Frost Flowers Research Articles

Study of the origin of atmospheric mercury depletion events recorded in Ny-Ålesund, Svalbard, spring 2003

An international campaign involving six teams was organized in Ny-Ålesund, Svalbard, in order to understand better the origin of atmospheric mercury depletion events (AMDEs). Special emphasis was given to determining the source region of the observed events and the physical and chemical processes leading to AMDEs. Five AMDEs were recorded during a one-month field experiment (10 April–10 May, 2003). The different events presented various characteristics, especially in terms of mercury species formation, atmospheric particle variations and meteorological conditions. After careful examination of each event, we postulate that two were probably due to advection of already depleted air masses and three were a product of local or regional chemistry. The roles of different surfaces (frost flowers, snow, ice aerosol in clouds) involved in heterogeneous reactions leading to AMDEs are also discussed. We speculate that ice clouds may explain the particle variations observed during the three more local events.

Enhanced Deposition and Bioaccumulation of Mercury in Antarctic Terrestrial Ecosystems Facing a Coastal Polynya

Mercury emitted by anthropogenic and natural sources occurs in the atmosphere mostly in the gaseous elemental form, which has a long lifetime in tropical and temperate regions. Once deposited in terrestrial and aquatic ecosystems the metal is partly re-emitted into the air, thus assuming the characteristics of global pollutants such as persistent volatile chemicals. In polar regions, during and after the sunrise, the photochemically driven oxidation of gaseous Hg by reactive halogens may result in areas of greatly enhanced Hg deposition. Mercury concentrations in soils, lichens, and mosses collected in a stretch between 74 degrees 30' S and 76 degrees 00' S, in ice-free coastal areas of Victoria Land facing the Terra Nova Bay coastal polynya, were higher than typical Antarctic baselines. The finding of enhanced Hg bioaccumulation in Antarctic terrestrial ecosystems facing a coastal polynya strongly supports recent speculations on the role of ice crystals ("frost flowers") growing in polynyas as a dominant source of sea salt aerosols and bromine compounds, which are involved in springtime mercury depletion events (MDEs). These results raise concern aboutthe possible environmental effects of changes in regional climate and sea ice coverage, and on the possible role of Antarctica as a sink in the mercury cycle.

Specific surface area, density and microstructure of frost flowers

Frost flowers often grow on new sea ice. They are thought to have a high specific surface area (SSA) that provides sites for heterogeneous reactions. We have measured the SSA of frost flowers using CH4 adsorption at 77 K and obtained a value of 185 (+80 −50) cm2/g, much lower than inferred by others. Their density is 0.02 g/cm3. We calculate that the total surface area of frost flowers is 1.4 m2 per m2 of ice surface, so that they do not increase the ice surface area significantly. Their role as sites for enhanced heterogeneous reactions should be reconsidered. Frost flowers also commonly grow on fresh water and the saline brine seen on young sea ice is not necessary for their growth. Photo‐ and electro‐micrographs reveal hollow and concave structures, typical of very fast growing crystals. The brine that wicks up frost flowers considerably perturbs their growth.

Halogens in the coastal snow pack near Barrow, Alaska: Evidence for active bromine air‐snow chemistry during springtime

We measured halide concentrations of snow and frost flowers in the vicinity of Barrow, Alaska. We find that the ratio of bromide to sodium in frost flowers is slightly enhanced (≈10%) as compared to sea water. In contrast, the ratio of bromide to sodium in some snow samples is more than an order of magnitude enhanced, and in other samples is more than an order of magnitude depleted. We interpret the bromide depleted snow as having been processed by heterogeneous chemistry and providing reactive halogen compounds to the atmosphere. The eventual end product of reactive bromine chemistry is HBr that is then deposited over a wide region, enhancing bromide in inland snow samples. Although frost flowers or open leads are likely to be the original source of halides that become reactive halogen gases, we find that the bromide release often occurs subsequent to production of aerosol from marine sources.

