Scientists have confirmed a 2.5 million-square km hole in the
low-level ozone layer over western China's Qinghai-Tibet
Plateau.
The hole formed in December 2003 over the plateau, which stands
at an average 4,000 meters above sea level, according to an article
in the reputable Chinese science magazine Scientific
Report.
Experts from the Chinese Academy of Sciences and the China
Meteorology Research Center have proved a significant decrease in
total column ozone.
The article is based on comprehensive research and analysis of
data from both ground monitoring and the Total Ozone Mapping
Spectrometer, a satellite-borne instrument used to measure global
ozone levels.
The scientists have been monitoring ozone changes over the
plateau since a dramatic loss in upper-level ozone was recorded in
summer 2003.
However, they are divided over whether another hole will form
over the plateau (the other two are over the Antarctic and the
Arctic).
The article says an area of 2.5 million square km air with a
total ozone of less than 220 Dobson Units (DU) was found over the
plateau from December 14 to 17, 2003, and hit a record low of 190
DU.
The international measuring system of Dobson Units prescribes
100 DU to equal a one-millimeter thick layer of pure ozone with
conditions of one standard atmospheric pressure and a temperature
of zero centigrade.
This is the first time that an ozone minimum-hole or extremely
low ozone has been witnessed over the region, the article says.
However, experts believed that the cause of the hole over the
plateau differs from that in the two poles, due to varied
atmospheric movements.
"The decrease of ozone over the plateau was caused by airflow
exchange in the sky. When low-ozone air currents in lower layer
enter the upper air layer, the overall ozone density is reduced,"
the article says.
The ozone holes over the Poles were caused by the global
"greenhouse effect."
Ozone is one of the gases forming the earth's atmosphere and is
the major shield against Ultraviolet-B (UVB) radiation, absorbing
approximately 90 percent of solar UVB. Excessive exposure to UVB
can cause skin cancer in humans and is a major contributor to
glacial melting.
(Xinhua News Agency May 4, 2006)