Subject: Stratospheric Ozone Depletion Apparently-To: cestvr@ces.iisc.ernet.in Status: R STRATOSPHERIC OZONE DEPLETION Background Ozone exists through all levels of the atmosphere, from the surface to about 100 kilometers (km) altitude. The majority of ozone is concentrated in the stratosphere, between about 10 and 30 km, in a region known as the ozone layer. The ozone layer is critical to life on Earth because it absorbs biologically-damaging solar ultraviolet radiation. Depletion of stratospheric ozone increases the amount of this ultraviolet radiation received at the Earth's surface, which would likely increase the incidence of skin cancer and melanoma, as well as possibly impairing the human immune system. Damage to terrestrial and aquatic ecosystems may also occur. The concentration of stratospheric ozone has been observed to be decreasing throughout much of the globe. The most dramatic decrease is observed over Antarctica each austral spring. Observations have demonstrated that the Antarctic ozone depletion is due to man-made chemicals, and the weight of evidence suggests that these chemicals likely cause much of the mid-latitude depletion as well. Heterogeneous chemistry (mixed-phase reactions), involving increased amounts of chlorine and bromine in the stratosphere, are key to the ozone decline. The sources of chlorine are largely chlorofluorocarbons, human-produced chemicals that are used as refrigerant, foaming, and cleaning agents. Bromine also has a large anthropogenic source. It is found in halons that are used in various types of fire extinguishers and in some agricultural fumigation. An international treaty, the Montreal Protocol on Substances that Deplete Stratospheric Ozone, has been signed by many countries, including the United States. This treaty calls for the phase-out of chlorofluorocarbons by the year 2000, although there are provisions for a faster phase-out if the science warrants. Because new advance in scientific understanding are constantly occurring, major scientific assessments are conducted periodically to determine if stricter provisions are necessary. The most recent assessment was conducted in 1991; some of the results are described below. Issues The Antarctic Ozone Hole: In the high-latitude stratosphere, the typical process of ozone production is through photochemical (with sunlight) reactions and of ozone loss is through transport into the troposphere (lower atmosphere). However, the huge decreases in stratospheric ozone discovered in the mid-1980s during the austral spring over Antarctica could not be explained by transport processes. Several scientific expeditions were mounted to investigate the cause of this "ozone hole." These expeditions showed that the Antarctic ozone hole is very likely caused by a combination of special meteorology and the presence of human- produced chlorine and bromine. The Antarctic stratosphere is extremely cold in the winter, allowing ice particles to form spontaneously, creating Polar Stratospheric Clouds (PSC's). The PSC ice crystals chemically change chlorine from a passive to an active form. Nitrogen attaches to the ice crystals and is transported away from the region, removing the compound chlorine normally reacts with to return to its passive state. The active chlorine acts as a catalyst to destroy ozone, when sunlight returns in the spring. This heterogenous chemistry terminates as the stratosphere warms and nitrogen-rich air is again transported into the Antarctic and allowing chlorine to switch back to its passive state. Ozone-rich can than be transported into the regions, enough to largely "fill" the hole. Global Stratospheric Ozone: Stratospheric ozone is measured from ground-based and satellite-based instruments. Data from these two sources continue to show a decline in the total amount of ozone in the atmosphere during the Northern Hemisphere winter. New evidence indicates that significant ozone decreases are also occurring in the spring and summer in both hemispheres and during the Southern Hemisphere winter. These decreases are observed mainly in the lower stratosphere, below 25 km, at middle and high latitudes, where heterogeneous chemistry occurs as in the Antarctic. The increased abundance of chlorine and bromine in the stratosphere is likely at the root of the ozone depletion. Evidence suggests that heterogeneous chemical reactions, similar to those involving ice crystals over Antarctica, can occur on the surface of sulphate aerosol particles which reside in the stratosphere. Another cause for part of the observed decreased ozone could be the transport of ozone-depleted air from polar regions into the middle latitudes. An Arctic Ozone Hole? The Arctic stratosphere rarely gets as cold as the Antarctic; however, PSC's have been observed there. Also, high concentrations of chlorine-containing chemicals have been observed in the Arctic stratosphere. While significant ozone losses have been observed in the Arctic, they are, thus far, much smaller than those in the Antarctic and therefore do not develop into an ozone "hole." NOAA and NASA are conducting an expedition during the winter of 1991-92 to learn more about the chemistry of the Arctic stratosphere. What can be done?: The ozone-depleting chemicals are being phased out of production in most countries, under the terms of the Montreal Protocol. Several countries, including the United States, have sped up the timetable for ceasing production to mid-1990's. Because of the important functions these ozone-depleting substances perform, substitutes are being developed. Some of the most likely substitutes do contain chlorine, but are more apt to react in the lower atmosphere so less chlorine would enter the stratosphere. Much research remains to be done to develop completely ozone-safe substitutes. NOAA's Contribution Contributes to the international debate on stratospheric ozone depletion, including the U.S. delegation, the Montreal Protocol, and several ozone scientific assessments. Conducts studies to better define the chlorine and bromine chemistry which destroys ozone and has been a major participant in Antarctic and Arctic research campaigns. Develops theoretical models to understand and predict the interaction of meteorology and chemistry. Monitors stratospheric ozone from ground- and satellite-based instruments. Evaluates the chemical and physical properties of proposed CFC substitutes and determines their impact on stratospheric ozone. TROPOSPHERIC OZONE