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How does the ozone layer protect Earth from radiation?

This is bad, but it would be a lot worse without the ozone layer. garth.kennedy/Flickr

SAVING THE OZONE: Part three in our series exploring on the Montreal Protocol on Substances that Deplete the Ozone Layer – dubbed “the world’s most successful environmental agreement” – explains why we need an ozone layer.

The ozone layer acts as a filter for the shorter wavelength and highly hazardous ultraviolet radiation (UVR) from the sun, protecting life on Earth from its potentially harmful effects. When the sky is clear, there is an inverse relationship between stratospheric ozone and solar UVR measured at the Earth’s surface. That is,the lower the ozone levels, the higher the solar UVR.

The level of UVR from the sun measured at the Earth’s surface varies linearly with latitude. There are higher UVR levels nearer the equator and lower UVR nearer the poles (see Figure 1).

Australia has high levels of solar UVR, due mainly to its geographical position. We have capital cities at latitudes ranging from 12°30’S (Darwin) close to the equator down to 42°52’S (Hobart). For comparison with some Northern Hemisphere locations, the south of France is 43°N and London is 51°32’N, while Melbourne at 37°46’S is as far from the equator as the coast of North Africa (37°16’N).

The southern hemisphere generally has higher levels of solar UVR than the northern hemisphere, because the Earth is approximately 1.7% closer to the sun in January (summer) than at the equinox and 1.7% further away in July (northern hemisphere summer). The intensity of solar UVR is proportional to the square of the distance, so this means solar UVR levels are already 3.4% higher in the southern Hemisphere than at equinox and 3.4% lower for an equivalent location in the northern hemisphere. However as the atmosphere in the southern hemisphere is cleaner than that in the northern and transmits UVR more readily, these differences are even larger for similar latitudes, approaching ~15%.

Figure 1. Measured solar UVR data versus latitude for a number of locations in different countries. The Southern Hemisphere sites are Australia and New Zealand as well as the Australian Antarctic Stations (at just below 70°S) and are marked as AAD and with UVR levels well above the other high latitude locations due to the effects of the ozone hole. Macquarie Island (at ~ 55°S) has annual UVR levels that are unaffected by the ozone hole. The Northern Hemisphere sites are the US (which has the highest data shown in this graph at Mauna Loa in Hawaii at 20°N and 3800m altitude), Japan and a number of European counties.

Australians are predominantly descended from fair-skinned individuals used to European conditions, so exposure to these high levels of solar UVR has resulted in very high rates of skin cancer within the population. Deaths are now more than 1800 per year with a cost to the health system of more than $300M annually.

Generally the higher the sun is in the sky, the shorter the path through the atmosphere and the higher the solar UVR levels. The maximum height of the sun in the sky changes slowly from day to day, but ozone over a location can change considerably from one day to the next due to natural variability. Levels can rise or fall by up to 100 Dobson Units (DU) in 24 hours.

For consecutive clear sky days, large but natural changes in ozone levels in the stratosphere above cities can affect the solar UVR at the surface significantly. There are differences of up to 30 to 40% from one day to the next, with measured daily UV index values increasing or decreasing inversely with large daily decreases or increases in ozone.

The ozone hole - discovered in the early 1980s - and its effects on solar UVR levels over the Antarctic and possibly further north could only add to the problem of population UVR exposures. The southern hemisphere has been affected more by ozone depletion than the northern hemisphere due to several geophysical and atmospheric factors which have lead to the annual appearance of the ozone hole over Antarctica.

Measurements of the solar UVR levels at the Australian Stations in the Antarctic (Casey, Davis and Mawson) show as the ozone hole passes overhead each spring, the annual levels of solar UVR at the stations have increased significantly. They are now equivalent to that received at numerous places in Europe. Interestingly, Macquarie Island, which is outside the reach of the Antarctic ozone hole, shows little in the way of increased annual solar UVR levels.

Because the annual ozone hole breaks up in spring, pockets of ozone-depleted air sometimes move northwards and pass over Australia adding slightly to the solar UVR levels there (this was first observed in the late 1980s). Recently there have been incidents of low ozone over Australia due to other atmospheric processes dragging low-ozone upper-atmospheric air down from equatorial regions (ozone is generally lower over the equator than at mid-latitudes). In such cases UV index levels at the ground are elevated and increase the potential for adverse health effects for populations living in these areas.

If not for the success of the Montreal Protocol it is very likely that the more densely populated areas of the globe would be subject to increased solar UVR with potentially severe consequences for (human) health.

This article was co-authored by Stuart Henderson, who works with Peter Gies in the UVR Group in the Radiation Health Services Branch at the Australian Radiation Protection and Nuclear Safety Agency. Stuart Henderson has a PhD in Applied Physics from RMIT.

Tomorrow: what are ozone depleting substances?

Read more on the Montreal Protocol’s 25th anniversary.

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