All data below are the result of the RIO interpolation technique, in which the measurement observations are interpolated to a 4x4 km² grid resolution. Locally, concentrations can be higher or lower.
The AOT60 (= Accumulated Ozone Exposure above a Threshold of 60 ppb =(120 µg/m³)) is an indicator to assess the effects of ozone for the population. The AOT60 sums the difference between the concentrations above 120 μg/m³ and 120μg/m³ of the daily maximum 8-hour mean concentrations. In contrast to the NET60 indicator, the AOT60 considers the size and duration of the exceedance. In the draft version of the first ozone directive (2002/3/EC) and the National Emissions Ceilings Directive, 5800 (μg/m³).hours was proposed as the medium-term objective (MTO) for 2010, which corresponds to a NET60 equal to 25 days. The long-term objective is 0 (μg/m³).hours. The AOT60 was not incorporated into the current European Ambient Air Quality Directive, but is a better indicator to quantify the excess burden on public health. This indicator was also incorporated into the Flemish MINA-4 plan (2011-2015) to illustrate the plan objectives.
Evolution of the percentiles
The figure below uses box plots to show the evolution of the minimum, the 25th percentile (P25), the 50th percentile (P50), the 75th percentile (P75) and the maximum of the AOT60 values for the Belgian population since 1990. The years 1990, 1995 and 2003 immediately strike as unfavourable ozone years for the Belgian people. The last few years were however relatively favourable where the highest values never exceed the European treshold of 5800(µg/m³).h. While the average AOT60 has never since failed to meet the European treshold, the long term goal of 0(µg/m³).h is exceeded in Belgium every single year.
Box plots of AOT60 in Belgium over the period 1990-2015. The spatial average is represented by the blue circles. All data were calculated using the RIO interpolation technique./em>
Click here for more information on how to interpret a boxplot.