<P> The total column amount of ozone generally increases as we move from the tropics to higher latitudes in both hemispheres . However, the overall column amounts are greater in the northern hemisphere high latitudes than in the southern hemisphere high latitudes . In addition, while the highest amounts of column ozone over the Arctic occur in the northern spring (March--April), the opposite is true over the Antarctic, where the lowest amounts of column ozone occur in the southern spring (September--October). </P> <P> The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), nitrous oxide (N O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br). While there are natural sources for all of these species, the concentrations of chlorine and bromine increased markedly in recent decades because of the release of large quantities of man - made organohalogen compounds, especially chlorofluorocarbons (CFCs) and bromofluorocarbons . These highly stable compounds are capable of surviving the rise to the stratosphere, where Cl and Br radicals are liberated by the action of ultraviolet light . Each radical is then free to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules . By 2009, nitrous oxide was the largest ozone - depleting substance (ODS) emitted through human activities . </P> <P> The breakdown of ozone in the stratosphere results in reduced absorption of ultraviolet radiation . Consequently, unabsorbed and dangerous ultraviolet radiation is able to reach the Earth's surface at a higher intensity . Ozone levels have dropped by a worldwide average of about 4 percent since the late 1970s . For approximately 5 percent of the Earth's surface, around the north and south poles, much larger seasonal declines have been seen, and are described as "ozone holes". The discovery of the annual depletion of ozone above the Antarctic was first announced by Joe Farman, Brian Gardiner and Jonathan Shanklin, in a paper which appeared in Nature on May 16, 1985 . </P> <P> To support successful regulation attempts, the ozone case was communicated to lay persons "with easy - to - understand bridging metaphors derived from the popular culture" and related to "immediate risks with everyday relevance". The specific metaphors used in the discussion (ozone shield, ozone hole) proved quite useful and, compared to global climate change, the ozone case was much more seen as a "hot issue" and imminent risk . Lay people were cautious about a depletion of the ozone layer and the risks of skin cancer . </P>

Where was it first discovered that the earths ozone layer was being depleted