In the stratosphere (upper atmosphere) there is layer of ozone which protects us from ultraviolet rays from the Sun. This layer also protects the oxygen present in the troposhere(lower atmosphere).
Ozone is destroyed by chemical reactions with a number of substances that occur naturally in the stratosphere e.g. nitrogen oxides (from microbes and the combustion of fossil fuels) and methane( produced by microbes from in swamps). Increasing man-made substances are now attacking the layer.
The decrease in the ozone layer was first discovered in 1983, when scientists with the British Antarctic Survey observed that the concentration of ozone in the strastosphere decreased rapidly over Antarctica each spring to be replenished by the end of November.
To be continued......