Elevated mercury measured in snow and frost flowers near Arctic sea ice leads

Elevated mercury concentrations have been reported in arctic coastal snow far from emission sources. The mercury is deposited during mercury depletion events (MDEs), a set of photochemical atmospheric reactions involving reactive halogens. The highest mercury concentrations are clustered near the coast, leading to speculation that sea ice or sea ice leads play a role in MDEs. The nature of this connection is not fully understood. We report mercury concentrations up to 820 ng/L in snow and frost flowers along sea ice leads near Barrow, Alaska. These concentrations are nine times higher than values from nearby coastal snow and are almost half of the mercury maximum contaminant level in United States drinking water. The high values were found only near leads that had convective plumes above open water suggesting that the same processes that produce a supersaturated environment for water vapor near sea ice leads may be instrumental in mercury deposition.

Sources and transport pathways of marine aerosol species into West Antarctica

AbstractSixteen high-resolution marine aerosol (Na+, SO42–) records from spatially distributed International Trans-Antarctic Scientific Expedition (ITASE) ice cores spanning the last ~200 years from the Pine Island–Thwaites and Ross drainage systems and the South Pole are used to examine sources (sea spray and frost flowers) and transport pathways of marine aerosols into West Antarctica. Factors contributing to the amount of marine aerosols transported inland include sea-ice extent, the presence of open-water features (polynyas, leads), wind strength and direction, and the strength and positioning of low-pressure systems. Analysis of SO42–/Na+ ratios indicates that frost flowers can contribute significantly (40%) to the Na+ budget of Antarctic ice cores. Higher Na+ concentrations in the Ross drainage system may result from greater production of marine aerosols related to frost flowers in the Ross Sea region in association with greater sea-ice extent and larger open-water areas. Significant positive correlations of sea-ice extent and the Na+ time series exist in some regions of West Antarctica. Higher wind speeds in winter and higher Na+ concentrations when sea-level pressure is lower indicate that intensified atmospheric circulation enhances transport and production of marine aerosols.

The origin of sea salt in snow on Arctic sea ice and in coastal regions

Abstract. Snow, through its trace constituents, can have a major impact on lower tropospheric chemistry, as evidenced by ozone depletion events (ODEs) in oceanic polar areas. These ODEs are caused by the chemistry of bromine compounds that originate from sea salt bromide. Bromide may be supplied to the snow surface by upward migration from sea ice, by frost flowers being wind-blown to the snow surface, or by wind-transported aerosol generated by sea spray. We investigate here the relative importance of these processes by analyzing ions in snow near Alert and Ny-Ålesund (Canadian and European high Arctic) in winter and spring. Vertical ionic profiles in the snowpack on sea ice are measured to test upward migration of sea salt ions and to seek evidence for ion fractionation processes. Time series of the ionic composition of surface snow layers are investigated to quantify wind-transported ions. Upward migration of unfractionated sea salt to heights of at least 17cm was observed in winter snow, leading to Cl- concentration of several hundred µM. Upward migration thus has the potential to supply ions to surface snow layers. Time series show that wind can deposit aerosols to the top few cm of the snow, leading also to Cl- concentrations of several hundred µM, so that both diffusion from sea ice and wind transport can significantly contribute ions to snow. At Ny-Ålesund, sea salt transported by wind was unfractionated, implying that it comes from sea spray rather than frost flowers. Estimations based on our results suggest that the marine snowpack contains about 10 times more Na+ than the frost flowers, so that both the marine snowpack and frost flowers need to be considered as sea salt sources. Our data suggest that ozone depletion chemistry can significantly enhance the Br- content of snow. We speculate that this can also take place in coastal regions and contribute to propagate ODEs inland. Finally, we stress the need to measure snow physical parameters such as permeability and specific surface area to understand quantitatively changes in snow chemistry.

