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An evaluation of ice nucleating properties across the silica group of minerals

K. R. Lever (ee16krl@leeds.ac.uk), A. D. Harrison (ee11ah@leeds.ac.uk), B. J. Murray (b.j.murray@leeds.ac.uk) and J. V. Temprado (eejvt@leeds.ac.uk)

1. Abstract

Mineral desert dusts are considered one of the most important ice nuclei particles (INP) (Atkinson et al., 2013). Atmospheric mineral dust is estimated to be made up with 47% clay minerals, 29% quartz, 13% feldspar minerals and other minerals making up the rest (Murray et al., 2012). Recently Atkinson et al. (2013) demonstrated the importance feldspars are for effective ice nucleation. Quartz samples have shown to nucleate ice relatively effectively. Boose et al. (2016) highlighted the range in activity found for quartz ability to nucleate ice. However, there has never been a systematic study of ice nucleation by a range of atmospherically relevant silica mineral samples.

2. Scientific rationale

Cloud droplets in the absence of INP can supercool to below -37°C (Murray et al., 2012). The existence of INP can lead to triggering of freezing at higher temperatures. There has been recent research into nucleation by mineral dusts and feldspar, but not many studies have been conducted on a vast range of the silica group, although it has been suggested that these may have potential importance in the atmosphere.

a. Ice nucleation

There are two pathways for ice formation: homogeneous and heterogeneous freezing. Homogeneous freezing is the nucleation of ice away from a surface and freezing within a super cooled liquid droplet of water. Whereas heterogeneous freezing is caused by water freezing upon a surface.

There have been 4 different type of heterogeneous modes characterized (Vali et al., 2015):

immersion freezing: - where an INP immersed in a liquid cloud droplet starts the freezing;

condensation freezing: - in which involves freezing during the droplet formation by the condensation on an INP;

contact freezing: - this refers to freezing due to contact of an INP with the surface of a droplet;

deposition nucleation: - which is the direct deposition of vapour on an INP leads to freezing.

Many observations have suggested that liquid droplets are present before ice crystal

formation from heterogeneous freezing. Thus, immersion nucleation is more dominant in mixed-phase clouds (Seinfeld and Pandis, 2016). Some particles are better for immersion freezing than others. INP makes the transition from liquid to solid more favourable.

In the upper troposphere water vapour can deposit upon solid particles. Water droplets can exist in a supercooled state within a cloud to temperatures as low as -37ºC in the absence of INP (Murray et al., 2012). It is particles, such as quartz, that can provide the important nuclei that then triggers the water droplets to freeze at warmer temperatures than without the nuclei.

Due to human activity, there has been a vast deviation in the atmospheric composition

since the industrial era. For pre-industrial times, it was suggested that cloud condensation nuclei (CCN) loading was similar in both the continental regions and the oceanic regions, however now

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the CCN over continental regions has strongly increased (Murray et al., 2012). However, it is not known whether the INP has changed due to anthropogenic activity.

b. Implications of ice nucleation

Clouds represent a crucial component in the Earth’s climate system and INP can have a large impact upon cloud dynamics. The phase state of the cloud affects the lifetime of the cloud and the presence of ice particles in clouds correlates to the cloud properties and their impact on climate. There is substantial understanding about the basics of cloud formation, however quantitatively predicting cloud properties is still challenging. This is partly down to the range of diversity of both the chemistry and amount of aerosol particles (Koop, 2013). The quantitative understanding about clouds is limited. Ice nuclei are important for the existence of lightening and the formation of seeds of precipitation droplets. It will be interesting to explore the various types of quartz and how the chemistry of the ice nuclei affects their activity.

INP have an important impact on mixed phase and cirrus clouds (Murray et al. (2012).

Measurements have shown that even though INP represent only 1 in 105 ambient particles in the troposphere, INP heavily influence cloud microphysical processes (Rogers et al., 1998). The Wegener-Bergeron-Findeisen process (also known as the Bergeron mechanism) describes the growth of ice at the expense of the surrounding liquid droplets. This is because the lower saturation vapour pressure of ice in comparison to liquid droplets favours transformation to the ice state. The more INP in existence in a cloud, the increase in ice crystal concentration and then as a result these tend to precipitate out, reducing the lifetime of the cloud. Thus, INP can lower the net cooling impact of clouds on the planet. The Bergeron mechanism can alter the cloud optical depth, radiative forcing, precipitation rate, cloud coverage and lifetime. However concrete evidence of this is yet to become apparent, as many cloud modelling studies have demonstrated opposing arguments. Also, mixed phase clouds do not always demonstrate the Bergeron mechanism.

