Cool Harvest: Frost on sea ice may boost atmosphere’s bromine

Frost flowers, the delicate crystals that sometimes grow atop fresh sea ice, can be a substantial source of ozone-destroying bromine in the lower atmosphere near the poles, researchers suggest.

CHILLY GARDEN. Frost flowers, such as these 2-centimeter-tall crystals on a patch of fresh ice in the Arctic Ocean, may be a major source of atmospheric bromine. S. Kern/Univ. of Hamburg

Over tropical and temperate seas, salt spray from breaking waves provides most of the low-altitude atmospheric bromine. The origin of bromine over ice-bound oceans has remained a mystery, but several lines of evidence now point to frost flowers as culprits. These arrangements of needlelike ice crystals often grow on the new ice that forms rapidly in frigid conditions.

When winds are calm and the humidity just above the fresh ice is high, a dense crop of frost flowers sprouts. The fragile structures can reach heights of 2 centimeters or so. If the air temperature is about –40°C, for example, frost flowers can accumulate over as much as 50 percent of the fresh ice in just 5 days.

When seawater freezes, many dissolved salts and ions, including bromine, concentrate in briny pools atop the new ice. The frost flowers act as wicks, drawing the liquid into their petal-like crystals. There, the concentrations of dissolved ions can reach about triple what’s normally found in the ocean, says Georg C. Heygster, a physicist at the University of Bremen in Germany. What’s more, he notes, the brine-soaked crystals provide a large surface area where chemical reactions can take place.

The team’s new analyses of ground-based weather data bolster this notion by showing that some polar air masses containing elevated concentrations of bromine monoxide (BrO) had on the previous day passed over areas of fresh sea ice, which probably sported frost flowers. Heygster and his colleagues report this finding in the Aug. 28 Geophysical Research Letters.

The reactions that produce BrO and other brominated gases are typically driven by sunlight. Satellite data of the polar-ocean region indicate that BrO shows up in especially large quantities only after sunshine returns to polar regions in the spring, the researchers point out.

“Until recently, frost flowers seemed like just a very pretty, but not especially important, phenomenon,” says Eric W. Wolff of the British Antarctic Survey in Cambridge, England. The new research provides, “for the first time, some real evidence that the areas where frost flowers are likely to occur are also the areas where [scientists] find high levels of BrO” in the lower atmosphere, Wolff notes.

Ice covered with frost flowers reflects radar differently than smooth ice does, says Mark R. Drinkwater of the European Space Agency in Noordwijk, the Netherlands. Scientists might next use high-resolution radar observations from satellites to more definitively link BrO plumes to patches of ice covered with frost flowers.

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