Quantifying the efficiency with which biological particles nucleate ice when immersed in supercooled water droplets

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Clouds composed of both ice particles and supercooled liquid droplets, known as mixed phase clouds, exist at temperatures above ~-35oC and cover a large portion of the planet. These clouds impact climate by both simultaneously warming the planet by trapping outgoing infrared radiation and cooling the planet by reflecting incoming visible light from the sun back to space. It is becoming increasingly apparent that mixed phase clouds are very sensitive to the number and type of particles, known as aerosols, present in the atmosphere. A lot of work has been done in the past to understand the role of aerosols on clouds that are entirely composed of liquid droplets and the Intergovernmental Panel on Climate Change (IPCC) attempted to quantify this impact, albeit with large uncertainties. However, the role that aerosols play in ice formation, which dramatically alters the properties of a cloud, remains very uncertain and the IPCC were not in a position to assess this forcing despite evidence that the impact is very large. Aerosols that can catalyse ice particle formation are known as ice nuclei; however their identity, concentration, global distribution and the efficiency with which they nucleate ice are all poorly quantified at present. There is mounting evidence from field studies that biogenic particles, such as bacteria, pollen or fungal spores, nucleate ice in clouds. It has been known for some time that about 25% of insoluble aerosols can be of biogenic origin, but their role in cloud formation remains highly uncertain. In the past few years technological advancements in field equipment have led to the discovery that a major fraction of particles which can serve as ice nuclei in the atmosphere are of biogenic origin. In an aircraft campaign, it was found that a third of the ice crystals in a cloud over Wyoming contained biogenic material (Pratt et al., Nature Geosci, 2, 398. 2009). In a separate study biogenic material dominated the ice nuclei populations above -25oC in the Amazon (Prenni et al., Nature Geosci, 2, 402, 2009). Hoose (Nature Geosci, 2, 385, 2009) suggests that these discoveries may represent 'the tip of the iceberg'. Hence, it is clear that biogenic aerosols are strongly correlated with ice yet their proper treatment in cloud and climate models is missing and their ice nucleation properties are very poorly characterised with huge gaps in basic knowledge. Modelling studies give conflicting results, with some models suggesting a major impact on cloud formation while others suggest a marginal impact of biogenic ice nucleation. The difference in model results and the discrepancy with the field data suggests that the laboratory data on which the models are based is inadequate. In fact, in their global model Hoose et al. (J. Atm. Sci, doi: 10.1175/2010JAS3425.1, 2010) use a crude estimate of the ice nucleating ability of fungal spores since there is no suitable experimental data on which to base the parameterisation. Given fungal spores account for 23% of the primary emissions of organic aerosol globally, their assumption will lead to major uncertainties in the model. Lab data for ice nucleation by pollen and bacteria are also very poor. In short, there is a large amount of biogenic material in the atmosphere, but we do not know how it impacts clouds and climate due to the paucity of basic data. In order to address this paucity of information we propose a set of experiments in which we make use of a unique instrument which Murray developed during his NERC fellowship. This instrument has and is being used to measure the efficiency with which mineral dust particles nucleate ice in the immersion mode. This work has resulted in the first quantitative measurements of ice nucleation by clay minerals (Murray et al. Atm. Chem. Phys. Disc. 4, 115, 2010). We plan to apply the same rigorous and quantitative techniques to fungal spores, pollen, and bacteria for the first time.