Conclusions
The causes of air pollution episodes for European cities can vary and depend on factors including emissions, prevailing wind patterns, topography, solar radiation and season.
Particle episodes have been experienced in winter times and ozone levels can be particularly high during summer periods.
Inversions, which lead to stagnant air, are particularly important in relation to episodes and in many cases are responsible for very high levels of pollution.
Regional transport of pollution, for example particles, can also lead to standards being exceeded.