The document discusses photochemical smog and tropospheric ozone. It explains that (1) emissions from burning fossil fuels and deforestation produce pollutants like NOx and VOCs that react with sunlight to form ozone in the troposphere. (2) High temperatures and concentrations of these pollutants near the equator exacerbate ozone formation. (3) Photochemical smog results from a mixture of ozone, NOx and VOCs in cities without wind dispersing the pollutants. It causes eye, skin and respiratory irritation in humans and damage to plants. Management strategies aim to reduce vehicle use and encourage public transport to lessen emissions.

1. Topic 6
Atmospheric Systems and Societies
6.3 Photochemical Smog
http://www.youtube.com/watch?v=
UtdKRvWC1yQ

2. Tropospheric Ozone
• CO, CO2, SOx NOx and volatile organic compounds
(VOCs) are released from the burning of fossil
fuels (especially in car engines)
• NOx and VOCs 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
• Smog can also be caused by the burning of trees
as part of deforestation

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. Thermal Inversion
• This is caused when dense, cool
air is trapped beneath less
dense, warm air
• It is a particular problem in
Mexico City
• The 2 layers of air can’t mix, so
the smog builds up near the
ground and is not swept away

6. 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

7. 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