Effects of reduction of NOx on the NO2 levels in urban streets

Nitrogen dioxide is a major problem in urban areas. Persons suffering from respiratory diseases, especially asthma patients, are sensitive to NO2 at high concentrations. Nitrogen oxide emissions from private cars have decreased due to the introduction of catalysts. However, the catalyst technique is today limited to petrol vehicles. Since heavy diesel vehicles, e.g. buses, contribute significantly to the NOx emissions, this will limit the reduction in emissions in many urban streets. Another factor is the continued increase in the car fleet in all countries. Most of the NOx from traffic is emitted as NO, which is thought to be harmless. NO is rapidly transformed to NO2 by a reaction with ozone in the air. Tropospheric ozone levels have been increasing in Europe for many years as a consequence of increasing pollution. Ozone in Danish streets is seen to be the limiting factor for the production of NO2. Thereby O3 is also the limiting factor for the NO2 levels in the streets. In this paper the above will be supported both by observations from the Danish air quality measurement programmes and by model simulations with the Danish Operational Street Pollution Model.