The Earth's Troposphere regularly experiences (naturally occurring *) ozone depletion events. The presence of atmospheric ozone is of extreme importance to organic life, as it substantially 'filters out' a large proportion of dangerous UV radiation from the Sun before it reaches the Earth's surface.

The most common location of these events is the air above the Arctic pole - and it usually happens in Springtime (March > May). The depletion events also occur above Antarctica.

The events were first described in 1989, and in some regions the ozone content can drop so low as to be undetectable.

After their discovery, it was soon confirmed that the principal chemical mechanism causing the loss of ozone was its reaction with bromine ions (and other halogens ions like chlorine and iodine) catalysed by sunlight.

The bromine is almost certainly originating from surface seawater, but the way in which bromine salts in the ocean liberate bromine as a gas, which eventually reaches the polar air, is currently unknown.

[…] the precise source of halogens remains an open question and further research is needed to close this gap. Suggestions have included sea salt deposits on snow, young sea-ice surfaces, sea-salt aerosol, concentrated brines on new sea-ice, and frost flowers. This question is one of the most debated ones in the community today.

Source [technical details] : Halogens and their role in polar boundary-layer ozone depletion Atmos. Chem. Phys., 7, 4375–4418, 2007

* Note: These naturally occurring events are not related to man-made ozone depletion caused by the use of CFCs etc.

Also see : Ozone depletion anomalyplugin-autotooltip__plain plugin-autotooltip_bigOzone depletion anomaly

According to a 2007 research project, the current atmospheric theories regarding depletion of the Earth's ozone 'layer' at the poles only account for around 40% of the actual depletion as measured by NASA. See: Is a major fraction of polar ozone loss due to a currently unknown mechanism ? (Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany)

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