Summary of topic 5.7
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Tropospheric ozone is formed when nitrogen oxides (NOx) and volatile organic compounds released from burning fossil fuels react with oxygen in the presence of sunlight. This causes eye and skin irritation and breathing problems in humans. In large cities, ozone levels are lower due to reactions with NO from vehicles forming photochemical smog. Photochemical smog forms in cities with low wind and stable atmospheric conditions. It is a mixture of ozone, NOx and VOCs produced when these pollutants react in sunlight. Summer smog is worse in sunny areas and winter smog occurs in inversion layers trapping pollution. Smog levels may be higher surrounding city centers where polluted air drifts.
Summary of topic 5.7
- 1. Topic 5 Pollution Management 5.7 Urban Air Pollution http://www.youtube.com/watch?v= UtdKRvWC1yQ
- 2. Tropospheric Ozone • NOx and volatile organic compounds (VOCs) are released from the burning of fossil fuels (especially in car engines) • These release oxygen atoms under the action of UV light which combine with O2 in the troposphere to form ozone • This is accelerated by high temperatures and sunny conditions – therefore the problems are exacerbated in conurbations close to the equator
- 3. Effects of Tropospheric Ozone • Ozone damages plants and in humans causes eye and skin irritation and breathing problems • It is very reactive and attacks man-made fibres • In large urban areas (like Mexico City) , tropospheric ozone is less of a problem, since ozone reacts with NO from car exhausts to form NO2 – however this causes separate pollution problems (photochemical smog) http://www.youtube.com/watch?v=JVcFps_gWpk http://www.youtube.com/watch?v=r6L_7QvJr64
- 4. Photochemical Smog • Caused by a mixture of ozone, NOx and VOCs • It is a problem in cities where pollutants are not swept away by winds (either due to low pressure or the existence of a thermocline) • Summer smog occurs in sunny areas where photochemical activity causes ozone formation • Winter smog occurs due to thermal inversion and the increased combustion of fossil fuels in cold conditions • Often smog problems are worse in areas surrounding city centres since: – Ozone forms relatively slowly allowing polluted air to drift away from where it was formed – Nox levels are higher in city centres and react with ozone
- 5. Management Strategies • Reduced use of private vehicles and trucks • Introduction of more efficient engines • Encouraging car-pooling and use of public transport • Switch from road to rail haulage • Change to more efficient and less polluting fuels
- 6. Questions 1. State the sources and effects of tropospheric ozone 2. Outline the formation of photochemical smog 3. Describe the effects of photochemical smog 4. Describe and evaluate strategies to control urban air pollution