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

由冰粒和过冷液滴组成的云被称为混合相云，存在于-35摄氏度以上的温度，覆盖了地球的大部分地区。这些云层既通过捕获发出的红外辐射来使地球变暖，又通过将从太阳传入的可见光反射回太空来冷却地球，从而影响气候。越来越明显的是，混合相云对大气中存在的粒子(称为气溶胶)的数量和类型非常敏感。过去已经做了大量工作，以了解气溶胶对完全由液滴组成的云的作用，政府间气候变化专门委员会(气专委)试图量化这种影响，尽管存在很大的不确定性。然而，气溶胶在冰层形成中扮演的角色仍然非常不确定，尽管有证据表明影响非常大，但气溶胶在冰层形成中所起的作用仍然非常不确定，政府间气候变化专门委员会无法评估这种强迫。能够催化冰粒形成的气溶胶被称为冰核；然而，目前对它们的特性、浓度、全球分布以及它们成核冰的效率都缺乏量化。来自实地研究的越来越多的证据表明，细菌、花粉或真菌孢子等生物粒子会使云中的冰成核。一段时间以来，人们已经知道大约25%的不溶气溶胶可能是生物来源的，但它们在云形成中的作用仍然高度不确定。在过去的几年里，野外设备的技术进步导致人们发现，大气中可以用作冰核的大部分颗粒是生物起源的。在一次飞机行动中，人们发现怀俄明州上空云层中三分之一的冰晶含有生物物质(Pratt等人，自然地理科学，2398)。2009年)。在另一项研究中，亚马逊地区零下25摄氏度以上的冰核数量以生物物质为主(Prenni等人，自然地球物理，2,402,2009)。Hoose(《自然地理》，2,385,2009)认为，这些发现可能只是“冰山一角”。因此，很明显，生物气溶胶与冰密切相关，但它们在云和气候模型中缺乏适当的处理，它们的冰核特性特征很差，基础知识存在巨大差距。模拟研究给出了相互矛盾的结果，一些模型表明对云的形成有重大影响，而另一些模型则表明生物成因冰核的影响很小。模型结果的差异以及与现场数据的差异表明，作为模型基础的实验室数据是不充分的。事实上，在他们的全球模型中，Hoose等人。(J.ATM。SCI，DOI：10.1175/2010JAS3425.1,2010年)使用真菌孢子成冰能力的粗略估计，因为没有合适的实验数据作为参数化的基础。考虑到真菌孢子占全球有机气溶胶初级排放量的23%，他们的假设将导致模型中的主要不确定性。关于花粉和细菌成冰的实验室数据也非常糟糕。简而言之，大气中有大量的生物质，但由于基础数据的缺乏，我们不知道它是如何影响云和气候的。为了解决这种信息匮乏的问题，我们提出了一系列实验，在这些实验中，我们利用了默里在他担任NERC研究员期间开发的一种独特的仪器。该仪器已经并正在用于测量矿物粉尘颗粒在浸泡模式下成核冰的效率。这项工作首次定量测量了粘土矿物的冰核作用(Murray等人)。自动取款机。化学。太棒了。光盘。4、115、2010)。我们计划首次将同样严格和定量的技术应用于真菌孢子、花粉和细菌。

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.