PREV'AIR pollution

Episodes >> Ozone

Examples of photochemical
pollution episodes

What is photochemical pollution? Photochemical pollution episodes are characterized by high concentration levels of a chemical compound: ozone. Ozone is a secondary pollutant : it is not directly emitted into the atmosphere but it builds up from other chemical species (nitrogen oxides and volatile organic compounds) associated to human activities (these are primary pollutants ). Photochemical pollution episodes result from both anthropic (i.e. of human origin) and natural factors. Indeed, ozone production derives from chemical reactions involving nitrogen oxides and volatile organic compounds, initiated by solar radiation (hence the expression "photochemical" pollution) and enhanced by high temperatures. Thus, the anticyclonic situation that prevailed during the first two weeks of August 2003 (see hereafter) was favourable to the development of a photochemical pollution episode. Another factor of influence is the wind: a light wind - and, more generally, stagnant conditions - prevents the pollutants to be dispersed in the atmosphere; as a consequence, secondary pollutants build up downwind of large emission centers. Finally, ozone lifetime in the lower part of the atmosphere is equal to a few days; ozone molecules might be advected dizains or even hundreds of kilometers downwind of their area of production.

At the end of June, the beginning of July, 2009 : Animation of the episode

At the end of June and at the beginning of July, the weather conditions characterized by high temperatures and low(weak) winds were convenient to a rather general degradation of the air quality in France.

The national system of forecast and surveillance of the air quality Prev' Air correctly planned during this period a net increase of the concentrations of ozone on the country, identifying several regions more particularly touched as the Ile-de-France, the Haute-Normandie, Provence-Alpes-Côte d'Azur and Languedoc-Roussillon.

Animation hour per hour of the concentrations of ozone analyzed on-surface (O3) on France according to the forecasts realized by PREV'air and the national observations for period from June 28th till July 3rd, 2009.

August 2003 : description of the episode

From the 1st to the 15th August 2003, anticyclonic conditions prevailed over Western Europe, thus favouring the development of a photochemical pollution episode. This episode was out of range with respect to its geographical extension, its duration and the ozone levels that were encountered. In France, 417 µg/m3 were reached near Marseille, and the population information threshold for this pollutant (i.e. 180 µg/m3, hourly mean concentration) was exceeded several times all over the French territory and in many neighbouring countries.

On the right-hand side, the map displays the ozone ground concentrations (in µg/m3) on August 8, 2003,  2 p.m. Isolines display the simulated concentrations - a posteriori - by the CHIMERE model; points refer to in-situ observations (carried out by local air quality monitoring networks). Exceedances of the 180 µg/m3 threshold are in black.

This picture shows the continental extension of the August 2003 photochemical episode. Almost all French regions were touched, including remote places (away from large emission centers such as cities,  industries) or usually clean areas (Atlantic sea-side or Brittany).

(Source IPSL/CNRS)

August 2003 : forecasts of the PREV'AIR systema name="II">

In 2003, the PREV'AIR forecasts accounted well for the course of the photochemical episode.

At the beginning

The three-day forecasts of July 31, 2003 show the shift from a slightly polluted situation (under the influence of the clean masses of air of oceanic origin) to already high ozone levels (see the forecasts for August 2, 2003) over almost the whole of France - except for the most North-Western part - and the North of Europe (Belgium, Netherlands, Nord-West of Germany) and Northern Italy.

In the middle of the episode

Based on the forecasts of August 8, 2003, concentrations were expecteed to exceed 180 µg/m3 over a large area, extending from North-East to South-West, from Netherlands to South-West of France, as well as over the South of England , Northern Italy and the Portuguese coast.

It appears that the forecast levels were overestimated compared to the observations. On the other hand, the extent of the polluted field was correctly estimated in the forecast maps.

The end

On August 16, 2003, the three-day forecasts show a progressive return to a slightly polluted situation under the influence of the clean air masses of oceanic origin.


Vautard, R., Honoré, C., Beelmann, M. et L. Rouïl, Simulation of ozone during the August 2003 heat wave and emission control scenarios, Atmospheric Environment, Volume 39, Issue 16, May 2005 , pages 2957-2967