12.2.2 Ultraviolet attenuation by a modern earth

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Atmospheric ozone occurs as a layer 20 to 30 km in altitude, and at a peak concentration of about 10 ppm. Ozone density varies according to latitude (highest over the poles, lowest over the equator), and also varies diurnally, with season, with altitude and possibly the 11-year sun cycle. Global factors result in a natural variation that can reach 25% at any one location over a year (Blumthaler 1993).

Stratospheric ozone is created continuously by an interaction between solar UV radiation and molecular oxygen, producing atomic oxygen:

O2 + UV → O + O (12.1)

Atomic oxygen rapidly recombines with O2 to form ozone:

O + O2 → O3 (12.2)

Most stratospheric ozone is removed by two slow-moving reactions (12.3 and 12.4) one of which is also dependent on absorption of UV radiation:

O + O3 → 2O2 (12.3)

O3 + UV → O2 + O (12.4)

Both of these reactions are enhanced by naturally occurring catalysts such as the oxides of nitrogen (NO) and hydrogen (OH). Chlorine and bromine oxides (ClO and BrO) can similarly catalyse removal of ozone. These two catalysts do not play a major part in the unperturbed stratosphere; rather, the large quantities of industrially produced organochlorine and bromine chemicals which reached the stratosphere during the late 1900s are believed to have led to localised reductions in ozone concentration, with chlorofluorocarbons (CFCs) and halons chiefly responsible (Beckmann 1991). Indeed, one of the most widely recognisable icons of ‘climate change’ is the satellite image of an ‘ozone hole’ over Antarctica each spring since the mid-1980s. This ‘ozone hole’ is a large area of intense depletion in stratospheric ozone 10 to 50km above the earth’s surface.

Concern over depletion of stratospheric ozone and an attendant increase in transmission of UV radiation led to international action via the Montreal Protocol of 1987, an international agreement which ultimately aims to eliminate production of CFCs and other ozone-degrading pollutants. Even if targets are achieved, atmospheric models predict that global ozone will be reduced somewhere between 1% and 4% by the year 2015 as existing ozone-degrading pollutants find their way to the stratosphere. A slow recovery in ozone levels is predicted thereafter (Fraser 1989).

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Figure 12.17 Upper portion: spectral irradiance of terrestrial and extraterrestrial solar radiation together with that generated by a typical UV sunlamp. Also shown is the calculated shift to lower UV wavelengths of terrestrial radiation due to ozone depletion of 16% and a generalised UV action spectrum for plant damage. Lower portion: relative absorption of UV radiation by nucleic acids, proteins and flavoproteins

(Lower portion based on Caldwell 1971; upper portion, generalised data from various sources)

As production and destruction of stratospheric ozone is largely dependent on absorption of solar UV, the ozone ‘layer’ essentially shields the earth’s surface from most UV radiation. The solar UV spectrum (Figure 12.17) extends from 100 to 400nm, and has been divided into three bands: UV-A, 315–400nm; UV-B, 280–315nm; and UV-C, 100–280nm. These divisions are somewhat arbitrary (the UV-A/UV-B boundary ranging from 315 to 320nm and the UV-B/UV-C boundary from 280 to 290nm) and relate to effects that each band has on biological systems.

The shortest UV wavelengths reaching ground level are in the UV-B range (280–320nm). This wavelength range represents up to 1.5% of extraterrestrial irradiance, and it is attenuated to 0.5% or less of total irradiance reaching the earth’s surface (Figure 12.17; Blumthaler 1993). While UV-B is only minor in terms of total solar irradiance at ground level, the high energy of UV photons make UV-B a photo-chemically active and biologically significant form of radiation. Proteins, DNA and RNA, absorb UV-B radiation strongly (Figure 12.17) and are thus prone to damage. Even small increases in UV-B irradiation arising from depletion in stratospheric ozone could have significant effects on biological systems.

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Figure 12.18 Reciprocal variation in atmospheric ozone over the South Pole (open symbols), and flavonoid content of the moss Bryum argenteum (solid symbols). Flavonoids provide photoprotection against UV-B irradiance which rose markedly during the 1960s, due to ozone depletion. Ozone concentration is indicated as Dobson units which represent the physical thickness of the ozone layer at a pressure of one atmosphere (e.g. 300 Dobson units = 3 mm). South polar stratospehric ozone suffers enhanced depletion each southern hemisphere spring, and the south polar 'ozone hole' region reached an area of about 24 million square kilometres on 7 October 1994

(Based on Markham et al. 1990; plus data from the Total Ozone Mapping Spectrometer (TOMS) aboard the Russian Meteor-3 satellite)

However, a paradox remains. Notwithstanding a consensus on stratospheric chemistry and UV-B among atmospheric scientists, could the ‘ozone hole’ be a ‘natural’ phenomenon? Ground-based measurements of ozone over the South Pole made between 1964 and 1986 imply that ozone depletion over the Antarctic is not necessarily a consequence of recent human activity and may have been influenced by ‘natural’ processes. This view has been supported by measurements of flavonoid levels in samples of the moss Bryum argenteum by New Zealand phytochemist Ken Markham and colleagues (Markham et al. 1990; Figure 12.18).

Flavonoids are synthesised in Bryum as a UV-B-screening pigment and synthesis is sensitive to small changes in UV-B irradiation. Samples of Bryum collected from the Ross Sea area between 1957 and 1989 show that flavonoid content rose markedly during the mid-1960s correlating with a reduction in ozone levels at the time, but ahead of any serious accumulation of CFCs. Key agents in this mid-1960s ozone depletion are believed to include altered sunspot activity, emissions from volcanic eruptions and atmospheric tests of nuclear weapons in the early 1960s. Analysis of flavonoid
levels from other historical plant collections may provide a key to determining whether past depletions of ozone and consequent increases in UV-B transmission have been ‘natural’ phenomena.

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