The graphs are based on the 99.2 percentile of daily mean concentration values corresponding to the 4th highest daily mean; they present the range of concentrations at all station types (in μg/m3) officially reported by the EU Member States and how the concentrations relate to the limit value set by EU legislation (marked by the red line).
The diagram indicates the lowest and highest observations, the means and the lower and upper quartiles. The lower quartile splits the lowest 25 % of the data and the upper quartile splits the highest 25 % of the data.
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