NO2 19th highest hourly mean
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.
For the protection of human health from short-term NO2 peak concentrations, the European legislation imposes an hourly limit value of 200 μg/m³. This hourly limit is to be exceeded no more than 18 times (hours) per year. The 19th highest hourly value corresponds to the 99.8th percentile of all hourly mean concentrations in one year. In Belgium, this hourly limit value is easily attained. Very locally, in busy traffic streets, the 200 μg/m³ limit may be exceeded a few times per year, but the 18-hour limit is never reached in any of the monitoring stations.
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 19th highest hourly mean NO2 concentration in Belgium. The spatial average for Belgium is represented by the blue circles.
Box plot of the 19th highest hourly mean NO2 concentration (μg/m³) over the period 1990-2015. The spatial average is represented by the blue circles. All data were calculated using the RIO interpolation technique.
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Evolution of regional averages
Since 1990 there has been little variation in the spatially averaged, minimum and maximum of the 19th highest NO2 hourly value. This means that the peak NO2 concentrations have not or have hardly decreased in the Flemish, Brussels-Capital and Walloon Regions in the last 20 years. One exception was the year 1997, when very high concentrations were measured in the stations of the Belgian Petroleum Federation in Antwerp in the month of January.The average 19th highest NO2 hourly value in Flanders, Wallonia and Brussels is 50 μg/m³, 39 μg/m³ and 85 μg/m³ respectively. The highest values occur only in large agglomerations such as Brussels and Antwerp. The WHO guideline value is also 200 μg/m³, but is not to be exceeded at any time. This value, too, is attained everywhere in Belgium.
Evolution of the maximum of the 19h highest NO2 hourly value in the three Regions and Belgium based on the RIO interpolation technique.