Ambient NO2 concentration profiles in Flanders using passive sampling
In most parts of Europe NO2 emissions from excessive road traffic, concentrated by confined spaces and limited dispersion, are often higher than the ambient guideline values. As a pollutant,NO2 has a number of adverse effects on human health and the environment. The European Union sets guideline and threshold values for various pollutants, to protect humans and the environment, of which NO2 is one. Flanders adopted these values as most countries did, and the monitoring and evaluation of the levels against these guideline values are mostly done by VMM (the Flemish Environmental Company). The air quality Framework Directive (96/62/EG) was drafted on the27th of September 1996 and instituted on the 21st of November 1996. New guideline values forNO2 will come into effect in 2010 (1999/30/EG). The future hourly guideline value is 200 µg m-3which may not be exceeded more than 18 times in a calendar year. The average annual guideline value must not exceed 40 µg m-3.Currently various pollutants are continuously monitored by means of fixed monster monitors and analysers, where after data is extrapolated to give an overview of the dispersion. In the 2003annual report the future guideline value has been exceeded in 7 locations in Flanders. Moreover, in a separate study it was reported that in various locations with high traffic density and low dispersion, this value was exceeded, even though the dispersion model did not indicate it. Hence, to test these and other locations against the future guideline value a total of 19points, in 6 different cities and towns in Flanders, were chosen to monitor the NO2 profile over a1 year period. Passive sampling, averaged over periods of 2 weeks, was used and comparisons with the fixed monitors in some of these locations were possible. The future annual guideline value of 40 µg m-3 (2010) was exceeded in 11 of the 19 measured locations. When high traffic density was accompanied by low dispersion the value was at its highest and the results confirmed the relationship between high traffic density and high NO2concentrations. A good correlation between duplicate measurements and those compared to the fixed monitors ensured accurate and precise results. Ozone-rich periods were noted for the highNO2 values as expected, while seasonal effects were also noted. Backward trajectories indicated the significance of air masses originating from heavily polluted areas.