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Ozone in the atmosphere

It might be as well to emphasise that "Ozone" [ O3 ] is not new! Although it has become part of the 'green' vocabulary, and is rightly a topic of concern, Ozone has, as far as we (humans) are concerned, always been a constituent part of the earth's atmosphere, albeit with variations in its distribution in time and space.

What should be of concern to us is that the concentrations at low altitudes (on / close to the surface) under certain atmospheric and human-influenced conditions can increase to uncomfortable, or even hazardous levels.

However, in the upper atmosphere (roughly between 12 and 50 km altitude, with a peak concentration in the 15 to 25 km altitude band), where Ozone is of benefit to all animal and plant life, total amount, and the regional distribution have varied alarmingly over short periods, over and above the natural variation that is a part of normal atmospheric processes: at these high altitudes, a sustained depletion of natural Ozone is a potential long-term catastrophe for life on this planet. Without a healthy, self-sustaining stratospheric Ozone layer, life as we know it on this planet would not be possible.

The rest of this note looks at each problem in turn:


a. Ozone at low levels:


There is always a small background level of naturally occurring Ozone, but that level can be significantly enhanced, because the unburnt residues involved in the combustion of hydrocarbon fuels, (particularly those concentrated in vehicle exhaust emissions), contain the ingredients to produce Ozone, when acted upon by strong sunlight, in combination with Nitrogen Oxides, also produced by transport exhaust, and in certain power station emissions.

Under light wind / low level stability conditions (see the uk.sci.weather FAQ entry "Stable and unstable air masses"), the Ozone ingredients (NOx; unburnt hydrocarbons), along with a mixture of other unsavoury gases, such as sulphur-dioxide, nitrogen-dioxide etc., accumulate, causing breathing problems, particularly in those at risk from asthma attacks, and other respiratory problems.

  • Nitrogen dioxide, Sulphur dioxide & Ozone irritate the airways of the lungs, increasing symptoms of those suffering from lung disease.
  • Particulate matter is very fine and can be carried deep into the lungs where they can cause inflammation and a worsening of the condition of people with heart and lung disease.
  • Carbon monoxide prevents the normal transport of oxygen by the blood. This can lead to significant reduction in the supply of oxygen to the heart, particularly in people suffering from heart disease.

Ozone is a particularly sneaky problem, as its peak of concentration often occurs after several hours of sunshine, when people are opening windows to benefit from the supposedly fine weather - in fact, concentrations of ground level Ozone tend to be significantly higher in the summer months, due to the strength of the sunshine - an essential ingredient for the production of low-level Ozone.

Further, the problem is not necessarily concentrated in the inner cities. Because many major road / motorway interchange complexes are situated in semi-rural areas, under conditions of near-stationary traffic, a rapid build-up of engine exhaust pollution can occur, which if the low-level atmospheric conditions are correct, will not be dispersed. Also, the build-up of the primary pollutants that produce Ozone often drift out of town, so rural / suburban residents in areas downwind of major conurbations are often more at risk from high Ozone pollution level events, due to concentrations that have built up in town earlier in the day.

There is also a paradox in that some emissions from vehicles actually combine with the emerging Ozone radicals and offset the production of the gas – so statistically, rural areas often experience high levels of Ozone pollution because of lower levels of traffic! (Of course, in light wind/hot/high stability conditions, other pollutants will cause problems in the cities, so this difference is a bit spurious – also, large cities act as ‘heat islands’ local / mesoscale inflow of air will drag Ozone pollution back into the towns.)

Ozone also attacks the cell membranes of plants, leading to stress. Significant 'die-back' of otherwise healthy-looking foliage, particularly noticeable in the canopy of mature trees, and is often the first sign of repeated high-Ozone attacks.

Ozone is a "greenhouse gas": that is, it contributes to the normally beneficial extra-warming of the planet. However, any increase over and above the 'natural' level would add to any perceived problem with enhanced global warming. However, the effect, compared to other gases of concern, e.g. CO2, methane and water vapour, is smaller, though still significant.

A LIST OF THE MOST PREVALENT POLLUTANTS, AND SOURCES.

POLLUTANT

SOURCE (MOST SIGNIFICANT)

Carbon Monoxide (CO)

Motor engine exhausts; some industrial processes.

Sulphur dioxide (SO2)

Power generators using coal or oil etc., that contain sulphurous elements.

Particulate matter

Vehicle exhausts; many industrial processes; incineration; power generation etc.

Lead (Pb)

Vehicle exhausts.

Nitrogen dioxides (NO2)
[ formed from Nitric oxide (NOx) after emission from vehicle exhausts - those burning fossil fuels]

Vehicle exhausts. power generators. Fertiliser plants.

Photochemical oxidants (including Ozone O3)

Reaction of NOx and unburnt hydrocarbons (from vehicle exhausts) to sunlight.

Carbon dioxide (CO2)

all sources that involve combustion.

Others:

Solvents (vapour), fuel combustion vapours etc.

 

For more details on air quality monitoring (and background to the problem), see: http://www.airquality.co.uk/
or: http://www.defra.gov.uk/environment/airquality/index.htm
and BBC CEEFAX (p417) and ITV TELETEXT (p156) services carry current data, as do some newspapers.


b. Ozone at high levels:


In the stratosphere, Ozone is an important constituent gas of the atmosphere for two key reasons:

1. The radiation received from the sun, our primary source of energy, irradiates the atmosphere across a broad band of wavelengths, but it is the ultra-violet part of the spectrum that concerns us here, accounting for about 7% of the total radiation received at the earth's upper atmosphere. [The other important wavelengths are: visible (41%) & infra-red (52%)].

