12e Ultra Violeta

	LABORATORIO DE FISICA DE LA ATMOSFERA \|
Universidad Mayor de San Andrés - La Paz Bolivia

Ultraviolet Radiation

At the beginning of the nineteenth century, Johannes Ritter discovered that the sun had wavelengths shorter than the violet spectrum and were therefore invisible to the human eye. This radiation was named "Ultraviolet" and is nowadays is sub-classified into UV-A, UV-B and UV-C.

UV-A, is a continuation of the visible radiation. The longest in length within the UV spectrum, UV-A's wavelength ranges from 400 to 320 nm (1 nanometer nm= 10^-9 m) and reachs the earth's surface. UV-A penetrates the skin to the level of the dermis and can damage collagen protein and elastin fibres. This causes premature ageing of the skin and shows as wrinkles, dryness and weathered skin. Scientists and doctors now believe that UV-A rays have a part to play in the cause of skin cancers, especially those of the non-melanoma type.
UV-B rays also known as biological UV have a wavelength ranging from 280 to 320nm. UV-B is responsible for the sunburn we feel if exposed to strong sunshine and is dangerous because we do not feel sunburn until 8-10 hours after exposure. Ozone in the outer stratosphere screens out some of the UV-B rays. In percentage terms about 5% of total UV radiation and 0.25% of the whole total solar radiation reaches the earth's surface. Even with the protective barrier of the ozone UV-B is harmful. These rays are strongest during the midday hours which is when we are most at risk of burning. Intensity and long term exposure to UV-B rays and frequency of sunburn are strongly associated with the melanoma type of skin cancer.
UV-C rays are the shortest within the UV spectrum. They are known to cause cancer, but fortunately these rays are absorbed by the atmosphere/ozone layer before they reach the earth's surface.

The intensity of ultraviolet radiation reaching the earth's surface depends on many factors, the most important of which are: the time of year and day i.e. the length of the sun from the earth and hours of daylight; latitude; altitude i.e. the radiation increases with altitude; ozone layer thickness; cloud cover; atmospheric contamination i.e.pollution creates surface ozone which reduces UV radiation (see Ozone); and "albedo".

Laboratory Instruments

Since 1995 we have been measuring ultraviolet radiation intensity in La Paz, with the following instruments:

a) Solar radiometer Light SL-501 "Biometer". This instrument measures UV radiation continuously in MED units. 1 MED = the UV-B radiation intensity required to cause irritation of the skin (medium opacity) within one hour of exposure. 1 MED is approximately 210 J/m2 and this scale is used in medicine (dermatology).

b) Brewer #110 spectrophotometer. This spectrophotometer measures UV intensity in the range of 290 nm to 325 nm, with a resolution of 0.5 nm. This means that it has the capacity to provide UV-B global and spectral information in physical absolute units. It also weights its measurements with sensibility curves (Mc Kinley-Diffey) making its measurements useful to dermatological work and UV Index calculus.

c) Two YES Radiometers: These instruments measure global UV-B in the range of 280-325 nm, continuously and in absolute physical units. These measurements are very useful when investigating the relationship between UV intensity and the variable altitude.

d) Two spectral NILUV: these spectral NILUV measure UV in four wavelengths and they are also useful in measuring ozone.

All of the instruments used are regularly maintained, tested and calibrated. The very delicate job of the inter-calibration of all the different instruments is carried out and put though three international calibrations.

Main research

1. UV-B Monitoring in La Paz

The laboratory has been continuously mon itoring the UV-B levels in La Paz, since December 1995. The UV- B Index is measured and data is retrieved through radial communication media. The effect of variables such as time of day, and year on the behaviour of UV-B radiation levels are analyzed. The following picture shows the typical seasonal behaviour of UV-B as measured using the Brewer Instrument. The measurements illustrate La Paz's typical climatology, sunny skies in autumn time and cloudy skies in summertime. Translated to (0-16) UV Index scal e we see that in autumn time UV intensity averages 7 and in summertime 12, with peaks of 15. The extreme high values obtained are in fact among the highest on earth and are influenced by altitude, air quality, horizon width and ozone thickness.

