Andrew M. Rankin

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

[1] 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. INDEX TERMS: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 1640 Global Change: Remote sensing; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 3360 Meteorology and Atmospheric Dynamics: Remote sensing. Citation: Kaleschke, L., et al. (2004), Frost flowers on sea ice as a source of sea salt and their influence on tropospheric halogen chemistry, Geophys. Res. Lett., 31, L16114, doi:10.1029/ 2004GL020655.

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.

A role for newly forming sea ice in springtime polar tropospheric ozone loss? Observational evidence from Halley station, Antarctica

Since March 2003, measurements of surface ozone have been made at the British Antarctic Survey Clean Air Sector Laboratory (CASLab) at Halley station in coastal Antarctica. Detailed measurements of boundary layer meteorology, as well as standard meteorological parameters, are also measured at the CASLab. Combining these data allows us to probe the transport pathway of air masses during ozone depletion events (ODEs). ODEs were observed at Halley on several occasions during Antarctic spring 2003. On some occasions, extremely rapid loss of ozone was observed (loss of 16 ppbv in 1 min on one occasion), which was associated with regional-scale transport. For each such event during 2003, the air mass originated in the southern Weddell Sea, an area of vigorous sea-ice production. On other occasions the development of the event and its recovery were strongly associated with the build-up and decline of a stable boundary layer. In these cases, air masses had had recent contact with a nearby open water lead where sea-ice production is known to occur. The data presented here are entirely consistent with the idea that halogens responsible for ozone loss are derived during new sea-ice formation from an associated surface such as brine slush or frost flowers.

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

Abstract It 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.

Ammonium and potassium in snow around an emperor penguin colony

Snow samples taken at various distances from the emperor penguin (Aptenodytes forsteri) colony near Halley station were analysed by ion chromatography. Extremely high ammonium concentrations were encountered at the colony itself, but fell off sharply with distance from the colony, reaching background levels within a few kilometres of the colony. A seasonal effect was also seen, with the highest concentrations found in spring when the colony was at its most active. Levels of potassium and other sea-salt ions were also elevated near the colony. The ratio of sodium to potassium was lower than that found in bulk seawater, and closer to that found in the penguins food source, indicating that the increased concentrations are due to emissions from the penguins and not merely to the proximity of open seawater to the site. The colony thus has a significant effect on the composition of the nearby snow, but this effect is strongly localised and is not likely to significantly influence snow chemistry at inland ice core drilling sites.

Aerosol profiling using a tethered balloon in coastal antarctica

The composition of air near ground level is not necessarily representative of the troposphere as a whole. In particular in the Antarctic, stratification from the strong inversions often present and scavenging by blowing snow in the lowest part of the boundary layer may lead to air masses aloft being of different composition than those at ground level. The difference in aerosol composition with height in the Antarctic has been shown for the first time with a lightweight sampling system capable of being hoisted to heights of a few hundred meters on a helium-filled blimp. The system was tested at Halley research station in the Weddell Sea region of coastal Antarctica. On more than one occasion, air with a strong sea-salt aerosol component was found aloft, despite the air at ground level showing little marine influence. Meteorological instruments carried on the blimp just prior to one such flight indicated the presence of a strong inversion at the time.