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The Atmo index

Definition
The Atmo index, symbolised by a giraffe, represents mean urban air quality using a single figure scale. The Atmo outlook varies daily according to air quality using a scale of 1-10 (1 = very good air quality, 10 = very bad air quality).

10
Very bad
9
Bad
8
Bad
7
Poor
6
Poor
5
Moderate
4
Good
3
Good
2
Very good
1
Very good
The Atmo index scale

Calculation method
Atmo bases its calculation on four subindexes characterising the four pollutants listed below:

  • Sulphur dioxide (SO2),
  • Nitrogen dioxide (NO2),
  • Ozone (O3),
  • Particles in suspension (PS).

The thresholds used to define subindex levels were set based on regulatory criteria on air quality:

Subindexes PS scale NO2 scale SO2 scale O3 scale
Average of
mean daily concentrations for the various sites		 Average of the hourly maxima for the various sites
1 0 à 9 µg/m3 0 à 29 µg/m3 0 à 39 µg/m3 0 à 29 µg/m3
2 10 - 19 30 - 54 40 - 79 30 - 54
3 20 - 29 55 - 84 80 - 119 55 - 79
4 30 - 39 85 - 109 120 - 159 80 - 104
5 40 - 49 110 - 134 160 - 199 105 - 129
6 50 - 64 135 - 164 200 - 249 130 - 149
7 65 - 79 165 -199 250 - 299 150 - 179
8 80 - 99 200 - 274 300 - 399 180 - 209
9 100 - 124 275 - 399 400 - 599 250 - 359
10 125 and more 400 and more 500 and more 240 and more
Atmo subindex determination grid

The final Atmo index is equal to the highest of the four above-determined subindexes. This calculation method, used since 1 January 2005, is defined by the French order dated 22 July 2004 governing air quality indexes.

Measuring stations taken into account
The measuring stations selected for calculating the Atmo index must be representative of background pollution. They are urban stations measuring sulphur dioxide, nitrogen dioxide and particles in suspension. Ozone measurement requires both urban and suburban sites.

For built-up areas in Auvergne, the Atmo index is calculated based on the following measurement sites:

Built-up area NO2 SO2 PS O3
Clermont-Ferrand Lecoq
Montferrand
Delille
Jaude		 Lecoq
Montferrand
Jaude		 Lecoq
Montferrand
Delille		 Lecoq
Montferrand
Delille
Gerzat
Royat
Aurillac Mairie Centre Centre Mairie
Aérodrome
Montluçon Château Centre Centre Château
Hippodrome
Le Puy-en-Velay Centre Fayolle Fayolle Centre
Vals-près-Le-Puy
Measurement sites used for calculating the Atmo index