Frost flowers on sea ice as a source of sea salt and their influence on tropospheric halogen chemistry

Frost flowers grow on newly‐formed sea ice from a saturated water vapour layer. They provide a large effective surface area and a reservoir of sea salt ions in the liquid phase with triple the ion concentration of sea water. Recently, frost flowers have been recognised as the dominant source of sea salt aerosol in the Antarctic, and it has been speculated that they could be involved in processes causing severe tropospheric ozone depletion events during the polar sunrise. These events can be explained by heterogeneous autocatalytic reactions taking place on salt‐laden ice surfaces which exponentially increase the reactive gas phase bromine (“bromine explosion”). We analyzed tropospheric bromine monoxide (BrO) and the sea ice coverage both measured from satellite sensors. Our model based interpretation shows that young ice regions potentially covered with frost flowers seem to be the source of bromine found in bromine explosion events.

Towards multi-sensor microwave remote sensing of frost flowers on sea ice

AbstractIt has been hypothesized that frost flowers could be important for tropospheric chemistry and, as a source of sea-salt aerosol, for the interpretation of ice-core data. Furthermore, frost flowers can cause severe errors in sea-ice products derived from remote-sensing data. Up to now there have been few datasets available relating frost-flower distribution and variability in the sea-ice-covered regions. We present a method for frost-flower detection using a combination of active and passive microwave sensors. The vertical polarized C-band radar backscatter of a young sea-ice region covered with frost flowers can be higher than the signal from multi-year ice or from the wind-roughened ocean. Therefore, the classification result of synthetic aperture radar (SAR) sea-ice images can be defective due to the influence of frost flowers. The ambiguity of the open-water and frost-flower backscatter signal was successfully resolved using additional information from the 85 GHz Special Sensor Microwave/Imager (SSM/I) channels. Results of the method were compared to aircraft measurements and observations in the marginal ice zone near Svalbard.

A reinterpretation of sea-salt records in Greenland and Antarctic ice cores?

AbstractIt has recently been shown that much sea-salt aerosol around the coast of Antarctica is generated not from open water, but from the surface of newly formed sea ice. Previous interpretations of ice-core records have disregarded the sea-ice surface as a source of sea salt. The majority of sea-salt aerosol at Halley research station originates from frost flowers rather than open water, and the seasonal cycle of sea salt in aerosol at Halley appears to be controlled by ice production in the Weddell Sea, as well as variations in wind speed. Frost flowers are also an important source of aerosol at Siple Dome, suggesting that variations in sea-salt concentrations in the core, and other cores drilled in similar locations, may be reflecting changes in sea-ice production rather than changes in transportation patterns. For Greenland cores, and those from low-accumulation inland sites in Antarctica, it is not simple to calculate the proportion of sea salt originating from frost flowers rather than open water. However, modelling studies suggest that a sea-ice surface source contributed much of the flux of sea salt to these sites in glacial periods, suggesting that interpretations of ice-core records from these locations should also be revisited.

A year‐long record of size‐segregated aerosol composition at Halley, Antarctica

Size‐segregated aerosol samples were collected with a cascade impactor at 2 week intervals for a year at the research station Halley, situated near the coast in the Weddell Sea region of Antarctica. Sea salt is a major component of aerosol throughout the year, and we estimate that at least 60% of the total sea salt arriving at Halley is from brine and frost flowers on the sea ice surface rather than open water. Chloride in sea‐salt particles is depleted relative to sodium in summer, consistent with loss of HCl as sea‐salt particles react with gaseous acidic species, but is enhanced in large particles in winter because of fractionation occurring during the production of new sea ice. Non‐sea‐salt sulphate peaks in the summer, with the majority being in small particles indicative of a gas phase origin. The distribution of methane sulphonic acid closely follows that of non‐sea‐salt sulphate. In the winter, non‐sea‐salt sulphate is frequently negative, especially on stages collecting large particle sizes, consistent with the source of sea salt during the winter being predominantly the sea ice surface rather than open water. Nitrate peaks in the spring and summer and shows some association with sea‐salt particles.

