The mechanisms of air pollution

There are several stages in the processes governing air pollution. Firstly there is the emission of pollutants, quickly followed by their dispersion and chemical transformation within the atmosphere.

Emissions
Pollutant emissions strongly impact on air quality. Primary pollutants, including nitrogen oxides, sulphur dioxide, carbon monoxide, particles in suspension and certain volatile organic compounds (VOC), are emitted directly into the atmosphere. They may be produced by fixed (urban heating systems, industrial, domestic or agricultural activities) or mobile sources, cars in particular. The production of primary pollutants decreases during summer as central heating systems are switched off and there is less traffic in town centres.

Transport and dispersion
Dispersion, i.e. the transport of pollutants from their source, is a key stage as it determines whether a pollutant will accumulate or be diluted in the atmosphere. Dispersion is influenced by several factors including weather conditions and local topography (altitude, relief, rivers and streams, etc.).

There are two types of dispersion:

• Vertical dispersion, related to the vertical temperature gradient of the surface layer, the atmosphere's lower layer influenced by the ground below it
• Horizontal dispersion, also referred to as transport, relating to wind speed and direction

For example, an anticyclonic (high-pressure) situation characterised by light winds, limits horizontal dispersion. In winter, low temperatures and clear skies also promote an inversion of the vertical temperature gradient (the air is warmer at altitude than it is at ground level), which inhibits vertical dispersion. This situation, which occurs frequently in Clermont-Ferrand, raises pollution levels through the accumulation of gases thus resulting in pollution peaks. Conversely, low-pressure situations, generally associated with stronger wind patterns, effectively disperse the pollutants throughout the atmosphere. Furthermore, rainfall, which results in moisture-laden pollutant deposits (flushing), reduce pollutant concentrations.

Chemical transformations
During dispersion, pollutants may be transformed by complex chemical reactions to form secondary pollutants such as ozone and certain VOC. Nitrogen dioxide may also be considered a secondary pollutant insofar as direct source emissions, although significant, are still considered marginal.

The formation of ozone requires strong sunlight and the presence of some precursor components, such as VOC. This leads to reactions between primary and secondary pollutants, the most common being the reversible reaction between ozone and nitrogen oxides (NO + O₃ ↔ O₂ + NO₂) which takes place in sunlight and when there are high concentrations of nitric oxide. This reaction can explain the lower ozone levels in urban areas during peak traffic periods (due to the destruction of ozone by reaction with nitric oxide).

Conversely, periurban stations located downwind of the town register maximal ozone peaks, because the absence of large emissions of nitric oxide means the polluted air becomes enriched in ozone the further it travels.