Mixed phase clouds and their ice nucleation can have an important effect on the Earth’s energy balance (Gettelman et al., 2010). This change is a result of a difference in the incoming and outgoing solar radiation. This is important to understand because, for example, more ice nucleation will cause two different outcomes. Ice particles will reflect more incoming radiation back out to space, whereas more ice nuclei will cause more outgoing radiation to be reflected down to earth again.

c. Types of ice nucleating particles

A wide range of INP have been distinguished by the ice nucleation community. Figure 1 shows a sample of 46 particles sampled from a wave cloud, demonstrating one instance of the INP particle distribution (Pratt et al., 2009). However, the proportions can vary substantially between samples. Sources of important INP that have been distinguished are mineral dusts, biological species, carbonaceous combustion products and volcanic ash (Murray et al., 2012). Silicates are the most abundant mineral type, the most commonly found in the earth’s crust is feldspar (Murray et al., 2012).

Recent work by Atkinson et al. (2013) demonstrated that feldspar minerals dominated ice

nucleation by mineral dusts and that clay minerals were less important ice nuclei; previously it had been thought that clay minerals were more important. Atkinson et al. (2013) also importantly determined that the global aerosol model showed the feldspar ice nuclei is globally distributed, thus feldspar could represent a large proportion of ice nuclei in the earth’s atmosphere. Harrison et al. (2016) further analysed feldspar and evaluated 15 different feldspar samples for their ability to nucleate ice. They demonstrated that alkali and potassium feldspars nucleate ice the most efficiently, in comparison to the plagioclase series (Harrison et al., 2016). Their work also showed the range of ice nucleating ability within the groups sampled. For instance, five out of six potassium-rich feldspars nucleated ice in a similar manner, whereas one potassium-rich feldspar sample was significantly more active. The study found that the hyperactive Na-feldspar lost activity

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with time, as it was suspended in water. With a decrease in activity of 16°C, over a period of 16 months. This effect also lowered the mean freezing temperature of the hyper active feldspar by 2°C (Harrison et al., 2016). However, the “standard” feldspar did not change its activity, within these experiments.

After feldspar, quartz is the next most abundant mineral in the earth’s crust and one that occurs in many igneous, sedimentary and metamorphic rocks (Deer et al., 1966). The erosion of rocks on the earth’s surface leads to the abundance of these minerals in the atmosphere. Quartz is present in most igneous rocks and its structure is formed of silicon dioxide tetrahedral as shown in figure 2a, which creates a tight arrangement of six membered loops. Thus, the structure is very strong and it does have a large resistance to erosion. Feldspar is made up of a network of tetrahedrals, as shown in figure 2b. Feldspar is a complex group of minerals, as they have a wide range of chemical compositions and crystal structures. Quartz is more abundant than feldspar in the atmosphere (figure 3).

Figure 1- the proportion of INP types sampled Figure 2- representations of quartz (a) and feldspar (b) as polygons in a wave cloud (Pratt et al., 2009) (Murray et al., 2012)

Desert dust is one of the most

important atmospheric INP aerosol types around the world. Desert dust is made up of a range of minerals, depending on the types of rock present. It was recently shown that feldspars are extremely important source of INP (Atkinson et al., 2013), but, it has also been noted that some quartz samples can nucleate ice relatively efficiently. However, there has never been a systematic study of ice nucleation looking at a range of atmospherically relevant silica mineral samples.

The development of ice nucleation in

clouds is currently insuffiently understood. It is important to understand the ability of the range of atmospheric dust compositions and their efficiency as INP. Figure 3 shows the vast range of average dust compositions.

