Arctic Ozone

The Natural Ozone Regime and the Arctic

Ozone is formed in the stratosphere (the atmospheric layer that lies between about 10 and 50 km above the earth's surface) when powerful ultraviolet rays in sunlight break up oxygen molecules (O₂), liberating oxygen atoms (O) that then combine with intact oxygen molecules to form ozone (O₃). Most of the stratosphere's ozone is produced over the tropics, where these ultraviolet rays penetrate most deeply into the atmosphere, but stratospheric winds distribute this ozone over the rest of the world. Eventually the ozone is destroyed by sunlight or by reactions involving chlorine, water vapour, nitrogen oxides, and other substances in the air, but in an undisturbed natural system there is always enough new ozone coming in from the tropics to replenish these losses. This does not mean that the amount of ozone in the stratosphere is always constant. It, in fact, varies noticeably from day to day, season to season, year to year, and place to place, but at any given place average natural ozone levels tend to be much the same from one decade to another.

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Ozone amounts above a point on the earth's surface are commonly measured in Dobson Units (DU), with 100 DU corresponding to 1 mm of ozone at ordinary surface temperature and pressure. Most of this ozone is in the stratospheric ozone layer, but a small amount is also present near the earth's surface, where it has been brought down by air currents from the stratosphere or created by chemical reactions involving air pollutants. Average yearly ozone values range from a low of 260 DU in the tropics to as much as 380 DU in the Arctic. In early spring, ozone amounts over parts of the Arctic commonly exceed 500 DU on some days (Figure 2).

That the highest ozone amounts in the world are found over the Arctic is, at first glance, surprising, but there are at least three good reasons why this is so. The first is the total absence of sunlight for several weeks during the Arctic winter. Without solar energy to split ozone molecules apart, the natural breakdown of ozone is slowed considerably. The second is that the flow of ozone towards the poles is generally greater in the winter. This seasonal bias appears to be related to seasonal shifts in surface weather systems and upper-level wind patterns that provide much of the energy that drives the poleward movement of air, and therefore ozone, in the stratosphere.

The third reason is that the stratosphere is deeper over the polar latitudes than anywhere else in the world. To understand why, it is necessary to look at how the atmosphere is structured vertically. The stratosphere sits on top of the troposphere, the bottom, more turbulent layer of the atmosphere where weather activity takes place. The troposphere commonly occupies as much as the bottom 18 kilometres of the atmosphere in the tropics but only about 8 km near the poles. This difference causes the boundary between these layers, known as the tropopause, to slope from the equator to the poles, at first gradually, then more steeply, like the bottom of a swimming pool. Because the Arctic is situated at the deepest part of the stratosphere, it can hold more ozone than the tropics or midlatitudes. As for the Antarctic, ozone values there, even before any depletion, have always been lower than those in the Arctic. The difference is due to the Southern Hemisphere's stratospheric winds being less effective at transporting the ozone poleward.

Ozone accumulates gradually in the Arctic stratosphere during the winter as the rate of resupply from the tropics exceeds the natural rate of destruction. With the arrival of spring, a change in the pattern of stratospheric winds usually brings a final increase in ozone values to 450 DU or more. The reapparance of the sun, however, increases the pace of ozone destruction, while the rate at which ozone is transported from the tropics falls off considerably and remains at a low level throughout the summer. Consequently, ozone levels decline from the early spring high of approximately 450 DU to about 300 DU in October.

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