You are here: Home / Air Quality / Measurements / Nitrogen Dioxide / History / Trends / NO2 19th highest hourly mean

NO2 19th highest hourly mean

NO2 annual mean | 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 1990-2020. The spatial average is represented by the blue circles. All data were calculated using the RIO interpolation technique.

Click here for more information on how to interpret a boxplot

Evolution of regional averages

Since 1990, there has been a decline in the spatial maximum of the 19th highest NO2 hourly value. This means that peak NO2 concentrations have decreased over the last 30 years, in the Flemish, Brussels Capital and Walloon Regions. 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. In 2020 the maximum of the 19th highest hourly value in Belgium, representative at 4x4 km², was 79 µg/m³, which is well below the European limit of 200 µg/m³, and is observed in Flanders. The average 19th highest NO2 hourly value in Flanders, Wallonia and Brussels is 42 µg/m³, 27 µg/m³ and 60 µg/m³ respectively. The highest values occur only in large agglomerations such as Brussels and Antwerp.

Evolution of the maximum of the 19h highest NO2 hourly value in the three Regions and Belgium based on the RIO interpolation technique.