The following simple graphic shows how ultra-violet radiation 'fits in' with the rest of the electromagnetic spectrum. (nm=nanometres) .

Gamma rays X-rays Ultra-violet Visible Infra-red Microwaves Radio
     200 to 400 nm  ~400 to 760nm  > 760nm    

 

Ultra-violet radiation has a complex relationship with high-altitude Ozone .... one particular uv wavelength band contributes to the formation of Ozone, and other wavelengths are involved in the dis-association of the same gas. In the process, sensible heat is released which contributes significantly to the heat budget at these Ozone-rich altitudes, with a peak of absorption (& therefore stratospheric warming) at 50km: if the Ozone levels were to be significantly depleted on a permanent basis, the stratosphere, and the troposphere below it (where all the 'active' weather is), would have a significantly different vertical temperature profile, with consequences for the whole atmospheric system.

The generally accepted ultra-violet ‘bands’ used in the high-level Ozone debate, and their possible / probable effects are:-

 CATEGORY  WAVELENGTHS (nanometres)  REACTIONS IN & WITH STRATOSPHERE  EFFECT UPON HUMANS, PLANTS ETC.
 UV-A:  315/320 - 400 nm  Not (significantly) absorbed by the stratospheric Ozone layer.  10-15% of ‘burning’: possible connection with the formation of malignant melanomas. These wavelengths are responsible for primary skin 'tanning' & skin ageing.
 UV-B:  280 – 315/320 nm  Absorbed by Ozone in the stratosphere. Ozone absorbs UV radiation without being reduced; the overall result being to convert UV radiation to heat. This is why the temperature of the stratosphere increases with increasing altitude. Any significant reduction in Ozone (caused by us) will have an impact upon the thermal character of the Stratosphere.  85 – 90 % ‘burning’; involved with both malignant & benign cancerous growths. Also linked to eye cataracts. Effects on growth of plants and marine life – but variable; generally though, any increase in UV-B thought to be harmful.
 UV-C:  200 - 280 nm  highly absorbed (by Oxygen molecules): involved with formation of Ozone (wavelengths < 240 nm in particular).  not (thought to be) significant, mainly because of its efficient absorption at high-altitudes: large-scale weakening of the Ozone layer though may revise this opinion.

 

2. The processes outlined above involve the 'absorption' of uv radiation preferentially in a wavelength band known to be important in the formation of human cancers: UV-B (280-320 nm). In studies in both this country & in North America, UV-B was found to be responsible for some 85 to 90% of ‘burning’: that is, increasing the risk of formation of cancerous growths on the skin. UV-B is also involved in a widely accepted link with eye cataracts. In very rough terms, the shorter the wavelength, the higher the impact upon living tissues at near-ground level, whether human or plant life. When the Ozone concentration is dramatically reduced (the so-called 'Ozone-hole' - actually an Ozone weakness), then more of this harmful radiation reaches the earth's surface, and without adequate protection, would eventually lead to an increase in several types of skin cancer, damage to human (and other) immune systems, and an increase in the incidence of eye defects.


UV radiation has always reached the surface of course, and there are naturally occurring variations in Ozone, which we can do nothing about. These occur on time-scales of hours (connected with upper tropospheric ‘weather’ disturbances), to seasons (due to varying solar radiation levels as the earth orbits the sun with its tilted axis.)

However, certain classes of man-made chemicals, used in such processes as refrigeration, foam packaging manufacture and (formerly), in aerosol sprays, find their way to the stratosphere, residing there for long periods, and accelerating the destruction of Ozone. Stratospheric circulation systems concentrate these Ozone minima to the polar regions. Protocols have now been put in place to phase out the problem substances, but it will be many years, perhaps decades, before the 'normal' background levels of Ozone are restored.

It is important to understand that at present (early 21st century), the occurrence of 'Ozone holes' is a high latitude problem, observed around the return of the spring sunshine to high-altitudes. The effect was first described in the mid-1980's (though probably a developing problem since the mid-1970's), and at that time, and even currently, is primarily associated with the Antarctic polar vortex; however, recently a similar, though not as yet dramatic, depletion has been noted in the Arctic sector.

Elongation of the polar vortices 'swing' a marked depletion of total column Ozone (i.e. Ozone found taking a sample from bottom to 'top' of the atmosphere) across major population centres, for example across the southern tip of South America & southern parts of Australia and New Zealand. However, within our (mid-latitude) bands, although we cannot be complacent, an 'Ozone-hole' as such does not occur: there has been though, a broad-scale steady loss of total column Ozone, which obviously will not help the situation.

Changes in life-style since the mid-1960's are a major contributory factor to the increased incidence of skin cancers etc: increased disposable income has encouraged more and more people to travel to the sunspots of the world.


Within the UK, the Health Education Authority indicated in its 1996 pamphlet that the increase in skin cancer reporting is probably due more to the habit of 'soaking up the sun' rather than altered levels of uva and uvb, although it doesn't rule out the latter as a cause. Certainly, over a period of 25 or more years, more people have travelled abroad for holidays in the summer months at lower latitudes, where the strength of the sun is notably stronger. Such widespread travel abroad would have been unthinkable to my parents' generation, and although uv radiation level increase has been detected, it is more likely that our self-inflicted increased exposure to strong sunshine has a lot to do with the problems of skin cancer.


for more information on the stratosphere & high-altitude Ozone see:
http://www.metoffice.gov.uk/research/stratosphere/index.html
and
http://www.faqs.org/faqs/ozone-depletion
and BBC CEEFAX (p418) provide forecasts which attempt to convey the ‘risk’ associated with exposure to the sun.