Ultraviolet radiation (or UV radiation) is electromagnetic radiation like visible radiation (which a human eye can see) or infrared radiation (which we feel as heat). Electromagnetic radiation can also be regarded as a flow of small energy packages traveling through space. These packages are usually referred to as "radiation quanta" or "photons". Some photons are more energetic than others. A "ultraviolet photon," for example, has more energy than a visible one and this, in turn, has more energy than a infrared one. So if a single UV photon hits your skin more energy has to be absorbed than if a infrared quantum impinges your body. Our eyes sense photons of different energy as different colors. "Red photons" are the least energetic photons that we can see, violet photons are the most energetic ones. Ultraviolet radiation is simply composed of photons with slightly higher energy than violet light. Photons, which are even more energetic than UV quanta, are called "X-rays." X-rays do not principally differ from visible light, except of their energy.
The energy of a photon can also be expressed by its "wavelength." A shorter wavelength means more energy. The unit most widely used in radiometry, the science of measuring radiation, to measure wavelength is "nanometer" or simply nm. One nm is equal to 0.000000001 meter. So it's a rather short distance. Using wavelength, electromagnetic radiation can be classified as follows:
|Wavelength in nm||Name||Comment|
|100 - 280||UVC||Most dangerous UV radiation; absorbed completely by the Earth's ozone layer|
|280 - 320||UVB||Dangerous UV radiation; absorbed partly by the Earth's ozone layer|
|320 - 400||UVA||Less dangerous UV radiation; only little absorbed by the ozone layer|
|380-780||Visible||This is what we can see|
Why is ultraviolet radiation dangerous?
We are all composed of molecules. The most famous is the DNA, which holds our genetic code. Molecules can be split by radiation, if the energy of the incident photons (see FAQ above) is sufficient. For many molecules, including the DNA, the critical threshold is in the UVB: The energy of photons in the UVA and visible is not sufficient to cause damage, however UVB photons will likely cause molecules to dissociate. Fortunately most damage can be repaired by our body. If, however, our immune system is weakened or we are exposed to high radiation levels (for example at the beach during noon) not all damage can be repaired. The consequences may be sunburn, snow blindness, or, in the worst case, skin cancer.
I have heard that more and more people suffer skin cancer. Is this caused by the ozone hole?
It is true that skin cancer incidence rates are increasing throughout the world. But it cannot be proven that ozone depletion observed during the last years or the "ozone hole" are the cause. Skin cancer needs decades to develop and the ozone hole was first observed in 1985. In addition, ozone depletion in populated areas of the Earth is much less than in areas affected by the ozone hole. For example, in northern mid-latitudes the downward ozone trend is in the order of 4% per decade (see FAQ on trends below). This leads in turn to an enhancement in skin damaging UV radiation of approximately 5-9%, which is far less than the increase in skin cancer incidence trends actually observed. This does not mean, however, that skin cancer is not caused by UV radiation; the opposite is true. Yet the natural variation in UV - introduced for example by differences in the geographic latitude of the city people are living in - is currently much higher than the additional effect that is introduced by ozone trends. For example, summertime UV values at the northern coast of Australia are about two times higher than in Melbourne, mainly because the sun is higher in the sky near the equator. Skin cancer rates consequently show a difference of a factor of four between north and south Australia, which can be expected from the latitudinal difference in UV. The actual reason for the increase in skin cancer rates seen today is more likely a change in leisure activities of people. Changed behavior, and not ozone depletion, may be the primary reason why rates in Australia - where hanging out at the beach is part of the lifestyle - are the highest in the world. If the lifestyle does not change (in fact, it did) ozone depletion will certainly be responsible for additional deaths but it is very difficult to statistically separate these cases from the natural background. One can argue that each person that died from skin cancer is one person too much. It is therefore important that both man-made ozone destruction is stopped and people are educated to protect themselves appropriately when being in the sun.
How is solar ultraviolet radiation measured?
The principal setup of an instrument measuring UV radiation is the following: First, radiation enters through fore-optics, which make sure that both radiation coming directly from the sun and also radiation scatted by air molecules is entering the remainder of the system. The next part is a filter, which allows only photons of a specific wavelength range to pass. There are many ways how in practice this filter is implemented. It may be a filter selecting the whole UV range. Or, the transmission of the filter resembles the (wavelength dependent) response of a specific biologic system (for example human skin). In this case, the instrument's signal is directly proportional to the biological effect that is caused by the incident radiation. A third instrument class are so called "spectroradiometers". They split up the whole UV range in several "sub-colors". A typical spectroradiometer used for solar UV measurements would for example measure the whole UV range in steps of 1 nm (see FAQ "What is ultraviolet radiation?" above). The result of the measurement is a UV "spectrum" rather than a single value. Radiation passing the filter is detected with an electronic element, which converts radiation into an electrical current. The current is finally amplified and logged, for example, by a computer.
What exactly is the "ozone hole" and how is it formed?
