Trend analysis of urban NO2 concentrations and the importance of direct NO2 emissions versus ozone/NOx equilibrium

The annual air quality standard of NO2 is often exceeded in urban areas near heavy traffic locations. Despite significant decrease of NOx emissions in 1986–2005 in the industrial and harbour area near Rotterdam, NO2 concentrations at the urban background remain at the same level since the end of the nineties. Trend analysis of monitoring data revealed that the ozone/NOx equilibrium is a more important factor than increasing direct NO2 emissions by traffic. The latter has recently been identified as an additional NO2 source due to the introduction of oxy-catalytic converters in diesel vehicles and the growing number of diesel vehicles. However, in Rotterdam over the period 1986–2005 direct NO2 emissions by road traffic only increased 3–4%. Due to the importance of the ozone/NOx equilibrium, it is concluded that local NOx emissions in Rotterdam need substantial reduction to achieve lower NO2 urban background levels. This is a relatively costly abatement strategy and, therefore, a “hotspot” approach aiming at reducing NOx emissions by local traffic measures is more effective to meet European air quality standards.