Large Particles in Arctic Stratosphere Help Explain Ozone Loss

John Bluck

NASA Ames Research Center, Moffett Field, CA

(Phone: 650/604-5026 or 604-9000)

E-mail: jbluck@mail.arc.nasa.gov

David W. Fahey

NOAA Aeronomy Laboratory, Boulder, CO

(Phone: 303-497-5277)

E-mail: dfahey@al.noaa.gov

Newly discovered large particles that contain nitric acid are helping
scientists better explain the process of ozone loss in the Arctic’s
stratosphere 6 to 30 miles above Earth.

An international team of 27 scientists published their findings about the
new class of particles in today’s issue of Science magazine. The Earth’s
ozone layer protects life below from harmful ultraviolet radiation coming
from the Sun that can lead to the formation of skin cancers.

“The size of these particles was surprisingly big, and they are part of a
process that is removing nitric acid from the stratosphere, eventually
leading to ozone loss,” said Hansjurg Jost, co-author of the Science
article and a scientist at NASA Ames Research Center, in California’s
Silicon Valley. Lead author of the paper is David W. Fahey of the National
Oceanic and Atmospheric Administration (NOAA), Boulder, CO.

“Ames teams contributed to these findings with crucial high-precision
measurements of trace gases and temperature,” said co-author Thaopaul Bui
of Ames.

The newly discovered class of particles has given scientists a better
understanding of the processes that “set the stage” for chlorine-caused
ozone depletion in the Arctic stratosphere. Scientists have observed
unusually low levels of ozone over the Arctic during recent winters.

Icy, nitric acid-containing “polar stratospheric cloud” (PSC) particles are
formed in the polar regions during winter, where they enhance the
destruction of ozone caused by human emissions of chlorine and bromine.
Fahey, a scientist at NOAA’s Aeronomy Laboratory, and colleagues, reported
the discovery of a new population of very large PSCs. They have diameters
of 10-20 microns (millionths of a meter), which is about 10-20 times larger
in diameter than typically observed in PSCs.

These particles have eluded detection to date because of their large size
and very small abundance in the atmosphere, scientists said. The PSCs are
laden with nitric acid and serve as reservoirs for nitrogen in the polar
stratosphere. As the particles sediment, or fall out of the atmosphere,
the stratosphere becomes “denitrified.”

The loss of nitrogen has consequences for ozone, because ozone-destroying
forms of chlorine and bromine are longer-lived in a denitrified
stratosphere. The discovery of this new class of large PSC particles helps
to explain a long-standing mystery that the extent of denitrification
observed in the polar stratosphere could not be accounted for by the
smaller (and slower-to-sediment) PSCs.

Scientists made their observations in the Arctic stratosphere from
January-March 2000. Instruments onboard a NASA ER-2 high-altitude research
aircraft measured reactive nitrogen species as the aircraft traveled toward
the North Pole and deep into the region of highest ozone loss.

The large-sized PSCs observed in some of the air samples contained 15-20
percent of the available reactive nitrogen in the Arctic stratosphere, and
were falling at a rate of 1-2 kilometers per day. These values demonstrate
the potential for significant denitrification by the large particles. The
2000 Arctic winter stratosphere was extensively denitrified, which set the
stage for significant chlorine- and bromine-caused ozone loss in the winter
and spring.

Cold temperatures promote the growth of large PSCs, enhancing the loss of
ozone by chlorine and bromine. Unusually cold winters, or climate shifts
that reduce stratospheric temperatures or alter the amount of water vapor
in the stratosphere, could prolong chemical ozone loss in the Arctic even
as chlorine levels fall in response to international curtailments in the
use of ozone-depleting chemicals. “Our findings suggest that we need to
learn more about how these large PSCs are formed, so that we’ll have a
better understanding of how the ozone layer will recover in the future,”
Fahey said.

Cooling of the stratosphere will likely increase ozone loss during Arctic
winters in coming decades, even as chlorine and bromine levels decrease as
a result of the Montreal Protocol, according to scientists. The buildup of
greenhouse gases, such as carbon dioxide, tends to trap more heat near the
Earth’s surface, while at the same time colder than normal temperatures are
experienced above, in the stratosphere, where ozone breakdown occurs,
researchers said.

The ER-2 science flights took place as part of two international field
experiments: NASA’s SAGE III Ozone Loss and Validation Experiment (SOLVE)
and the European Commission-sponsored Third European Stratospheric
Experiment on Ozone.