The term "ozone hole" describes a period of severe ozone depletion centered over Antarctica. The phenomenon occurred first in 1980 and can now be observed every year since then in the months September - December. During October, when the ozone hole is most pronounced, about 70% percent of the ozone molecules in the air column between ground level and top of the atmosphere have disappeared over certain regions of Antarctica. In some altitudes of the stratosphere (i.e. the atmospheric layer between 12 and 20 km altitude), where usually ozone concentrations are highest, all ozone molecules are virtually gone. Ozone destruction mostly takes place on the surface of so-called Polar Stratospheric Clouds, which only form in the very cold Antarctic Stratosphere during winter time. The chemical reactions leading to the observed ozone loss involve chlorine chemicals as a catalytic converter. Although some natural sources have been identified, the predominant part of chlorine originates from Chloroflurocarbon (CFC) molecules, which were found until recently in many industrial products, including aerosol cans. Although the Montreal Protocol and several amendments of this international treaty have almost stopped the production of CFC on a global scale, there are still enough CFCs in the stratosphere to keep the ozone hole "alive" for another 50 years. In fact, observations in the last years have shown that the stratosphere now also cools down in the Northern Hemisphere and Polar Stratospheric Clouds can now also be observed over the North Pole. Some scientists therefore think that similar process, which lead to the ozone hole over the South Pole, will in the near future also cause severe ozone depletion over the Arctic. This may have significant consequences because far more people live at high latitudes of the Northern Hemisphere compared to the Southern Hemisphere.
What are the trends in ozone and UV?
The following information has been extracted from the WMO/UNEP Scientific Assessment of Ozone Depletion: 1998, the most up-to-date and comprehensive document about the current state of the ozone layer.
Ozone trends in midlatitudes (25º-60º)
The midlatitudes include most of Europe and the United States. In this latitude belt, the total ozone amount in the atmosphere has decreased significantly between 1979 and 1991, with estimated linear downward trends of 4.0%, 1.8%, and 3.8% per decade, respectively, for northern midlatitudes in winter/spring, northern midlatitudes in summer/fall, and southern midlatitudes year round. However, since 1991 the linear trend observed during the 1980s has not continued, but rather total column ozone has been almost constant at midlatitudes in both hemispheres. The observed ozone losses from 1979 to the period 1994-1997 are about 5.4%, 2.8%, and 5.0%, respectively, for northern midlatitudes in winter/spring, summer/fall, and southern midlatitudes year round.
Ozone trends in high latitudes, including Antarctica and the Arctic
In Antarctica, the total ozone amount in the months September and October has continued to be 40% to 50% below the pre-ozone-hole values of approximately 320 Dobson Units (This is the unit usually used to measure the thickness of the ozone layer). During periods of a week or so, the decrease in ozone can be up to 70%, with virtually all ozone molecules disappeared in altitudes of 15 km.
In the Arctic, ozone levels were about 100 DU below 1960-1970 averages in recent winters, with shorter-period differences exceeding 200 DU. This is equivalent to about 20% to 45% less ozone compared to the 1960s and early 1970.
Ozone trends in the equatorial regions (20ºS-20ºN)
None, or at least no trends that are statistically significant.
Trends in UV radiation
There is an inverse relationship between surface UV radiation and the overhead amount of ozone, which can be well quantified. As a rule of thumb, a 1% decrease in ozone leads to an approximately 1.1% increase in sunburning ultraviolet radiation and a 2.2% increase in DNA damaging radiation. These relationships may be modified, however, by cloud cover and other factors. From satellite data sampled between 1979 and 1992 it has been determined that annual sunburning (or "erythemal") UV radiation levels increased by 3.7 ± 3% at 60ºN and 3.0 ± 2.8% at 40ºN per decade. Larger decadal increases were observed in the Southern Hemisphere: 3.6 ± 2% at 40ºS and 9.0 ± 6.0% at 60ºS. From ground-based measurements in midlatitudes it has been estimated that radiation levels at 300 nm have increased by 1.5% per year during 1989 and 1997. At 305 nm, the increase was 0.8%. Although these UV changes are consistent within the uncertainty limits with those estimated from satellite data, ground-based data from suitable stable and well calibrated instruments are not yet long enough to determine decadal trends. A major objective of my work is to improve the accuracy of ground-based UV measurements and to provide sufficiently long time-series to the scientific community.
I thought ozone is bad and causes health problems. Now I hear that a reduction of ozone increases hazardous UV radiation. How does this fit together?
It makes a big difference where ozone is in the atmosphere. Ozone is a toxic gas. If it is close to the ground and we can breathe it is "bad", if it is high up in the atmosphere it is "good" and protects us from harmful UV radiation. High concentrations of ozone can be formed close to the ground in the presence of air pollution (mostly NOx and CO molecules produced from industry and traffic) and high levels of solar radiation. These conditions mostly occur during summer in cities, forming "Summer Smog". In principle, near-ground ozone also protects from solar UV radiation. But even during episodes of severe Summer Smog only a small fraction of the total atmospheric ozone abundance is located in the first kilometer above the ground. The additional ozone amount can only slightly offset stratospheric ozone depletion. It is therefore not a good idea to promote traffic to protect from UV!
Where can I find more information about UV and atmospheric ozone?
See my links page.