Figure 3- the average of atmospheric dust compositions from using XRD analysis (Murray et al., 2012)

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3. Specific objective

The overarching aim of this project is to determine the ice nucleation activity of a wide range of characterised silica minerals using immersion mode experiments, this will be achieved by:

i) Characterizing a suite of previously collected samples. The characterizing of the

crystal structure will be determined and the specific surface area will be calculated. This will help enable analysis on how the crystal structure could affect the ability of INP. The surface area of INP in the atmosphere is important to correlate with the surface area used in the experiments. This will be done by using the X-ray Diffraction (XRD) method and the Raman microscope method.

ii) Make use of immersion mode experiments to survey different types of silica mineral’s nucleating activity. This will be done undertaking experiments in the laboratory using a wide range of samples of silica by immersing a sample within water, then analysing the freezing temperature of the group of droplets as they are cooled.

iii) Using lab data to determine the activity of the mineral in the atmosphere. The laboratory data will enable more reliable conclusions about silica minerals to be drawn and their ability to nucleate ice.

iv) Computer modelling. The method developed in Atkinson et al. (2013) will be further used to analyse the global distribution of INP. Using the models developed in Atkinson et al. (2013) will help estimate the INP concentrations in the atmosphere.

4. Background

Mineral dust, metallic particles, some biological particles and anhydrous salts have been

confirmed as INP by laboratory experiments (Seinfeld and Pandis, 2016). There are a wide range of sources for atmospheric mineral dust deposits, ranging from Africa, the middle east and Asia. Interesting early laboratory work analysing snow crystals revealed that mineral dust was often found at their centre (Kumai, 1961). Mineral dust is the outcome of eroded crustal rock. Then convective local and global weather systems, transport dust around the world.

There is a lack of understanding of the effect of ice on cloud properties (Murray et al., 2012). In

the Intergovernmental Panel on Climate Change (IPCC) fifth assessment they concluded that there were large uncertainties and a lack of knowledge of the anthropogenic fraction on INP (Stocker et al., 2013). As a result, they were unable to form a reliable conclusion.

Due to quartz’ chemical and

physical resilience to corrosion, quartz is an abundant detrital mineral, thus it becomes concentrated because of sedimentary processes and creates various sands and sandstone, and is often a cementing medium between sediments (Deer et al., 1966). Due to quartz’s hard structure and lack of cleavage, it is resistant to much weathering. Figure 4 shows the relationship between the temperature and pressure with varying silica minerals. The alpha quartz represents species stable at atmospheric temperatures, up to 573ºC, whereas the beta quartz represents species stable from 573ºC, up to 870ºC (Deer et al., 1966).

Figure 4- the stability relationship of the range of silica minerals (Deer et al., 1966)

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There are thought to be several properties that make for an effective INP (Pruppaqcher and Klett, 1997):

Insolubility: - water absorption could help disintegration of the particle itself.

Size: - there has been a correlation to the larger particles and ice nucleation

Chemical bond requirement: - water must be able to create chemical bonds with the INP surface.

Crystallographic characteristics: - a good INP should be able to facilitate the template of ice. This means that the nucleant should have a crystallographic face or a crack/defect at the specific active site (Murray et al., 2012).

However, the above stated guidelines for effective INP should be used as a guide and not as definitive. This is due to our current limited understanding of the best INP. This demonstrates the need for experimental exploration, evaluating a wide range of potential INP.

Figure 5 demonstrates the estimation of

ice nuclei concentrations in a range of samples and shows both quartz and feldspar to be high temperature ice nuclei. However, the exact process of how an atmospheric droplet crystallises into ice is not well understood. It is thus hoped that the results from laboratory and in-situ measurements will be able to in the future a better and reliable understanding of the role of the various INP.

5. Wider implications

Further exploring into the importance of different INP, will lead to better understanding about

the spatial distribution of a variety of INP. Thus, the implications they have on our climate, and their impact on climate change. As stated the IPCC could not reach a reliable conclusion on how INP currently affect our climate and how they will influence future climate change. Undertaking work such as the proposed work here, could enable the IPCC to take more confidence in the future, in what makes for an effective INP.

The process of water droplets coalescing together with INP, is an area of research of much

interest. However, it could still be decades before a clear understanding of the impact of heterogeneous ice nucleation emerges (Koop, 2013). In the long run the findings could benefit more than just the atmospheric science sector. For instance, it could help in applications for engineering, the cryostorage of biological foods and tissues and in the prevention of icing up of pipes on aircraft.

