Monitoring ozone depletion

Stratospheric ozone in the so called ozone layer plays a vital ecological role by filtering out most biologically harmful ultraviolet radiation. However, since the discovery of a large ozone hole over the Antarctic region in 1985, it has been established that decreases in the ozone layer are occurring over other parts of the world and more severely. Resulting biological damage of varying degrees has also been found. Monitoring for ozone depletion must be continued and improved as much as possible in order to assess past and present damage, estimate future damage, and have an accurate basis from which to effectively combat ozone depletion.

Stratospheric ozone shields humans and other organisms from harmful ultraviolet (UV) rays. Excessive exposure to UV rays causes skin cancer, cataracts and immunosuppressive diseases in humans and animals. It also causes degradation of materials such as rubber, wood and plastics, and slows plant growth.

The first evidence that CFCs damaged the Earth's ozone layer emerged in the 1970s, and thereafter continued to accumulate. The UN Environment Programme (UNEP) and the World Meteorological Organization (WMO) have been instrumental in highlighting the damage caused to the Earth's ozone layer.

Ozone has been cumulatively reduced by more than 14% between 1969/70 and 1993 over continental parts of the northern middle latitudes. During the 1992-1993 season of depletion, ultraviolet light increases of 12% in the middle latitudes were recorded. The mid-latitude season of depletion now extends into the summer months. Significant decreases in ozone concentrations in spring and summer in both hemispheres at the high-latitudes have also been detected. In 1994, the UN Environment Programme's (UNEP) Scientific Assessment Panel released the following findings: in recent years the growth rates of CFC-11, and CFC-12 and halons in the atmosphere has been slowing down, and total tropospheric chlorine increased by about 60 parts per thousand (ppt) in 1992, compared to 110 ppt in 1989. Tropospheric concentrations of bromine in halons increased by about 0.25 ppt in 1992, compared to 0.85 ppt in 1989, whilst the concentration of carbon tetrachloride is actually decreasing. Tropospheric chlorine in HCFC's increased by 5 ppt in 1989, and about 10 ppt in 1992. Decreases in ozone abundances of around 4-5% per decade at mid-latitudes in both hemispheres continue to be observed. Little or no atmospheric ozone reductions are observed in the tropics (20N-20S). Peak total chlorine/bromine loading in the troposphere is expected to occur in 1994, and the stratospheric peak will follow in 3 to 5 years. The Antarctic ozone holes of 1992 and 1993 were the biggest in area and the deepest (minimum amounts of ozone). At the end of September 1994 and during October, the Antarctic ozone hole, with less than 200 meters atmospheric centimetre, covered over 21 million kilometres. Over numerous points of this hole values of a few units below 100 Dobson units, or about 70% depletion, were measured. At the same time, large increases in UV light have been observed at ground level. The ozone layer over the Arctic has depleted.

In October 1994, Environment Canada reported that between 1989 and 1993, ozone values were reduced by 11% in winter, with a 90% increase in UV-B, and by 7.4% in summer periods, with an increase by 30% in UV-B radiation.

Exceedances of the UK Standard for ozone, of 50 ppb measured as a running 8 hour mean, were recorded at all 16 rural sites and 30 out of 34 urban and roadside sites in 1996. "High" concentrations (greater than 90 ppb) were recorded at 26 out of the 50 sites, most frequently at rural sites (on 5 days at Aston Hill and on 4 days at Sibton). No site recorded "very high" concentrations.

The damage to the ozone layer caused by CFCs, halons and other ozone-depleting gases, depends on their residence time in the atmosphere and the rapidity with which they contribute to the decomposition of ozone. These two factors determine the Ozone Depletion Potential (ODP) of ozone-depleting gases. An ozone depletion pressure index can be formed by multiplying the emissions of CFCs and halons by their ODP and then summing across the gases. Since production of these substances is assumed to be correlated with emissions, and since production data is usually more readily available than emissions data, it is prudent to use production rather than emissions data in the index. The weights deriving from the ODP have scientific backing, and therefore overcome the subjectivity inherent in the weights found in many types of environmental indices.