Introduction

Why is the ozone hole over Antarctica? That is one of the first questions that comes to mind when people think about the ozone hole. Every winter and spring since the late 1970s, an ozone hole has formed in the stratosphere above the Antarctic continent. In recent years this hole has become both larger and deeper, in the sense that more and more ozone is being destroyed. As summer approaches, the hole repairs itself, only to reform during the following spring.

Measuring the Ozone Hole

The most common ozone measurement unit is the Dobson Unit (DU). The Dobson Unit is named after atmospheric ozone pioneer G.M.B. Dobson who carried out the earliest studies on ozone in the atmosphere from the 1920s to the 1970s. A DU measures the total amount of ozone in an overhead column of the atmosphere. Dobson Units are measured by how thick the layer of ozone would be if it were compressed into one layer at 0 degrees Celsius and with a pressure of one atmosphere above it. Every 0.01 millimetre thickness of the layer is equal to one Dobson Unit. The average amount of ozone in the stratosphere across the globe is about 300 DU (or a thickness of only 3mm at 0�C and 1 atmospheric pressure!). Highest levels of ozone are usually found in the mid to high latitudes, in Canada and Siberia (360DU).

Why is the Hole over the Antarctic?

Observed ozone over the British Antarctic Survey station at Halley Bay first revealed obvious decreases in the early 1980s compared to data obtained since 1957. The ozone hole is formed each year when there is a sharp decline (currently up to 60%) in the total ozone over most of Antarctica for a period of about two months during southern hemisphere spring (September and October).

Man-made emissions of CFCs occur mainly in the northern hemisphere, with about 90% released in Europe, Russia, Japan, and North America. Gases such as CFCs that are insoluble in water and relatively unreactive are mixed throughout the lower atmosphere and rise from the lower atmosphere into the stratosphere; winds then move this air poleward.

Normally, chlorine and bromine is inactive, locked up in stable compounds, and does not destroy the ozone. However, during the Antarctic winter months (June to August) when the region receives no sunlight, the stratosphere becomes cold enough (-80�C) for high level [ice] clouds to form, called Polar Stratospheric Clouds (PSCs). These PSCs provide an ideal catalytic surface on which the chlorine can react with the ozone, thus destroying the ozone layer. This reaction requires sunlight, and therefore only begins when the Sun returns to Antarctica in spring (September to October), before the PSCs have had a chance to melt. The ozone hole disappears again when the Antarctic air warms up enough during late spring and summer

Annual Decline in October Stratospheric Ozone over Antarctica

During the southern hemisphere winter, Antarctica is isolated from the rest of the world by a natural circulation of wind called the polar vortex. This prevents atmospheric mixing of stratospheric ozone, thus contributing to the depletion of ozone. Although some ozone depletion occurs over the Arctic, meteorological conditions there are very different to Antarctica and so far have prevented the formation of ozone holes as large as in the southern hemisphere.