6. Methodology and approach

Figure 5-Ice nuclei concentrations were estimated using the abundance of a range minerals (Atkinson et al., 2013)

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i) Characterizing a suite of previously collected samples.

The silica samples have been gathered by Alex. These include a wide range of silica samples, with varying crystal structure and varying chemical impurities. The samples will be analysed for their composition by using both XRD and Raman microscope. Next samples were crushed and ground. The samples will then be characterised by XRD. Once enough samples have been ground down to a reasonable size, the surface area of the quartz will need to be determined.

ii) Make use of immersion mode experiments to survey different types of silica

mineral’s nucleating activity.

Brunauer, Emmett and Teller (BET) is a technique that will be used for the measuring surface area of the quartz samples. This is based upon using the amount of gas the sample absorbs. This means it determines the overall specific external and internal surface area of disperse/porus solid in respect to the amount of gas absorbed by using the BET method. Nitrogen will be used for the absorptive at ~77 K, for measuring the surface area. This temperature is important as it is below the critical temperature, thus condensable. This means that the nitrogen condenses upon the surface in a monolayer, then using the known size of the gas molecules and the amount of nitrogen absorbed, the surface area of the sample can be recorded.

Next each sample will be weighed, and a specific amount will be added to a water. To enable

suspension of the sample within the water, each sample will be mixed. Next the sample will be

pipetted 1l in volume onto hydrophobic glass slides. The glass slide will be filled with droplets equi-distance apart, to stop each droplet interfering with neighbouring ones. A flow of dry nitrogen through the chamber above the droplets will be blown onto the glass slide with the sample on, as to stop the formation of condensation and frost accumulation, which could otherwise trigger freezing in neighbouring droplets. This was demonstrated by Whale et al. (2015). The temperature of the glass slide will then be reduced until each droplet is frozen. The nucleation events will be recorded using a camera and this then allows the evaluation of the specific time each droplet froze. This will then allow the determination of what proportion of the droplets have frozen as function of temperature.

To compare the range of samples for their ability to nucleate ice, the number of active sites will

be standardised to the surface area for that available for nucleation. Connolly et al. (2009) presented an equation that can be used to calculate the ice nucleation active site density ns(T) (equation 1). Where n(T) is the number of droplets frozen at a given temperature, N is the total number of droplets and A is the surface area of the nucleator per droplet.

𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 1: 𝑛(𝑇)

𝑁= 1 − 𝑒−𝑛𝑠(𝑇)𝐴

iii) Using lab data to estimate the INP concentration in the atmosphere.

After the laboratory experiments, analysing the ability for silica minerals to nucleate ice are

completed, then the activity of the minerals will be determined.

iv) Computer modelling using GLOMAP.

As demonstrated in Atkinson et al. (2013), GLOMAP is run within the chemical transport model TOMCAT. It is a size and composition-resolving two-moment microphysical aerosol scheme. GLOMAP has been used in the past to study the atmospheric processing of mineral dust and in Atkinson et al. (2013) it was also used to include an extra eight mineral types.

7. Programme of research (to include a time plan)

Activity Predecessor Time estimates Expected completion

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Proposal - 6 weeks 9th February 2017

Laboratory experiments Proposal 4 months May 2017

Literature review Laboratory experiments

3 months of 2nd March 2017

Computer project Literature review 4 months 11th May 2017

Dissertation Computer project 4 months 10th August 2017

Seminar and poster session Dissertation 2 months 25th August 2017

8. Resources required, and where they come from (to include a budget if appropriate)

The main resources required will be the number of silica samples, that are bought from around the world. Alex has sought these and they have come to around £100. Whilst undertaking the experiments in the lab, there is a variety of equipment needed. These include pipettes, glass slides etc. Access to the XRD machine, the Raman microscope and the microliter Nucleation by Immersed Particle Instrument (µl-NIPI) will all be necessary to carry out the necessary experiments.

