Aerosols & the Environment
What is air?
By volume air comprises 78% nitrogen and 21% oxygen. The remaining 1% is mainly a mixture of 0.9% argon, 0.03% carbon dioxide (CO2) varying amounts of water vapour and trace amounts of hydrogen, ozone (O3), methane (CH4), carbon monoxide (CO), helium, neon, krypton and xenon.
The composition of air is not constant. It varies over time and by geography, altitude and temperature. It has only been for the last 100 million years that the air contained enough oxygen to support air breathing land animals. Today, for instance, water vapour content in the lower atmosphere can vary from virtually 0% in the deserts to more then 4% in a tropical rain forest.
What are the main environmental atmospheric issues?
There are three main areas of concern:
1. Stratospheric ozone depletion
2. Global warming
3. Tropospheric pollution
Why is the stratospheric ozone layer important?
Ozone in the stratosphere plays a vital role in filtering the sun’s ultraviolet (UV) radiation, which would damage life on earth if too much reached ground level. About 90% of the earth’s ozone is in the stratosphere, in what is known as the ‘ozone layer’.
The main concern about ozone depletion is that more UV radiation can lead to an increase in skin cancer and can reduce plant and crop growth.
What is the ‘hole’ in the ozone layer and what caused it?
First noticed over the Antarctic during late winter/early spring of 1978, the word ‘hole’ is used to describe a periodic thinning of the ozone layer by over 50%. This lasts for about 2 months, then normal levels are restored. A less marked and seasonal reduction has also been recorded in the Northern Hemisphere. Natural events, such as volcanic eruptions, are significant contributors to the thinning. So are man-made chemicals such as the chlorofluorocarbon family (CFCs).
CFCs are man-made chemicals developed in the 1930s for use as refrigerants. Unlike the chemicals that they replaced, CFCs are non-flammable, stable, low in toxicity and extremely safe under normal conditions of use. This same stability means that they are not destroyed in the troposphere but instead drift upwards to the stratosphere, where they are broken down by the strong sunlight. This releases chlorine (CI) which adds to the natural depletion cycle of ozone.
Before their environmental impact was understood, their stability meant that CFCs were often very suitable aerosol propellants. Their other uses included refrigeration, air-conditioning, insulation (closed foam cells), production of open cell foam (for furnishing etc) and as solvents.
What has been done to reduce the use of CFCs?
Concern about the possible effects of CFCs was first raised in the 1970s but it was not until 1987/88 that scientists agreed the relationship between CFCs, halons and ozone depletion.
In Europe, despite the lack of scientific consensus relating to CFCs’ environmental impact, the European aerosol industry signed a voluntary agreement with the EC in 1980 to reduce the annual CFC use by 30% compared with 1976 and not to exceed this level despite market growth.
In 1987, following 18 months of negotiation, the United Nations Montreal Protocol was signed which included plans to phase out all ozone depleting substances over a period of several years (by the end of 1995 for most products).
In 1988, the UK aerosol industry responded rapidly (in advance of legislation) with a commitment to phase out CFCs in all retail products (90% of the total) by the end of 1989. The rest of Western Europe took similar action. From about 1990, this meant that only medical aerosols used CFCs (principally metered dose inhalers used by asthma sufferers) with the intention to phase out CFCs once effective alternatives had been confirmed. Today in Europe, CFC use is prohibited.
What is Global Warming?
Global warming (and global cooling, for that matter) isn’t new. Global warming is mainly a natural effect which warms the Earth and its immediate atmosphere (the biosphere). Without it, life on Earth as we know it would not exist. If there was no global warming, the average temperature of the Earth would be
-18oC or about the same as that inside a deep freezer.
Global warming occurs because some gases in the atmosphere act in a similar way to a pane of glass in a greenhouse. They let in solar radiation but prevent heat loss by absorbing some of the infra-red radiation emitted from the surface of the Earth.
Although average temperatures have risen about 0.6oC over the past 100 years, they were higher in the 11th to 13th centuries (when vines grew all over south-east England). Conversely, in prehistoric times, global cooling is thought to have lead to the ice ages.
Although not all scientists agree, the majority think that unless man-made greenhouse gas emissions are reduced, the result will be a 3oC increase in the glbal temperature by 2100, which could change climate patterns. Fortunately, the International Community is beginning to respond to this challenge through such measures as the ‘Kyoto Protocol’.
What are the main greenhouse gases?
To contribute to global warming, gases need to persist in the atmosphere. Some have a life of well over 100 years. The hydrocarbon propellants that are now used in most aerosols, and solvents that are used in some of them, only have a life of a few days. Therefore, they contribute little to global warming.
The most significant man-made greenhouse gases are carbon dioxide, methane, nitrous oxide and ozone (all of which also occur naturally). Man-made chlorofluorocarbons (CFCs) have a strong global warming potential but because of their ozone depletion properties, they are no longer used in aerosols.
Hydroflurocarbons (HFCs) are a potential replacement for CFCs. They do not contain any chlorine and so do not deplete ozone. They are most likely to be used in asthma treatment and a few critical industrial aerosols. However, as they can crontribute to global warming, the British Aerosol Industry has voluntarily agreed not to use HFCs wherever a safe, more environmentally friendly and practical alternatives exist.
What are the main causes of Tropospheric Air Pollution?
While there are many natural causes, the principal man-made contributors are:
1. Oxides of sulphur (SOx)
2. Oxides of nitrogen (NOx)
3. Carbon monoxide (CO)
4. Ammonia (NH3)
7. Volatile organic compounds (VOCs)
Oxides of nitrogen (NOx) and sulphur (SOx – principally SO2) both contribute to the formation of acid rain which can result in damage to aquatic life, crops, trees and erosion of buildings. International controls are already in place limiting emissions of SO2 and NOx.
Carbon monoxide arises mainly from the incomplete combustion of fuel (largely road transport) and is now controlled increasingly through the compulsory fitting of catalytic converters to road vehicles.
VOCs contribute to creation of excess ozone in the troposphere.
Given sunlight, NOx and VOCs in the right proportions and calm air conditions, ozone can build up to form ozone ‘episodes’. These are like ‘clouds’ of ozone which may last for 2 or 3 days and travel considerable distances. In some sunny and heavily polluted environments, such as Athens and Los Angeles, ozone smog can be very persistent and significantly exceed maximum recommended levels.
High levels of ground level ozone can damage plant life, crops and buildings, Ozone can also have short term effects on humans, especially those with breathing difficulties such as asthmatics, the old or people taking strenuous exercise.
At the moment there is no globally accepted definition of a VOC. The United Nations definition is organic compounds of anthropogenic nature, other than methane, that are capable of producing photochemical oxidants by reactions with nitrogen oxides in the presence of sunlight.
More than 200 VOCs have been identified which can contribute to ozone formation. Some VOCs contribute more to the formation of ozone and so are described as being more reactive or having greater POCP (photochemical creation potential).
The reactivity is very important when considering industry’s contribution to ozone formation. While the use of aerosols accounts for about 3% of all man-made VOC emissions in the UK, the VOCs are low POCP types and so their contribution to ozone formation is proportionately less.
Leaving aside methane, it is estimated that man-made emissions of VOCs in the UK account for two-thirds of ozone production, the balance being due to methane, carbon monoxide and natural emissions such as those from trees.
While aerosol products make a small contribution to tropospheric ozone formation, their impact is at a very low level and equivalent to that of many other similar products.
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