Surface characteristics and atmospheric footprint of springtime Arctic leads at SHEBA

Observations of several freezing leads that occurred in spring near the Surface Heat Budget of the Arctic Ocean (SHEBA) ice station were made. The leads that formed during this study were between 3 and 400 m wide. Ice production in the leads less than 20 m wide was predominantly through congelation growth, while both frazil ice production and congelation ice growth was observed in the wider leads. The production of frazil ice and its advection downwind allowed open water to persist in the wider leads for between 5 and 24 hours, depending on the crossing angle of the wind. The surface energy budget of a wide freezing lead was estimated from observations and with a model that resolves the coupling between surface turbulent fluxes and ice growth across the lead. Both estimates of the net heat flux agreed with the increases in ice thickness observed throughout the 24‐hour period, though the modeled net heat flux deficit was 50% larger. The larger net heat flux deficit obtained with the model can be attributed to the simulation of larger turbulent heat fluxes. It was found that the surface roughness length for nilas given by Guest and Davidson [1991] was too large resulting in excessive surface cooling at night. Using a smaller roughness length improved the nighttime bias but resulted in a warm bias during the day. The daytime warm bias was due, in part, to neglecting the impact of frost flowers on the surface albedo. Additional uncertainty in the treatment of solar absorption by nilas also likely contributed. The modeled ice thickness and skin temperature were also affected by the treatment of the oceanic heat flux, which acted to warm the surface. The length of time that a lead affects the atmosphere is determined by lead surface conditions, atmospheric stability, wind speed, fetch, and upwind temperature. Under lead‐perpendicular winds the atmospheric influence of a 400 m wide lead extended more than 2.5 km downwind. Sensible heat fluxes observed 70 m downwind of the lead were a function of across lead fetch, upwind stability, and open water fraction. The sensible heat fluxes measured at this site were elevated above background values for nearly two days despite 11.5 cm of ice growth in the lead.

Frost flowers: Implications for tropospheric chemistry and ice core interpretation

This paper discusses the chemical composition of frost flowers and their accompanying slush layers and the evidence for their role as a salt source in processes important to atmospheric chemistry and ice core interpretation. Analysis of Antarctic frost flowers shows that they are highly saline and fractionated in sea‐salt ions, with sulfate being depleted strongly relative to sodium. Because frost flowers give a bright return on satellite scatterometer images, the times and places of their formation can be identified. When winds blow towards an aerosol sampling station from areas identified by the scatterometer as covered with flowers, the collected aerosol is also depleted in sulfate. Because the flowers have a large salinity, bromide concentrations are elevated in frost flowers relative to seawater. With their high surface area, it is possible that bromine is released to the atmosphere from frost flowers, with consequent implications for tropospheric ozone depletion. The finding that quantities of fractionated sea salt are available at the sea–ice interface in the winter months and may be transported inland as aerosol also has implications for the interpretation of ice core records. Analysis of one near‐coastal core shows that the majority of the sodium comes from a fractionated source rather than from open water. Hitherto, strong sea‐salt signals in ice cores have been attributed to increased open water and more efficient transport inland, perhaps due to stormier weather. At least in coastal regions, however, these signals may be related instead to the increased formation of sea ice and frost flowers.

Frost flowers as a source of fractionated sea salt aerosol in the polar regions

Frost flowers collected from the surface of new sea ice near the Brunt Ice Shelf, Antarctica, show depletion in sulphate and sodium relative to other sea water ions. This is consistent with loss of mirabilite (Na2SO4) during formation of the brine from which the frost flowers grow. Aerosol generated from frost flowers would have higher sodium:sulphate ratios than aerosol generated from sea water. This would explain low values of non‐sea‐salt sulphate encountered in winter aerosol, and winter layers in ice cores, at coastal Antarctic sites. Calculations confirm that the frost flower source should be significant compared to an open water source for coastal regions.

