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Polar clouds take a 'bite' out of meteoric iron

April 14, 2004

Polar clouds are known to play a major role in the destruction of Earth's protective ozone layer, creating the springtime 'ozone hole' above Antarctica. Now, scientists have found that polar clouds also play a significant role in removing meteoric iron from Earth's atmosphere.

Polar clouds have been widely studied in recent years, because of their potential to provide an early indication of climate change in the upper atmosphere. The clouds were first observed over northern Europe in 1886, and appear to have increased in frequency since then. They are also being observed more often at lower latitudes (about once a year over the southern UK, in late June or early July). The findings about removal of iron could help researchers refine their models of atmospheric chemistry and global warming.

Using a sensitive laser radar (lidar) system, laboratory experiments and computer modeling, researchers from the University of East Anglia and the University of Illinois studied the removal of meteoric iron by polar clouds in the Earth's mesosphere that they observed during the summer at the South Pole.

"Our measurements and models have shown that another type of reaction that takes place in the upper atmosphere - this time related to ice particles - plays a very important role in the processes that influence the chemistry of metal layers in this region," said Chester Gardner, a professor of electrical and computer engineering at the University of Illinois and one of the co-authors of the paper published in this week's edition of the journal Science (16 April 2004).

First deployed over Okinawa, Japan, to observe meteor trails during the 1998 Leonid meteor shower, the Illinois lidar system uses two powerful lasers operating in the near ultraviolet region of the spectrum and two telescopes to detect laser pulses reflected from the atmosphere. The system was moved to the Amundsen-Scott South Pole Station in late 1999.

"Simultaneous observations of the iron layer and the clouds revealed nearly complete removal of iron atoms inside the clouds," Gardner said. "Laboratory experiments and atmospheric modeling done by our colleagues at the University of East Anglia then showed that this phenomenon could be explained by the efficient uptake of iron on the surfaces of ice crystals."

Polar mesospheric clouds are the highest on Earth, forming at an altitude of about 52 miles. The clouds form over the summertime polar caps when temperatures fall below minus 125 degrees Celsius, and overlap a layer of iron atoms produced by the breaking up of small meteoroids entering the atmosphere.

"At such cold temperatures, the iron atoms stick when they bump into the ice crystals," said Professor Gardner. "If the removal of iron is rapid compared to both the input of fresh meteoric iron and the vertical transport of iron into the clouds, a local depletion or 'bite-out' in the iron layer will result."

To examine whether the observed bite-outs could be fully explained by the removal of iron atoms by ice particles, John Plane, a professor of environmental sciences at the University of East Anglia, and graduate student Benjamin Murray measured the rate of iron uptake on ice.

In their laboratory, Plane and Murray first deposited a layer of ice on the inside of a tube. At one end of the tube, iron atoms were generated by a laser focused onto an iron target, while at the other end, a second laser measured how much iron made it through the tube.

"By changing the temperature in the tube, we could compare how much iron was absorbed by the ice and calculate the rate of sticking," said Professor Plane. "Once we knew how efficiently the iron atoms stick to the ice, our next question was whether there was enough ice surface in the polar clouds to deplete the iron and cause the dramatic bite-outs revealed in the lidar observations."

The researchers answered this question by carefully modeling the size distribution of ice particles depending on altitude. They concluded there was sufficient surface area to remove the iron. "Because iron atoms stick so efficiently when they hit the ice surface, all of the iron in the vicinity of a large cloud is removed in a couple of hours", reports Plane. "This is one of the most striking direct observation of this kind of process in the atmosphere, the more so considering that these ice particles have a radius of about 5 millionths of a centimetre."

"Our results clearly demonstrate the importance of ice particles in the chemistry of this region of the atmosphere," said Professor Gardner. "Not too many years ago we learned how important polar stratospheric clouds were to the chemistry of the ozone layer. Now we are seeing something very similar happening at higher altitudes."

In addition to Gardner, Plane and Murray, the team included research scientist Xinzhao Chu from the University of Illinois who made the measurements at the South Pole. The National Science Foundation, the Royal Society and the Natural Environment Research Council funded the work.

East Anglia, University of