Gráfica de los valores diarios de UV (1996-1998)

Daily UV radiation graphic (1996-1998)

2. UV-B and altitude.

The laboratory is also engaged in a study of the relationship between altitude and UV-B levels. This research is possible given the unique location and peculiar top ography relating to La Paz and its surroundings. The topography of the city and its environs range from heights of 2,800m a.s.l. (in the valley) to 5300 m a.s.l. (peak of Chacaltaya mountain) with the straight - line distance between those points, and our laboratory, of less than 20 km. Measurements confirm that\~ the intensity of\~ UV-B increases by 4.8% every 300 meters. This means that between sea level and La Paz City UV-B intensity increases by The effects of the ultraviolet radiation (UVR) on biological organisms. In response to concern over the increase in UV levels reaching the earth surface due to ozone layer thinning the laboratory started a series of studies on the effects of the UVR and in particular UV-B on biological organisms.

3. The effects of UV levels on higher plants linked to the national food supply chain.

This study was first carried out at laboratory scale during 1997. The experiment involved the cultivation of quinoa (Chenopodium quinoa, Willd), a species representative of the plants grown by farmers of the Bolivian Altiplano region. One set of plants was cultivated under open and natural conditions. The other set was cultivated under the same conditions of soil, temperature, light amount, moisture and water; but protected from the solar UVB radiation. We monitored the growth of the plants, and analysed their production and levels of key compounds such as chlorophyll, carotenoids and flavonoids. At the end of the experiment we examined the plant harvests to examine the differences in the amount of protein produced.

It has been proven that some quinua plants (a very important local crop) are more resistant than others to the effects of UV-B levels. This implies that different varieties should be used according to seasonal solar intensity and the exisitence of dramatic climatic changes such as those related to the "El Niño" phenomenon.

4. The effects of UV (A&B) on the plankton (phyto- and zoo-) of the high-altitude lakes.

These studies are being carried out in collaboration with the Estación de Fotobiología "Playa Unión" of Rawson (Argentina)and theLaboratorio de Limnología del Instituto de Ecología. The focus of the research is to find and understand the adaptive mechanisms developed by the plankton (phyto- and zoo-) of the high-altitude lakes in order to cope with the very high (relative) solar UV levels in their environments.

The plankton of Lake Titicaca's (3800 m asl) have been studied and two measuring campaigns carried out. The data collected and analysed has resulted in a scientific paper which has been published in "Freshwater Biology" magazine. It is interesting to note that the phyto-plankton of the Southern oceans' are under similar (enhanced) UV levels, during the "ozone hole episodes"; the difference being that the plankton of our region (e.g. from the Lake Titicaca) are under such levels continuously. In the future it is our intention to initiate research into the physiology relating to the protective mechanisms adapted by the plankton.

5. The effects of UV-B radiation on man-made materials.

Similar to its effects on biological organisms, the solar UV-R enhancement over the next few decades has and will also affect the performance of man-made materials exposed to sunlight. In order to obtain a base line of the main effects, and with a view to being able to control them, the laboratory has carried out some studies.

The first study was on the solarisation of Agrofilm™, which is a common plastic film widely used by the Altiplano's farmers as a roofing material for greenhouses. The main results obtained are follows; over a period of six months the plastic looses its property to block the UV-B radiation and at the same time it becomes very inefficient in the transmission of visible light. This has significant implications on agricultural production.

A second study looked at the effects of the solar UV-B radiation and its role as a corrosion trigger for exposed base metals. In the case of copper UV-B acts as a buffer, presumably due to electron activation. Several hypotheses have been made regarding this phenomenon, and other experiments will help to substantiate or question these.

A third study looked at the effect of solar UV radiation on textile fibres. Our aim is to understand the fragilisation process which occurs when textile fibres (both natural and manmade) are exposed to UV-R and to propose solutions to this problem.

We are currently involved in producing a base-line which could be adapted for use in the economic sector (industry, commerce, etc.) with a view to working towards greater efficiency in resource usage.

6. Education and diffusion campaigns.

It is important to us that our work is published and that the dangers relating to long-term exposure to UV radiation are communicated to and understood by the general public, particularly those people living at high altitudes e.g. La Paz.

Following an international symposium in 1997 a booklet was published outlining the effects of exposure to UV-B on human health, plant-life, animals and man-made materials. The University in association with local and national media and other institutions has launched several educational campaigns on the subject of the dangers of UV radiation, including a leaftlet campaign explaining the dangers of UV and how to protect yourself from its harmful effects. We have and continue to published, both nationally and internationally on the subject. See Publications

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