Causes of seasonal and daily variations in aerosol sea-salt concentrations at a coastal Antarctic station

Two years worth of daily aerosol data has been collected from Halley station, Antarctica, between February 1991 and February 1993. The seasonal cycle of sea-salt aerosol was found to peak during the winter months, with an annual mean of 162 ng m -3. Specific site characteristics are used to explain this relatively low value. The winter sea-salt source does not appear to be solely due to the presence of open water. Comparison of individual high salt concentration events in the data, with 3 hourly meteorological records, shows that sea-salt loadings are not linked to high wind speeds, but more moderate ones. The high sea-salt loadings are associated with a change in wind direction that opens up an area of water and then switches to bring sea-salt inland. It is hypothesised that the exposed areas of sea water, which are rapidly frozen in winter creating areas of local, freshly formed ice with a surface covering of concentrated brine, are the source of the winter sea-salt. Fractionation of the sea-salt component in individual high concentration events, is used to reinforce the theory that a surface skim of highly saline brine, on fresh ice, is the winter sea-salt source. The presence of frost flowers is thought to aid incorporation of sea-salt into the atmosphere. Implications for the interpretation of sea-salt data in ice cores are highlighted.

Laboratory measurements of sea ice: connections to microwave remote sensing

The connections between laboratory measurements and remote-sensing observations of sea ice are explored. The focus of this paper is on thin ice, which is more easily simulated in a laboratory environment. The authors summarize results of C-band scatterometer measurements and discuss how they may help in the interpretation of remote-sensing data. They compare the measurements with observations of thin ice from ERS and airborne radar data sets. They suggest that laboratory backscatter signatures should serve as bounds on the interpretation of remote-sensing data. They examine these bounds from the perspective of thin ice signatures, the effect of temperature, and surface processes, such as frost flowers and slush on these signatures. Controlled experiments also suggest new directions in remote-sensing measurements. The potential of polarimetric radar measurements in the retrieval of thickness of thin ice is discussed. In addition to the radar results, the authors discuss the importance of low-frequency passive measurements with respect to the thickness of thin ice.

Detection of features in first-year pack ice by synthetic aperture radar (SAR)

Observations by specialized sonars moored beneath drifting pack ice in the Beaufort Sea have been used to guide interpretation of SAR imagery under Arctic winter conditions. The sonars measured the draft, bottom-side topography and kinematic history of the ice field during the same time that SAR images were acquired by ERS-1 (satellite-based C -band) and STAR-2 (aircraft-based X -band) radar systems. Inter-comparison of the observations revealed that ridges crossing first-year pack ice cannot be detected reliably at the low incidence angles used by ERS-1. However, at the high incidence angles typical of airborne SAR, there is sufficient contrast to enable unambiguous discrimination of ridges and intervening floes. A strong correlation was observed between the spatial frequency of ridges and the average draft of the ice field. Therefore, a count of ridges intersecting a line across a SAR image can be used as a proxy for ice draft. A linear relation was observed between radar brightness and draft of ridges. This is probably a consequence of a progressive filling of the radar pixel by the ridge sail as the draft of the ridge increases. The relation between the frequency and maximum backscatter of ridges viewed by radiometrically calibrated SAR has potential as a surrogate for the keel frequency-draft function. Estimates of ridge frequency derived via SAR can, however, be biased, since both ridges and young leads appear as bright filaments in imagery. With ERS-1, there is a large variation in the microwave backscattering coefficient of sea ice during the early growth under cold conditions. This variation provides a clear indication of ice age and stage of development. For older pack ice, ERS-1 can provide only an aggregate measure of the backscatter from unresolved ridges and level floes in a selected area. The strength of this backscatter is a poor indicator of age or thickness because the relative contributions by ridges, frost flowers and smooth floes cannot be determined.

