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| Block 8: Environmental Issues and Public Health - Air Pollution Chapter 2: An Overview Of The Main Air Pollutants And Their Impacts (Continued) |
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Deposition of pollutants takes place onto buildings, vegetation and other surfaces, and rain tends to scrub out pollutants from the atmosphere - the acidic pollutants, sulphuric acid and nitric acid may form "acid rain". The relationship between measured ambient pollutant levels and source emissions is therefore complex.
| All pollutants undergo dispersion, chemical transformation and deposition in the lowest layer of the atmosphere - the troposphere. The troposphere extends to an altitude of about 16 to 18 km over the tropics, reducing to about 10 km over the poles and contains about 80% of the total air mass. All weather phenomena occur in this layer. Mixing between the troposphere and higher levels of the atmosphere (stratosphere and above) is negligible, therefore the dispersion of pollutants occurs almost exclusively within the troposphere. Note that the vertical scale in Figure 2.6 is greatly exaggerated. |  |
| Figure 2.6: The troposphere: The lowest layer of the atmosphere, 10 to 18km in thickness |
At locations that are remote from pollutant sources, air concentrations reach "background" levels. Background level concentrations refer to measurements done far from pollution sources. Due to the mixing and dispersion processes in the atmosphere, background levels may represent concentrations that are low but significantly different from the unpolluted air of pre-industrial periods. The distance from the source(s) required to reach ‘background’ levels depends on several factors, including the strength of the source, the rate of chemical transformation or deposition of the pollutant, and the prevailing meteorological conditions. The Cape Point monitoring station is regarded as a background station even though it is about 100 km from the major sources of the City of Cape Town; in contrast, the pollution plumes from the large Eskom power stations (Mpumalanga Highlands) are measurable more than 1000 km from the sources.
Table 3 gives some "background level" (Clean Air) concentrations compared to polluted air levels.
| Gas | Concentration (ppb) | ||
|---|---|---|---|
| Clean Troposphere | Polluted Air | ||
| Sulphur dioxide (SO2) | 1 - 10 | 20 - 200 | |
| Carbon monoxide(CO) | 120 | 1 000 - 10 000 | |
| Nitrogen dioxide (NO2) | 0.01 - 0.05 | 50 - 250 | |
| Ozone (O3 | 20 - 80 | 100 - 500 | |
| PM10 (units: µg/m3) | 0 (?) | 30 - 600 | |
| VOCs | ? | 500 - 1 200 | |
| Lead (Pb) (units: µg/m3) | 0.0005 - 0.003 | 0.4 - 2+ | |
The concentration of pollutants in urban air is one to three orders of magnitude (10x to 1000x) greater than levels in "background" or unpolluted air.
For example, Cape Point "background’ concentrations" (Figures 2.7 and 2.8) for ozone and CO may be compared with values within the City of Cape Town.
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| Figure 2.7: Background CO concentrations at Cape Point (Courtesy E Brunke) | Figure 2.8: Background ozone concentrations at Cape Point (Courtesy E Brunke) |
By contrast, 24 hour average CO levels in at the Drill Hall monitoring stations are about 2000 ppb, compared with about 55 ppb at Cape Point (Figure 2.7); hourly average ozone concentrations in the City are 0 to 120 ppb. Peak ozone levels in the City are about five times greater than at Cape Point. In some cities, peak ozone values of more than 200 ppb are not uncommon.
