A very different kind of a frozen surface also encountered in polar regions is sea ice.
While snow is generated in the atmosphere by condensation and nucleation of water vapor, sea ice forms by freezing of ocean water at low air temperatures. The freezing of ocean water involves several physical and thermodynamical processes since the ocean water contains sea salt at relatively high concentrations. The main sea salt components are chloride (Cl-), sodium (Na+), and sulfate (SO4
2-) with average concentrations of 1.94, 1.0, and 0.27 g per kg of ocean water [Millero, 2006]. These three ions contribute 94 % of the total sea salt. While the sea salt can easily be dissolved in the liquid ocean water, much smaller amounts of salt ions can be incorporated in the sea ice crystals [Thomas and Diekmann, 2003]. Therefore, the sea salt ions are rejected from the ice lattice during the freezing process. The ions remain dissolved in a liquid forming a salty brine, which is collected in microscopic brine inclusions in the sea ice. Eventually these inclusions form an entire network of pores making the vertical transport of the brine into the ocean water or to the top of the sea ice possible [Eicken et al., 2000]. Therefore, new sea ice is covered with the brine with salt concentrations significantly higher than encountered in the ocean water. Due to the high salt concentrations of the brine, it remains liquid even at temperatures well below the freezing point of water.
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Figure 4: Photographs of frost flowers on the Arctic Ocean. The photographs were taken on 25 March 2003 (75.9° N, 27.0° E).
An additional feature of new sea ice is the generation of dendritic ice crystals, which form in the polar regions under calm wind conditions [Perovich and Richter-Menge, 1994]. These so-called frost flowers (Figure 4) appear upon condensation of water vapor from a supersaturated layer above the sea ice surface on solid irregularities [Perovich and Richter-Menge, 1994; Martin et al., 1995]. Depending on the temperature gradient between the relatively warm sea ice surface and the colder air temperatures, the frost flowers can quickly cover large fractions of newly formed sea ice. Laboratory experiments demonstrated that at T = –30 °C the growth rate can be as large as 10 % area coverage per hour [Martin et al., 1996]. Interestingly, frost flowers also contain high concentrations of sea salt [Drinkwater and Crocker, 1988; Perovich and Richter-Menge, 1994; Martin et al., 1995; Rankin et al., 2002], although they are initially formed by the condensation of water vapor. The observations have demonstrated that the overall salinity of the frost flowers can be a factor of almost 5 higher than the salinity of the ocean water [Drinkwater and Crocker, 1988].
The transport of the sea salt ions into the frost flowers is only possible within the QLL of the single crystals. Within this liquid layer the ions can migrate from the brine layer on the sea ice surface into the frost flower crystals. The driving force of this migration is the so-called thermomolecular pressure gradient, which induces a transport of liquid water and the ions from warmer to colder regions within the frost flowers [Wettlaufer and Worster, 1995]. Since the temperature gradient in the air above the sea ice is also imprinted in the frost flowers (Figure 5), the coldest portion of the frost flower crystals are the highest tips with the largest distance from the warm sea ice surface [Martin et al., 1996].
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Figure 5: Photographs of an artificial frost flower grown in the laboratory at air temperatures between –25 and –30 °C (left regular digital picture, right IR picture). The horizontal dimension of the frost flower is on the order of 10 cm. The same structures can be recognized in both pictures. In the IR picture the strong temperature difference between the warmer sea ice surface (–9
°C) and the cold frost flower (temperature at tips between –14 and –20 °C) can be observed.
The observations of Rankin et al. [2002] demonstrated that in general the enrichment of the single sea salt components in the frost flowers were comparable to the enrichment of the salinity. The main exception was SO4
2-, which showed much lower enrichment factors. This change in composition was attributed to the formation of mirabilite (Na2SO4⋅ 10 H2O), which precipitates at a temperature of –8 °C [Untersteiner, 1986].
Due to the higher amount of Na+ available in the brine compared to SO4 2-, the precipitation has a stronger effect on the SO4
concentration compared to the Na+ concentration.
