September 5, 2003
Ministry of the Environment publisized the state of ozone depletion in the stratosphere over Antarctica, and other related observation results by analyzing the data collected to date by ILAS-II mounted on the ADEOS- II launched in December 200.
Stratospheric temperatures over Antarctica in 2003 remain at the lowest range since the 1980s, when the ozone hole first became noticeable. Large amounts of Polar Stratospheric Clouds (PSCs) have occurred since late May because of the low temperatures. These clouds are believed to play a central role in the formation of the ozone hole.
Observation results confirmed that about 30% of ozone has already been destroyed over Antarctica as of the end of August 2003. This is the worst ever ozone depletion that has occurred at this time of the year. The ozone hole over Antarctica is expected to become even larger in the coming months of September and October.
1. Improved Limb Atmospheric Spectrometer-II (ILAS-II)
The Ministry of the Environment has developed ILAS-II to monitor stratospheric ozone and other atmospheric trace gases related to ozone destruction, investing 3.5 billion yen since 1995. ILAS-II is a successor to ILAS, which is mounted aboard the Advanced Earth Observing Satellite (ADEOS), also known as Midori, launched in 1996.
The ILAS-II was mounted on the ADEOS-II (Midori II) and launched on December 14, 2002 from the Tanegashima Space Center (TNSC) of the National Space Development Agency of Japan (NASDA).
ILAS-II is a unique sensor compared to those made by the US and European countries so far. Its main features include:
- Simultaneous measurement of various substances relating to the depletion and recovery of the ozone layer, including ozone, nitric acid, nitrogen peroxide, dinitrogen monoxide, methane, water vapor, chlorine nitrate, and various aerosols.
- Highly precise and continuous measurement over a long period (3-5 years) at various altitudes (at one km intervals between 10 to 60 kilometers above the Earth's surface) with particular focus on the North and South Pole areas where ozone depletion is especially notable.
ILAS-II underwent functional testing at the beginning of 2003 and since April 2 of that year, has been successfully gathering data on a continuous basis.
2. State of the stratosphere above Antarctica in 2003
The stratospheric temperatures above Antarctica in 2003 have been extremely low contrary to the sudden rise in temperature at the end of September 2002, which caused the ozone hole to split into two. As shown in Fig. 1, temperatures measured at the stratosphere at approximately 20 km above Antarctica since mid-June 2003 have been the lowest observed since the 1980s, when the ozone hole first became noticeable. This trend continued even into September.
When the temperature of the stratosphere at about 20 km above the Earth's surface drops below -85ºC, clouds made up mainly of ice, typical of the polar areas, are believed to form. These are known as Type-II Polar Stratospheric Clouds (PSCs). They are considered to play a central role in the formation of the ozone hole.
Fig. 2 shows the occurrence of PSCs at 20 km above Antarctica observed by ILAS-II. These data show that PSCs have occurred frequently above Antarctica since the second half of May 2003. The frequency of occurrence was of the largest scale since the 1980s when the ozone hole first became apparent. This suggests that Antarctica might experience large-scale ozone depletion later in the year.
3. Ozone depletion above Antarctica as observed by ILAS-II
According to observations by the total ozone mapping spectrometer (TOMS), a sensor mounted on the US satellite Earth-Probe, it has been confirmed that ozone depletion over Antarctica has already occurred (Fig. 3). Because TOMS observes directly downward from the upper level, it can only detect the total amount of ozone from the Earth's surface to the midair region. It cannot detect at what altitudes the ozone is being destroyed.
On the other hand, data gathered by ILAS-II in 2003 (Fig. 4) showed that the average ozone level at 15-25 km over Antarctica between August 20 and August 26 was clearly low compared to the observed values in June and July. At this altitude, ozone depletion had taken place in wide areas over Antarctica.
Fig. 5 shows the correlation between ozone and dinitrogen monoxide (N2O), a tracer (gas that is chemically stable and does not change due to photochemical reaction), which have been observed simultaneously by ILAS-II. This also indicates that the ozone over Antarctica has already begun to decline over a large area as of August 2003. Roughly speaking, when compared to the June data, the August data showed that 30% of the ozone has already been destroyed. Continuous and simultaneous observation of dinitrogen monoxide and ozone over Antarctica using a satellite sensor aboard ILAS-II was the world's first use of such technology. The method is expected to provide important data on ozone depletion in a quantitative manner in the future.
As explained above, data obtained to date by ILAS-II shows that at altitudes between 15-25 km, where PSCs actually occur, the ozone hole mechanism, activating chlorinated compounds discharged from man-made chlorofluorocarbons interact with sunlight resulting to destruct ozone in the advance of spring, has already been triggered over Antarctica. This year has seen the lowest temperatures since the 1980s, when the ozone hole first became noticeable. If the occurrence of the largest amount of PSCs in the last twenty years coincides in timing with the maximum concentration of chlorinated compounds originating from CFCs in the stratosphere, an ozone hole of the largest scale may develop over Antarctica sometime around September and October 2003. ILAS-II and TOMS are expected to monitor the ozone layer closely.
4. Outlook of the ozone hole in 2003
The ozone hole appears in early spring (September to October in Antarctica). When summer comes, the reaction that triggers ozone depletion does not occur as readily, and the ozone hole disappears as it runs into air from the mid-latitudes. The substances that stop the ozone depleting reaction are highly responsive nitrogen oxides (NO, NO2, HNO3, ClONO2, N2O5), which react to active halogen compounds. They are collectively known as NOy. When the PSCs exist, HNO3, a main element of NOy, dissolves in the PSCs and precipitates by gravity, therefore disappearing from the stratosphere. This phenomenon is called denitrification. When this denitrification occurs, even a small amount of ozone-depleting substances (halogen compounds) can effectively destroy ozone, maintaining the ozone depletion reaction cycle into early summer (November-December in Antarctica.)
Fig. 6 shows the correlation between the observed values of NOy and dinitrogen monoxide (N2O) at an altitude of 15-30 km measured by ILAS-II in May (black) and August (red) of 2003. Both NOy and N2O are tracers, the concentrations of which are known to remain unchanged in the process of air movement. The two are also known to be closely related. When PSCs occur, HNO3 is taken into PSCs (denitrification), reducing the amount of NOy substantially, while N2O level remains unchanged. Using this, changes in the concentrations of NOy can be projected.
Before the occurrence of PSCs in May 2003, there is a strong correlation between NOy and N2O, as shown by the dotted lines. On the other hand, data from ILAS-II indicated that over 90% of NOy over Antarctica had been lost in August. It was the first time in recorded history that denitrification of such a large scale occurring over Antarctica in August has ever been observed.
This denitrification has occurred because the 2003 winter temperatures in Antarctica had remained in a low range, as compared to past years. As a result, the ozone hole over Antarctica in 2003 is expected to be larger compared to past years and will last longer. It is important to utilize ILAS-II to continuously monitor denitrification and PSCs over Antarctica in the future, in tandem with observation of the ozone and dinitrogen monoxide.
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