Like ozone hole, polar clouds take bite out of meteoric iron

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 mesosphere. The discovery 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 Illinois at Urbana-Champaign and the University of East Anglia in Norwich, England, studied the removal of meteoric iron by polar mesospheric clouds 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 Illinois and one of the co-authors of a paper to appear in the April 16 issue of the journal Science.

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 ablation of meteoroids entering the atmosphere.

“At such cold temperatures, the iron atoms stick when they bump into the ice crystals,” Gardner said. “If the removal of iron is rapid compared to both the input of fresh meteoric ablation 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 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 flow tube. Iron atoms were then generated by laser ablation of an iron target at one end of the tube. 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 sticking coefficient,” Plane said. “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 as a function of altitude. They concluded there was sufficient surface area to remove the iron.

“Our results clearly demonstrate the importance of ice particles in the chemistry of this region of the atmosphere,” Gardner said. “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 Environmental Research Council funded the work.