* Air Pollution, Climate Change and Ozone Depletion  Notes

Box 1 - Differences Between Combustion and Photochemical Smogs

Combustion
Photochemical
Typical time of day for formation Early morning or night-time Late afternoon
Typical time of year.
Winter, clear cold weather leading to a radiation fog Hot sunny day (summer)
Pollutants responsible Primary pollutants SO2, soot from coal burning or NOx from cars in the case of the 1991 London Episode. Release of NOx and VOCs from cars in the morning rush hour undergo photooxidation to produce secondary pollutants such as ozone in the afternoon.

Figure 3. Scematic diagram of the production of ozone during a photochemical smog episode. VOCs (CO), NOx and sunlight can participate in the cycling between OH and HO2 radicalswith the subsequent production of ozone.

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Figure 3. schematic diagram of the production of ozone during a photochemical smog episode. VOCs (CO), NOx and sunlight can participate in the cycling between OH and HO2 radicals with the subsequent production of ozone

Figure 2. Data taken from the NETCEN data archive from the Bristol area on 27th July 2001.
Figure 2. data taken from the NETCEN data archive from the Bristol area on 27th July 2001

The graph shows how the level of NO is high during the morning rush hour suppressing the formation of ozone. In the afternoon ozone levels begin to build as NO levels are lower and sunlight can initiate ozone production.

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Figure 1. a schematic of the evolution of the1952 London combustion smog

A: The sky was clear and there was little wind early in the morning of December 5th, the weather was cold and Londoners were burning a lot of coal, generating smoke (soot) and SO2 as were power stations.
B: The air near the ground was colder than the air above and an inversion layer was formed, a radiation fog began to form. The fog reflected incoming sunlight away, maintaining the cold surface temperatures.
C: The smoke filled up this shallow layer near the ground and London was plunged into artificial darkness for five days. The sunlight could not penetrate the ‘smog’ and so the inversion persisted. Eventually a change of wind direction brought relief with strong winds and broke the stranglehold of the smog.

Figure 1. A schematic of the evolution of the1952 London combustion smog.