Contact Freezing: a Review of Experimental Studies
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Open Access Atmos. Chem. Phys., 13, 9745–9769, 2013 Atmospheric www.atmos-chem-phys.net/13/9745/2013/ doi:10.5194/acp-13-9745-2013 Chemistry © Author(s) 2013. CC Attribution 3.0 License. and Physics Contact freezing: a review of experimental studies L. A. Ladino Moreno1,*, O. Stetzer1, and U. Lohmann1 1Institute for Atmospheric and Climate Science, ETH Zurich, Universitätstrasse 16, 8092, Zurich, Switzerland *now at: the Department of Chemistry, University of Toronto, Toronto, Ontario, Canada Correspondence to: L. A. Ladino Moreno ([email protected]) Received: 8 March 2013 – Published in Atmos. Chem. Phys. Discuss.: 21 March 2013 Revised: 22 August 2013 – Accepted: 23 August 2013 – Published: 2 October 2013 Abstract. This manuscript compiles both theoretical and ex- to initiate ice formation below 0 ◦C. Thus, aerosol particles perimental information on contact freezing with the aim to are important in cloud formation (e.g., lifetime, droplet size, better understand this potentially important but still not well cloud phase and cloud albedo) and therefore influence the quantified heterogeneous freezing mode. There is no com- hydrological cycle (Lohmann and Feichter, 2005). Most of plete theory that describes contact freezing and how the en- the precipitation in mid-latitudes originates via the ice phase ergy barrier has to be overcome to nucleate an ice crystal but reaches the surface as rain (melting of ice crystals) (Lau by contact freezing. Experiments on contact freezing con- and Wu, 2003; Lohmann and Feichter, 2005; Lohmann and ducted using the cold plate technique indicate that it can ini- Diehl, 2006). IN are mostly solid aerosol particles, either in- tiate ice formation at warmer temperatures than immersion soluble or crystalline. IN are thought to have a similar crys- freezing. Additionally, a qualitative difference in the freez- talline structure to ice and/or the possibility to form hydrogen ing temperatures between contact and immersion freezing bonds and to possess active sites (i.e., crevasses, imperfec- has been found using different instrumentation and differ- tions, corners and/or steps onto the particle surface). Possible ent ice nuclei. There is a lack of data on collision rates in physical and chemical influences are summarized in Prup- most of the reported data, which inhibits a quantitative cal- pacher and Klett (1997) and Vali (1999) (e.g., water uptake, culation of the freezing efficiencies. Thus, new or modified particle morphology, hygroscopicity and presence of ions be- instrumentation to study contact nucleation in the laboratory tween the particle layers). Natural aerosol particles such as and in the field are needed to identify the conditions at which bioaerosols (e.g., bacteria, pollen and fungi), volcanic ash contact nucleation could occur in the atmosphere. Important and soil particles (e.g., mineral dust and clays) have been questions concerning contact freezing and its potential role found to be good IN. Amorphous organic aerosols, such as for ice cloud formation and climate are also summarized. citric acid, levoglucosan and raffinose (Murray et al., 2010; Wagner et al., 2012; Wilson et al., 2012), secondary organic aerosols (Wang et al., 2012; Ladino et al., 2013); and crys- talline particles, such as ammonium sulfate (Abbatt et al., 1 Introduction 2006) or hydrated sodium chloride (Wise et al., 2012) may also serve as IN. Artificial particles such as silver iodide Clouds play an important role in the global radiative bud- (AgI) have been used in the laboratory and in cloud seeding get (Trenberth et al., 2009) as they cover around 70 % of studies (Wieringa and Holleman, 2006) because they were the Earth’s surface (Stubenrauch et al., 2010). Depending on found to be efficient IN. Diehl and Mitra (1998), Gorbunov cloud type, clouds can either cool and/or heat the Earth’s sur- et al. (2001) and Möhler et al. (2005) found that soot parti- face. Clouds reflect shortwave (solar) radiation cooling the cles can also act as IN, whereas other studies suggest that this Earth, and they can absorb and re-emit longwave radiation is not always the case (DeMott et al., 1999; Dymarska et al., emitted by the Earth’s surface back towards the surface caus- 2006; Friedman et al., 2011). Therefore, predicting the IN ing a warming. Aerosol particles can act as cloud conden- activity, if any, of atmospheric soot is limited by poor current sation nuclei (CCN), and a much smaller fraction of atmo- understanding. spheric aerosol particles act as heterogeneous ice nuclei (IN) Published by Copernicus Publications on behalf of the European Geosciences Union. 