Natural Ultraviolet Radiation

Abstract

Solar radiation consists of a wide range of electromagnetic waves emitted as visible light (40 %), as well as invisible radiations, viz., ultraviolet (10 %) and infrared (50 %). The fraction of ultraviolet radiation of sun’s rays comprises three wavebands, viz., UV-A (315–390 nm), UV-B (280–315 nm), and UVC (100–280 nm), each with energy of impact corresponding to the respective wavelengths. Entry of these UV wavebands into the earth’s atmosphere is differentially restricted by an “ozone layer” that spans the lower region of the stratosphere. Due to such restricted penetration of UV radiation into earth’s atmosphere, UV exhibits limited presence (only 0.5 % of total solar UV) in earth’s surface. Of the UV radiation penetrating in earth’s atmosphere, UV-A comprises 90 % and UV-B comprises 5–10 %; UV-C is totally absent in earth’s atmosphere. Of these, UV-B presents the most deleterious fraction with UV-A (of lower energy) following suit. Five layers of atmosphere, each with specific composition, temperature, and function, envelop the earth. Of these, the two biologically most important layers are troposphere and stratosphere: the troposphere representing the closest overlay on earth’s surface constitutes the weather determining region, while the stratosphere, lying just above, houses the UV screening ozone layer. UV restraining ozone layer developed through evolution of oxygenic life forms on earth pioneered by appearance of Cyanobacteria about 3 billion years ago. In that age, molecular oxygen resulting from oxygenic photosynthesis of Cyanobacteria as well as other later evolved photosynthetic organisms led to the formation of ozone layer through reaction between molecular oxygen and atomic oxygen under influence of UV that, being a fraction of sunshine, is highest at the equator. Ozone thus formed accumulates as a layer in the stratosphere being thicker at the poles with gradually decreasing thickness toward the equator, UV penetration varying as a function of wavelength, through the ozone layer. The penetration of UV radiation is affected by various factors such as latitude (as a factor of ozone thickness), altitude (as a factor of air rarification), variation in sun’s zenith angle relating to solar movement, diurnal, seasonal variation, and aerosol/cloud cover variation.

The temperature requirement for life support, early in earth’s evolutionary history, was provided by IR radiation that caused warming of earth’s supercool condition. This aided in origin and perpetuation of life on earth. Ironically in recent times, the same (IR) radiation, being trapped by enhanced “greenhouse gases,” accumulating due to adverse anthropogenic activities, is contributing to destruction of life through the process of global warming. Global warming on the earth’s surface has a two-pronged destructive effect; this warming of earth simultaneously causes cooling in the stratosphere, a condition that enhances ozone destruction, thereby augmenting depletion of the ozone layer.

As a result of gradual ozone thinning, during the later part of the last century, an “ozone hole” was detected, first in the coldest Antarctic (polar) region and subsequently in the Arctic region. Appreciable ozone thinning has also been recorded at the mid-latitudes. During the later part of the past century, reports on ozone thinning/ozone hole associated with concomitant increase in UV fluence on earth’s surface raised an alarm, and reports rapidly (particularly in the post-ozone hole era) started coming up on the harmful effects of UV radiation on living organisms. Realization that a gradual thinning of the protective ozone layer is occurring due to anthropogenic effect related release of damaging gases like CFC and related compounds as well as the greenhouse effect in the atmosphere prompted the UN to take urgent regulatory measures for reducing/reversing this state of global (UV related) predicament. Several regulations have been made mandatory through regulations of Kyoto Protocol and Montreal Protocol to control ozone thinning and atmospheric degradation worldwide. Since the universal acceptance of the Montreal Protocol, in 1987 there has been growing evidence for the recovery of the stratospheric ozone layer, albeit with apprehension of continuing low level destruction of the ozone layer.