It is well known that halides like chloride and bromide can be converted to reactive halogen compounds due to heterogeneous reactions [McConnell et al., 1992; Fan and Jacob, 1992; Vogt et al., 1996]. Crucial reactions in the case of bromide (Br-) are the formation of hypobromous acid (HOBr) in the gas phase, which is readily absorbed at surfaces, and the oxidation of the corresponding hypobromite anion (BrO-) to molecular bromine (Br2) in the presence of Br- and sufficient acidity. The solubility of Br2 is rather low leading to a release of this compound back to the gas phase. In the gas phase, Br2 is quickly photolyzed by UV and visible radiation producing bromine atoms (Br). A similar mechanism is also feasible for chloride (Cl-). Such a mechanism can occur on any environmental surface with the appropriate properties. However, new sea ice covered with frost flowers seems to offer ideal conditions for this mechanism: the brine as well as the frost flowers contain high concentrations of sea salt including Cl- and Br -and the specific surface area is drastically increased due to the prickly structure of the single crystals (Figure 4). For example, measurements of the specific surface areas of frost flowers resulted in a value of 200 cm2 g-1 [Domine et al., 2005].
The relation between sea ice formation and release of reactive halogens to the atmosphere was explored in Publ. 3.4.2 using remote sensing data. If a Br atom reacts with O3 bromine monoxide (BrO) is formed. This molecule can be detected using satellite observations [Richter et al., 1998; Wagner and Platt, 1998]. Enhanced BrO
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concentrations are regularly observed over the frozen Arctic and Antarctic Oceans during springtime [Richter et al., 1998; Wagner and Platt, 1998]. In Publ. 3.4.2 a thermodynamic model was presented to derive potential frost flower areas. This model utilizes several further remote sensing data like calculated open water areas or assimilated global data sets of the air temperature. The spatial agreement of open water areas with low air temperatures are a prerequisite for frost flower formation and are expressed as potential frost flower areas. In several case studies it was demonstrated that large potential frost flower areas are strongly related to air masses containing elevated BrO levels if the transport is taken into account using trajectories [Publ. 3.4.2].
A further indication that new sea ice is connected with the halogen activation process is the occurrence of tropospheric ozone depletion events. Such events were first observed almost 20 years ago [Barrie et al., 1988] in the Arctic. It is now well known that such events occur regularly in springtime in both hemispheres [e.g. Tarasick and Bottenheim, 2002; Wessel et al., 1998] and that they are related to the elevated tropospheric BrO concentrations observed by remote sensing techniques [Richter et al., 1998; Wagner and Platt, 1998]. The O3 destruction is caused by several catalytic cycles involving the reactive halogen compounds [Platt and Hönninger, 2003; Publ. 3.4.3]. Therefore, the depletion of ozone can be regarded as an indicator of vigorous halogen activation processes. In Publ. 3.4.1 we presented a time series of O3 concentrations measured in springtime in the marginal ice zone of the Arctic Ocean. Low O3 concentrations were encountered during numerous periods lasting for several days. Further analysis of the conditions during the onset of the longest O3 depletion event indicated that the O3
decrease was not caused by a change in air mass transport [Publ. 3.4.3]. It was demonstrated that the observed O3 decrease was a local phenomenon probably initiated by the local release of reactive halogens in the marginal ice zone. Further analysis indicated that larger areas with newly formed sea ice characterized the ice edge region at the time of the O3 measurements [Publ. 3.4.3]. Since air temperatures remained very low, the formation of frost flowers on the newly formed sea ice was very likely.
Nevertheless, all these observations do not reveal, which of the specific surfaces formed through the freezing of ocean water (e.g. brine, frost flowers) are responsible for the halogen activation. It has also been suggested that aerosols generated in the new sea ice areas and subsequently deposited on adjacent snow surfaces could be the active sites for the halogen release mechanism [Avallone et al., 2003; Simpson et al., 2005]. Currently, the observations are too limited to resolve this question. Even if the specific source of the reactive halogen species is unknown, the observed O3 decrease can be used to estimate concentrations of halogen atoms [Publ. 3.4.3]. These induced concentrations are extremely high in agreement with the observed rapid O3 decrease.