oxides of nitrogen are significant pollutants present in emissions, especially from diesel vehicles. Generally, the ambient concentrations of oxides of nitrogen (no_x) in many ban areas in developed countries have fallen steeply over the past ten years as old high-emitting diesel technologies have been replaced with new ones. Surprisingly, the levels of nitrogen dioxide (no₂), the more harmful of the two oxides, the other being nitric oxide (no), have not acted in the same way. For instance, in Germany, while the ambient no_x levels have decreased by about 50 per cent since 1987, the no₂ concentrations are hovering at the same level as during the 1990s (see graph: Constant trouble).

The results, published by the Institute for Energy and Environmental Research (ifeu), Heidelberg, Germany, put a question mark on the emission control technologies being used for diesel vehicles. In Germany, no₂ levels exceed the safe limit in many locations. It is known that even a short-term exposure to no₂ for up to three hours may cause cough and worsen respiratory illness. Long-term exposures cause increased susceptibility to respiratory infection and lung damage. Moreover, scientists at the University of California, usa, found high no₂ levels apparently raise the risk of sudden infant death syndrome (Archives of Disease in Childhood, Vol 90, No 7, July 2005).

Better or worse? According to ifeu, most diesel vehicles are equipped with emission control devices such as oxidation catalysts or passive catalysed diesel particulate filters (cdpf), which can increase no₂ emissions. Used to control particulate matter (pm) emissions from diesel engines, cdpf oxidise no to no₂ to remove the soot caught in the filter. But the device often produces more no₂ than that required to keep it soot-free.

The no₂ to no₂ ratio, which is merely 5-10 per cent from the engine, can shoot up to 20-70 per cent after the emissions pass through cdpf depending on factors such as the type of filter used, sulphur level in diesel fuel and the duty cycle, according to the California Air Resources Board (carb).

The other common emission control device is the oxidation catalyst. It is mainly used in cars to convert toxic pm, carbon monoxide and hydrocarbons to harmless compounds. But it also invariably converts no to no₂, though the overall no₂ levels may remain the same.

It is ironic that the emission control devices enhance no₂ levels. How will the developed countries tackle the problem? The ifeu points out the no₂ rise should be taken into account for pollution reduction strategies. The Association for Emissions Control by Catalyst, Brussels, says balanced emission control systems should not produce more no₂ than needed. But these are mere guidelines.

As of now, the approach closest to a solution is that adopted by carb for in use vehicles. It has sought to split the no₂ emissions into two