10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Activation of Intact Bacteria and Bacterial Fragments Mixed with Agar as Cloud Droplets and Ice Crystals in Cloud Chamber Experiments
KAITLYN J. SUSKI, David Bell, Naruki Hiranuma, Ottmar Möhler, Dan Imre, Alla Zelenyuk, Pacific Northwest National Laboratory
Abstract Number: 998 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract Bacteria has been extensively studied for its ability to nucleate ice in the immersion regime at modestly supercooled temperatures. Previous studies have shown that while whole bacteria cells are efficient ice nucleating particles (INPs), smaller bacterial fragments and even individual proteins from bacteria can also serve as INPs. Experiments conducted at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber at the Karlsruhe Institute of Technology (KIT) investigated the ice nucleation activity of two strains of cultivated bacteria (pseudomonas syringae and PF CGina) by forming a cloud via expansion at temperatures between -5 and -12 °C. We find that bacterial fragments mixed with agar growth media activate as cloud condensation nuclei (CCN) and INPs, while intact bacteria cells were not observed in cloud droplet and ice crystal residuals.
Cloud droplet residuals were sampled using a pumped counterflow virtual impactor inlet (PCVI) and ice crystal residuals were sampled using an ice-selecting PCVI (IS-PCVI). A single particle mass spectrometer (miniSPLAT) was used to characterize the size and chemistry of aerosol particles and cloud residuals. The aerosol in the AIDA chamber before and after the expansions had a bimodal size distribution with agar mixed with bacterial fragments comprising the smaller size mode and whole bacteria cells making up the larger size mode. Three expansions were performed with two bacteria strains and all expansions showed that the cloud droplet residuals had the same size distribution and mass spectral signatures of the bacteria fragments mixed with agar. Additionally, the ice crystal residuals were also composed of bacteria fragments mixed with agar. Due to the fact that an immersion mode INP must first become incorporated into a cloud droplet, whole bacteria cells were unable to serve as INPs in these experiments due to their limited CCN activity.
Previous studies have suggested that bacteria can range from hydrophobic to slightly hydrophilic due to the varying makeup of the bacteria cell wall. Thus, if bacteria are not hygroscopic, it could explain why they are less CCN active than bacteria fragments that are not completely enveloped in a cell wall and are mixed with a very hygroscopic material, in this case agar. These results suggest that in clouds where water vapor is limited, competition for water vapor between weakly hygroscopic intact bacteria and other more hygroscopic particles could result in a negligible amount of intact bacteria serving as CCN and INP in the immersion mode. In the laboratory, bulk immersion freezing measurements do not capture this competition for water vapor as particles are submerged in a well of water or a water droplet artificially overcoming this barrier to CCN activation. Also, if agar is adding to the hygroscopicity of the bacteria fragments in cultivated bacteria and enhancing their CCN and thus immersion INP activity, the role of agar must be quantified to understand these laboratory data in relation to real clouds.