9746 L. A. Ladino Moreno et al.: Contact freezing: a review of experimental studies To understand ice formation in mixed-phase clouds, it is et al., 1983; DeMott, 1995) and the CoLlision Ice Nucleation crucial to study each of the four known heterogeneous ice nu- CHamber (CLINCH, Ladino et al., 2011b)), a wind tunnel cleation modes (deposition nucleation, condensation freez- (Pitter and Pruppacher, 1973; Levin and Yankofsky, 1983; ing, immersion freezing and contact freezing) in detail. In Diehl and Mitra, 1998; Diehl et al., 2002; Von Blohn et al., addition, the influence of secondary ice formation should 2005), a cold plate (Fukuta, 1975a; Rosinski and Nagamoto, also be taken into account. The preference of one freezing 1976; Durant and Shaw, 2005; Shaw et al., 2005) and an mechanism over another depends on IN composition, tem- ElectroDynamic Balance (EDB, Svensson et al., 2009; Hoff- perature and supersaturation with respect to ice and/or wa- mann et al., 2013a, b). The collisions of the aerosol parti- ter and the presence of liquid supercooled droplets. Depo- cles with droplets have been simulated in different ways. For sition nucleation occurs when water vapor deposits onto an example, Shaw et al. (2005); Durant and Shaw (2005) and IN. In contrast, condensation freezing occurs when water Fornea et al. (2009) performed their experiments using the vapor condenses around the particle at temperatures below cold plate technique where the aerosol particles were brought 0 ◦C to form a supercooled liquid droplet which subsequently into contact with the drops mechanically. In contrast, in the freezes. Immersion freezing takes place when an IN is im- wind tunnel (e.g., Pitter and Pruppacher, 1973; Levin and mersed within a liquid droplet at temperatures where it does Yankofsky, 1983; Diehl and Mitra, 1998; Von Blohn et al., not freeze and subsequently the liquid droplet is cooled down 2005), EDB (Svensson et al., 2009; Hoffmann et al., 2013a, and initiates ice formation. The last heterogeneous freezing b) and cloud chamber studies (e.g., Langer et al., 1978; De- mode is contact freezing. Rau (1950); Fletcher (1969, 1970); Mott et al., 1983; DeMott, 1995; Ladino et al., 2011b) the Cooper (1974) and Fukuta (1975a) presented some of the aerosol particles were naturally scavenged from the air by first ideas on the concept of contact freezing. Contact freez- the liquid drops. ing is defined as the process in which freezing of a super- Several detailed reviews on ice nucleation have been pub- cooled droplet results from the collision with an aerosol par- lished (e.g., Mossop, 1963; Vali, 1985; Pruppacher and Klett, ticle (Vali (1985) and definitions by the International Com- 1997; Hoose and Möhler, 2012; Murray et al., 2012). The ice mission on Clouds and Precipitation (ICCP) and the Interna- nuclei concentrations, freezing pathways, proposed mecha- tional committee on Nucleation and Atmospheric Aerosols nisms and hypotheses to explain the laboratory observations (ICNAA)). Ice formation can be enhanced by contact freez- have been revised. However, there is no paper that compiles ing within mixed-phase clouds since both aerosol particles the available information on contact freezing, which is sug- and supercooled cloud droplets may be present. Inside these gested to be the pathway by which ice forms at the warmest clouds, the interstitial aerosol particles collide with the su- temperatures for a given IN type based on the available lab- percooled liquid droplets by different physical forces such oratory data (e.g., Hoose and Möhler, 2012). Although con- as Brownian motion, inertial impaction, interception, elec- tact nucleation can be a result of scavenging processes, the troscavenging, thermophoresis and diffusiophoresis (Green- reason of the high measured freezing onset temperatures is field, 1957; Slinn and Hales, 1971; Beard, 1974; Wang et al., still unknown. While some field studies support the atmo- 1978). spheric relevance of contact freezing (e.g., Auer Jr., 1971; In the past, a lot of work has been done to study the con- Hobbs and Atkinson, 1976; Hobbs and Rangno, 1985; Ans- ditions relevant for the different heterogeneous modes of ice mann et al., 2005; Seifert et al., 2011), some field studies formation. Here we only discuss studies of contact and im- (e.g., Twohy et al., 2010) and modeling studies (e.g., Cui mersion freezing with which contact freezing will be com- et al., 2006; Phillips et al., 2007) found the opposite. Contact pared. These two freezing modes are frequently compared; freezing has been observed to take place at the cloud top and however, this comparison is not trivial and requires more at cloud edges where dry ambient air is mixed with cloudy attention. Several experiments on immersion freezing us- air due to entrainment. This causes droplet evaporation and ing different instrumentation (e.g., the cold plate technique hence an increase in the collision rates due to thermophore- (Koop et al., 1998; Shaw et al., 2005; Vali, 2008; Rigg et al., sis. For example, Hobbs and Rangno (1985) proposed that 2013), the Differential Scanning Calorimeter (DSC, Marcolli contact freezing was responsible for the glaciation of the in- et al., 2007), the wind tunnel (Pitter and Pruppacher, 1973; vestigated cumulus clouds and the enhancement of ice for- Diehl et al., 2002; Von Blohn et al., 2005), the Immersion mation took place at the top of the cloud due to entrainment.