9. Outcome (publication, dissemination of data, archiving)

There is an envisagement of a publication to be brought out, as a result of the study detailed above. This is with a title similar to: “The evaluation of ice nucleating properties across the silica group of minerals” which would be a submission along with Alexander Harrison, Benjamin Murray and Jesus Vergara Temprado. An example of a recent paper bought out similar to the study I wish to explore was on feldspar. Of which was titled “Not all feldspar are equal: a survey of ice nucleating properties across the feldspar group of minerals”, Harrison et al. (2016).

10. References Atkinson, J.D., Murray, B.J., Woodhouse, M.T., Whale, T.F., Baustian, K.J., Carslaw, K.S., Dobbie, S., O’Sullivan, D. and Malkin, T.L., 2013. The importance of feldspar for ice nucleation by mineral dust in mixed-phase clouds. Nature, 498(7454), pp.355-358. Connolly, P.J., Möhler, O., Field, P.R., Saathoff, H., Burgess, R., Choularton, T. and Gallagher, M., 2009. Studies of heterogeneous freezing by three different desert dust samples. Atmospheric Chemistry and Physics, 9(8), pp.2805-2824. Deer, W. A., Howie, R. A. and Zussman, J (1966). An introduction to the rock forming minerals. London: Longman group limited. Pp.340-355. Gettelman, A., Liu, X., Ghan, S.J., Morrison, H., Park, S., Conley, A.J., Klein, S.A., Boyle, J., Mitchell, D.L. and Li, J.L., 2010. Global simulations of ice nucleation and ice supersaturation with an improved cloud scheme in the Community Atmosphere Model. Journal of Geophysical Research: Atmospheres, 115(D18). Harrison, A.D., Whale, T.F., Carpenter, M.A., Holden, M.A., Neve, L., O'Sullivan, D., Vergara Temprado, J. and Murray, B.J., 2016. Not all feldspars are equal: a survey of ice nucleating properties across the feldspar group of minerals. Atmospheric Chemistry and Physics, 16(17), pp.10927-10940. Herbert, R.J., Murray, B.J., Dobbie, S.J. and Koop, T., 2015. Sensitivity of liquid clouds to homogenous freezing parameterizations. Geophysical research letters, 42(5), pp.1599-1605. Koop, T. and Mahowald, N., 2013. Atmospheric science: the seeds of ice in clouds. Nature, 498(7454), pp.302-303.

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Kumai, M., 1961. Snow crystals and the identification of the nuclei in the northern United States of America. Journal of Meteorology, 18(2), pp.139-150. Murray, B.J., O'sullivan, D., Atkinson, J.D. and Webb, M.E., 2012. Ice nucleation by particles immersed in supercooled cloud droplets. Chemical Society Reviews, 41(19), pp.6519-6554. Pratt, K.A., DeMott, P.J., French, J.R., Wang, Z., Westphal, D.L., Heymsfield, A.J., Twohy, C.H., Prenni, A.J. and Prather, K.A., 2009. In situ detection of biological particles in cloud ice-crystals, Nat. Geosci., 2, 397–400. Pruppacher, H.R. and Klett, J.D., 1997. Microphysics of Clouds and Precipitation: With an Introduction to Cloud Chemistry and Cloud Electricity, 954 pp. Rogers, D.C., DeMott, P.J., Kreidenweis, S.M. and Chen, Y., 1998. Measurements of ice nucleating aerosols during SUCCESS. Geophysical research letters, 25, pp.1383-1386. Seinfeld, J. H. and Pandis S. N. (2016). Atmospheric chemistry and physics. 3rd ed. Hoboken, New Jersey: Wiley. 746-747. Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.) IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp, doi:10.1017/CBO9781107415324. Vali, G., DeMott, P.J., Möhler, O. and Whale, T.F., 2015. Technical Note: A proposal for ice nucleation terminology. Atmospheric Chemistry and Physics, 15(18), pp.10263-10270. Whale, T.F., Murray, B.J., O'Sullivan, D., Wilson, T.W., Umo, N.S., Baustian, K.J., Atkinson, J.D., Workneh, D.A. and Morris, G.J., 2015. A technique for quantifying heterogeneous ice nucleation in microlitre supercooled water droplets. Atmospheric Measurement Techniques, 8(6), pp.2437-2447.