Ozone depletion describes two distinct but relate phenomena observed since the late 1970s: a steday decline of about four percent in the total amount of ozone in Earth's stratosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone around Earth's polar regions. The latter phenomenon is referred to as the ozone hole. In addition to these well-known stratospheric phenomena, there are also springtime polar tropospheric ozone depletion events.
The details of polar ozone hole formation differ from that of mid-latitude thinning but the most important process in both iscatalytic destruction of ozone by atomic halogens. The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbon refrigerants, solvents, propellants, and foam-blowing agents (chlorofluorocarbon(CFCs), HCFCs, freons, halons). These compounds are transported into the stratosphere by winds after being emitted at the surface. Both types of ozone depletion were observed to increase as emissions of halocarbons increased.
CFCs and other contributory substances are referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280–315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone generated worldwide concern, leading to adoption of the Montreal Protocol that bans the production of CFCs, halons, and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in sunburn, skin cancer, cataracts, damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion.
Ozone layer depletion
The most-pronounced decrease in ozone has been in the lower stratosphere. However, the ozone hole
is most usually measured not in terms of ozone concentrations at these levels (which are typically a few parts per million) but by reduction in the total column ozone above a point on the Earth's surface, which is normally expressed in Dobson units, abbreviated as "DU". Marked decreases in column ozone in the Antarctic spring and early summer compared to the early 1970s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer.
Reductions of up to 70 percent in the ozone column observed in the austral (southern hemispheric ) spring over Antarctica and firs reported in 1985 (Farman et al.) are continuing. Since the 1990s, Antarctic total column ozone in September and October continued to be 40-50 percent lower than pre-ozone-hole values. A gradual trend toward "healing" was reported in 2016.
In the Arctic, the amount lost is more variable year-to-year than in the Antarctic. The greatest Arctic declines, up to 30 percent, are in the winter and spring, when the stratosphere is coldest.
Reactions that take place on polar stratospheric clouds (PSCs) play an important role in enhancing ozone depletion. PSCs form more readily in the extreme cold of the Arctic and Antarctic stratosphere. This is why ozone holes first formed, and are deeper, over Antarctica. Early models failed to take PSCs into account and predicted a gradual global depletion, which is why the sudden Antarctic ozone hole was such a surprise to may scientists.
In middle latitudes, it is more accurate to speak of ozone depletion rather than holes. Total column ozone declined to about six percent below pre-1980 values between 1980 and 1996 for the mid-latitudes of 35–60°N and 35–60°S. In the northern mid-latitudes, it then increased from the minimum value by about two percent from 1996 to 2009 as regulations took effect and the amount of chlorine in the stratosphere decreased. In the Southern Hemisphere's mid-latitudes, total ozone remained constant over that time period. In the tropics, there are no significant trends, largely because halogen-containing compounds have not yet had time to break down and release chlorine and bromine atoms at tropical latitudes.
Large volcanic eruptions have been sown to have substantial albeit uneven ozone-depleting effects, as observed (for example) with the 1991 eruption of Mt. Pinotubo in the Philippines
Ozone depletion also explains much of the observed reduction in stratospheric and upper tropospheric temperatures. The source of the warmth of the stratosphere is the absorption of UV radiation by ozone, hence reduced ozone leads to cooling. Some stratospheric cooling is also predicted from increase in greenhouse gases such as CO2 and CFCs themselves; however he ozone-induced cooling appears to be dominant.
Predictions of ozone levels remain difficult, but the precision of models' predictions of observed values and the agreement among different modeling techniques have increased steadily. The World Meteorological Organization Global Ozone Research and Monitoring Project-Report No. 44 comes out strongly in favor of the Montreal Protocol, but notes that a UNEP 1994 Assessment overestimated ozone loss for hte 1994-1997 period.
0 comments:
Post a Comment