A laboratory study of the effect of frost flowers on C band radar backscatter from sea ice

C band images of Arctic sea ice taken by the ERS 1 synthetic aperture radar show transitory regions of enhanced radar backscatter from young sea ice. Published field observations associate this increase with frost flower growth and the capture of blowing snow by the flowers. To investigate the first part of this phenomenon, we carried out a laboratory experiment on the response of C band radar backscatter to frost flowers growing on the surface of newly formed saline ice. The experiment took place in a 5 m by 7 m by 1.2 m deep saline water pool located in a two‐story indoor refrigerated facility at the Cold Regions Research and Engineering Laboratory. Sodium chloride ice was grown in this pool at an air temperature of −28°C. The frost flowers first appeared on the ice surface as dendrites and then changed to needles as the ice sheet grew thicker and the surface temperatures became colder. The frost flowers reached to a height of 10–15 mm, and beneath each cluster of frost flowers a slush layer formed to a thickness of approximately 4 mm. Far‐field radar measurements of the backscatter from the ice were made at incident angles from 20° to 40° and at approximately 6‐hour intervals throughout the 3‐day period of the experiment. A backscatter minimum occurred early in the flower growth at the time coincident with an abrupt doubling in the ice surface salinity. Once the full flower coverage was achieved, we removed first the crystal flowers and then the slush layer from the ice surface. The results for these cases show that the crystals have little impact on the backscatter, while the underlying slush patches yield a backscatter increase of 3–5 dB over that of bare ice. The laboratory results suggest that this relative backscatter increase of approximately 5 dB can be used as an index to mark the full areal coverage of frost flowers.

SAR observations of Arctic freeze-up compared to SSM/I during ARCTIC'91

Abstract Ice concentration in the Arctic derived from ERS-1 Synthetic Aperture Radar (SAR) and Special Scanning Microwave/Imager (SSM/I) images are compared. The satellite data are compared to video images and in situ measurements. The data were acquired during the freeze-up period of the ARCTIC'91 expedition. The studied areas were characterized by melting conditions and new ice formation with frost flowers. The ERS-1 SAR images are classified by a local averaging method and a segmentation method. Parameters for the methods are derived from the backscattering distributions. Temporal sequences and meteorological information are used for consistent results. Ice concentration derived from SAR are compared with the SSM/I ice concentration (NASA team algorithm) and ship observations. SSM/I may underestimate the ice concentration by 20 per cent due to thin ice formation and melting conditions while SAR may overestimate. However, by using the SAR estimate of the different ice classes we believe it is possible t...

The temperature dependence of frost flower growth on laboratory sea ice and the effect of the flowers on infrared observations of the surface

This paper describes a laboratory study of frost flower growth on young sea ice at different temperatures and the effect of these flowers on the surface temperature observed with an infrared radiometer. The flowers grew on sea ice which formed in a salt water tank at room temperatures of −20, −24, and −30°C, with an additional experiment at −16°C, where no flowers appeared. The growth habit and height of the observed crystals depended on the existence of a region of supersaturated vapor adjacent to the surface and on the range of temperatures in the surface boundary layer. The source of the surface brine from which the flowers grew was probably brine transport within the ice toward the cold upper surface driven by the thermomolecular pressure gradient. The evaporation of vapor from this liquid into the atmospheric boundary layer provided the supersaturated region adjacent to the ice surface. Two kinds of flowers were observed; at −20 and −24°C, dendritic crystals grew approximately between the −12 and −16°C isotherms, and, at −30°C, rod‐like flowers appeared between −16 and −25°C. These limits correspond to earlier work on crystal growth from the vapor. In each case, the maximum flower height approximately equaled the height of the isotherm corresponding to the colder temperature limit for each crystal type, −16°C for the dendrites and −25°C for the rods. The effect of the flowers on the radiometer surface temperature was as follows: because the flowers protrude 10–20 mm above the surface into the boundary layer, the infrared temperature of the flower‐covered ice was about 4–6°C colder than that of the same ice cleared of flowers. We also found that the insulating effect of the flowers caused the ice surface temperature beneath the flowers to be 1–2°C warmer than the surrounding bare ice. The importance of the flower growth is that infrared satellite observations of thin ice in winter will be colder than the actual surface temperature, which may account for the absence of warm young ice in infrared satellite images.