Frost flowers – implications for tropospheric chemistry and ice

core interpretation

Andrew Rankin1, Eric Wolff1 and Seelye Martin2

(1) British Antarctic Survey, Cambridge, UK.

(2) University of Washington, Seattle, USA.

Thanks

• Victoria Lytle, U. Tasmania – collection of frost flowers near Mertz Glacier

• BAS Halley wintering staff – collection of frost flowers near Halley

• David G. Long, Brigham Young University, NASA-scatterometer images

• Mark Drinkwater, ESA, interpretation of scatterometer images

Outline

• What are frost flowers?

• Importance as source for sea salt aerosol and ice cores

• Importance as surface for Br-related chemistry

What are frost flowers?

Widespread, whenever new sea ice forms

Important part of sea-air interface

Until now, no chemical studies

Source of sea salt aerosol?

• Numerous studies show that winter aerosol and snow, at least in coastal Antarctica, are significantly depleted in sulfate relative to sea water, ie nss SO4

2- is negative

• Ratio of SO42-:Na in sea salt aerosol closer to 0.1 cf 0.25

for seawater

• Sea salt maximum in winter despite much greater sea ice extent

Hypothesis: Sea ice surface (esp. frost flowers) is main source of sea salt, which assumes this surface is highly saline and depleted in sulfate

Frost flowers and brine are highly saline, diluted only when mixed with fresh snow

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Frost flowers and some brine are highly depleted in SO42-

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At Halley, all episodes of salty aerosol, depleted in sulfate, are associated with frost flowers; but this is not a sufficient condition

Each vertical bar represents one aerosol filter; grey ones are significantly depleted in sulfate. Black horizontal bars are periods of frost flowers near Halley.

Frost flower bloom seen on scatterometer (NASA QSCAT)

Back trajectories from Halley (ECMWF) pass over frost flower area

Halley aerosol filter shows high salt, depleted in sulfate

Back trajectories not over frost flowers, then flowers area decays

Halley aerosol is now low in salt and not obviously depleted in sulfate

Majority of Na in the core comes from the fractionated mode

Dolleman Island ice core

Conclusions for ice cores

• Beyond reasonable doubt, frost flowers and sea ice surface are main source of sea salt aerosol in coastal Antarctica

• This conclusion may be extendable to inland sites

Frost flowers contain high concentrations of bromide, though only in line with their increased salinity. Br- is likely to be on the surface however cold the ice

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Surfaces for Br-related chemistry

Surfaces and sources for Br-related chemistry

• Frost flowers vs sea water– Surface areas likely to be greater than that of fresh snow (~2 m2 g-

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– Where flowers present, estimate ~ 0.05 g cm-2 (Perovich & Richter-Menge 1994)

Surface areas of order 1000 m2 m-2

Concentrations higher than in sea water or old sea ice, both of which have relatively flat surface areas

Likely that at least in winter half-year, frost flowers present a greater Br-laden surface than sea water around Antarctica

Surfaces and sources for Br-related chemistry

• Frost flowers versus snowpack– Surface areas of snowpack to a few cm depth

are very large– But concentration of Br- in snowpack at most a

few ng g-1, compared to 200 000 ng g-1Br- in frost flowers

Frost flowers are expected to provide much more Br than snowpack can in coastal Antarctica

• Further study is needed but it seems probable that frost flowers provide by far the biggest surface area of available Br-

• Enhanced filterable Br and low-level tropospheric ozone loss episodes are compatible with the episodic (few days) nature of frost flowers

Conclusions• Sea ice surface (esp. frost flowers) is the main

source of sea salt aerosol in coastal Antarctica• Frost flowers are probably the greatest surface

area of Br-rich material available for reactionsObvious next steps• Relating Br/ozone events with QSCAT frost

flower events• And determining source of sea salt in central

Antarctica