the reason for this ice nucleating ability. Furthermore, recent studies suggest, that these INA entities maintain their ice nucleating ability even when being separated from their original carriers (bacterial cell or pollen grain, Hartmann et al., 2013; Augustin et al., 2012). This opens the possibility of accumulation of such INA macromolecules in e.g. soils. If such soils are then dispersed into the atmosphere due to e.g. wind erosion or agricultural processes, the biological entities, internally or externally mixed with the soil dust particles, could induce ice nucleation at temperatures higher then -20°C.

To explore this hypothesis, we investigated the ice nucleation behavior of mineral dust particles internally mixed with INA macromolecules. Specifically, we mixed pure mineral dust with INA biological material and quantified the immersion freezing behavior of the resulting particles utilizing the Leipzig Aerosol Cloud Interaction Simulator (LACIS, Hartmann et al., 2011). Therefore we produced an Illit suspension and mixed it with suspensions of SNOMAX or birch pollen washing water with a ratio of one to one. An atomizer (following the design of TSI 3075) is used to generate droplets from the mixed suspension. Subsequently these droplets are dried by passing them through a silica gel diffusion dryer. The dried particles are size selected (chosen mobility diameter of 500 nm) using a differential mobility analyzer (DMA). When passing through the LACIS flow tube, the particles are activated to droplets and due to further cooling the droplets may freeze. It should be noted that inside LACIS each droplet contains only one single size segregated particle. At the outlet of LACIS, the Thermally Stabilized Optical Particle Spectrometer (TOPS-ICE, developed and built at TROPOS, Clauss et al., 2013) determined the number and the phase state of the hydrometeors enabling us to calculate temperature dependent ice fractions (number of frozen droplets divided by the total number of frozen and unfrozen droplets).

Additionally, single particle mass spectrometry and electron microscopy are used to determine the fraction of biological material on the dust particles. Results indicate that the freezing of particles consisting of dust and biological material is governed by the biological component.

Acknowledgement: This work is partly funded by the German Research Foundation DFG within the framework of the Ice Nucleation research UNiT (INUIT).

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