Eventually, the chlorine atoms react with methane to form HCl. Near the bottom of the stratosphere at 16 km, relative HCl levels are quite low, while near the top of the stratosphere at 60 km the levels are quite high. These increased chlorine concentrations lead to greater ozone loss in the stratosphere. The growth of the chlorine and bromine levels in the stratosphere has produced very large losses of ozone over the polar regions, producing the famous Antarctic ozone hole. The top panels show ozone levels for years in the 1970s, when chlorine and bromine levels were low, while the bottom panels show recent ozone levels.
Recent years have ozone levels that are less than half of what was observed previously.
Losses of a similar magnitude have been observed over the Arctic during the last few years. Because of the different climates of the Arctic and Antarctic, the Arctic ozone levels are naturally higher, yet both polar regions have shown large declines over the last decade. Total column ozone is shown for various years in the 1970s and 1990s, as measured in Dobson units. Polar ozone losses are caused directly by chlorine and bromine catalytic reactions. The chlorine and bromine reactive forms are exacerbated by chemical reactions that take place on the surfaces of cloud particles.