The map shows the change in the average number of ozone exceedence days from April to September observed at regional background stations (EMEP monitoring stations) from the period 1990-1994 to the period 1999-2004, excluding the abnormally warm summer of 2003. A day is counted as an exceedence day when the daily maximal 8-hr-average ozone concentration exceeds 60 ppbV.
The number of exceedence days at each location is plotted as a filled coloured dot at the location’s coordinates, with the colour changing according to the following levels: [< -12, -12 to -8, -8 to -4, -4 to 0, 0 to + 4, 4 to 8, 8 to 12, > 12] . At locations where the number of exceedence days is in the range [-1, 1], the dot is replaced by an open circle in the colour of the original interval it belonged to.
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