Immersion freezing of biological particles

Abstract

ice nucleation in mixed-phase clouds can already occur at a temperature higher than - 20 °C (Seifert et al., 2010). In contrast, laboratory studies showed that, e.g., mineral dust and soot particles, which are two of the major components of ice crystal residues (e.g., Pratt et al., 2009), function as ice nuclei (IN) at lower temperatures. One possible explanation for the observed temperature differences might be the presence of biological particles (one out of three particles have biological origin, Pratt et al., 2009) acting as IN already at temperatures above - 20 °C. Biological particles such as bacteria and pollen possess membrane proteins and non-proteinaceous macro-molecules, respectively, acting as template for ice cluster formation reducing the energy barrier required for ice nucleation. Nevertheless, the dominating freezing mechanism, the properties inducing ice nucleation, and the relative importance of biological particles in the atmosphere are still unclear. In our study, the immersion freezing behaviour of different size-segregated biological particles is investigated at the laminar flow tube LACIS (Leipzig Aerosol Cloud Interaction Simulator, Hartmann et al., 2011). SNOMAX®, outer membrane vesicles (OMV) and surface material of pollen was used for the analysis. SNOMAX® is considered as convenient surrogate for bacterial IN and contains ice nucleation active proteins. Bacterial strains, here extracted and cultured from rain samples (Temkiv et al., 2011), have the ability to produce outer membrane vesicles, which might also contain ice nucleation active proteins. Surface material washed off from pollen grains (e.g. birch) includes macro-molecules which induce ice nucleation (Pummer et al., 2011). Figure 1 shows the ice fraction fice (number of frozen droplets per sum of frozen and unfrozen droplets) as function of temperature for all biological particles investigated. For 800 nm SNOMAX® the freezing curve is very steep in a temperature range from -5 °C to -10 °C and levels off to constant values of about 0.4, i.e., 40 % of SNOMAX® particles are ice active. The ice fractions observed for the OMV containing particles where close to the detection limit (in the order of 1 % and lower). This indicates that only a small fraction of the OMV particles nucleate ice. However, considering the small size of the OMV particles (ranging between 50 nm and 160 nm as shown in Temkiv et al., 2011) it might be possible that compared to SNOMAX®, the significantly reduced IN ability is mainly due to size effects. The IN contained in the washing water of birch pollen grains nucleated ice below -18 °C with fice curve having a slightly shallower slope than observed for SNOMAX®. About 80 % of the 800 nm particles might include at least one ice nucleation macro-molecule. In conclusion, investigating the ice nucleation behavior of quasi monodisperse biological IN, SNOMAX® was found to be the most ice nucleation active substance, followed by the residues of birch pollen washing water. Outer membrane vesicles featured the lowest ice nucleation potential. This work is part of the DFG research unit INUIT, under contract WE 4722/1-1. Hartmann, S. et al. (2011) Atmos. Chem. Phys., 11, 1753-1767. Pratt, K. et al. (2009) Nat. Geosci., 2, 397-400. Pummer, B. G. et al. (2011) Atmos. Chem. Phys. Discuss., 11, 27219-27241. Seifert, P. et al. (2010) J. Geophys. Res.-Atmos., 115, 13. Temkiv, T. S. et al. (2011) Bacteria in Clouds. PhD dissertation, Graduate School of Science and Technolopgy, Aarhus University, Denmark.