The declared objective of the European Air Quality Directive (1999/30/EC), its derivative directives and the implementations in national law by the member states (in Germany, the Federal Pollution Control Act, 'Bundesimmissionsschutzgesetz' or BImSchG for short) is to maintain ambient-air quality where it is good and improve it in other cases, and precisely this principle is applied with particular vigour in the case of Europe's major cities.
It is with this end in mind that limits values and alarm thresholds for the concentrations of certain pollutants in ambient air have been laid down; the figures apply to airborne pollutants from road traffic in particular and increasingly strict limits will be introduced as time goes on. At the present time, the permitted annual averages are 40 micrograms per cubic metre of air for nitrogen oxides and 20 micrograms for particulate matter (PM). If the limits are overshot, the authorities in the zones in question are required by law to impose measures that will effectively reduce the pollutant concentrations in the air in order to protect health.
Increasingly strict European and national laws are being imposed at a time when the situation in German cities is characterised by an increase in traffic volume. Although due in part to growth in personal mobility, this increase is fostered primarily by the swelling volume of goods traffic carried on the public roads.
The EU limits for airborne pollutants are exceeded in many German cities, with the thresholds for nitrogen oxides (NOx) and particulate matter (PM10) commonly proving the most problematical. Many urban authorities have reacted by imposing clean air plans in line with the new regulations set out in the Federal Pollution Control Act, (BImSchG). The city authorities are responsible for traffic-control and zoning plans and for public transport, so in the first instance it is up to these same authorities to develop and implement effective measures to ensure compliance with the EU rules and regulations.
Both technical and non-technical measures are called for in the road-traffic scenario to reduce emissions of diesel soot (a primary contributor to airborne particulate matter) and nitrogen oxides. One verifiably successful measure in this respect has proved to be the introduction of low emission zones (LEZs), which only low-emission vehicles are permitted to enter and drive about in. This is why Germany's federal government is so eager to encourage the installation of diesel particulate filters designed to enable even older vehicles to achieve good exhaust-emission levels.