Abstract
Keywords: exhaust emissions, nitrogen oxide, nitrogen dioxide, realworld driving, heavy duty, urban environment
Emissions of nitrogen oxides from motor vehicles are an important source of air pollution that affects not only acidification of the natural environment, but also has a severe impact on air quality in urban surroundings all over the world. Therefore, since late 1980's, legislative efforts has been targeted towards bringing down the levels of these substances that are mainly originating from the reactions of the intake air nitrogen and oxygen during the hightemperature combustion process in internal combustion engines.
However, in recent years the ambient air quality monitoring in many European cities has shown that the recorded levels of NO2 in urban environments exposed to exhaust gases from motor vehicles have not much improved, if changed at all. This has happened, despite the fact that more stringent NOx emission control has been implemented in all new motor vehicles and many types of quite effective emission reduction systems are being employed by all OEMs.
As many studies have shown that exposure to NO2 has more adverse health effects than those of NO, it is of primary concern, even if after a transformation in air, all NO will eventually be turned into NO2. Because this transformation usually takes place within minutes to a few hours, depending on many atmospheric parameters, emissions are usually diluted and transported away from the immediate release and exposure areas thus alleviating the illeffects. However, if more NO2 is directly emitted in urban air, elevated concentrations are an inevitable result.
Usually, as legislative standards only regulate NOx, the sum of NO and NO2, there has only been a few studies presenting NO2 emission rates, or reporting of NO/NO2 ratio in the NOx emissions. Therefore, in 2006 VTT initiated an effort to register typical levels of NO2 associated with different types of motor vehicles. For this purpose, NO and NO2 were determined separately using realworld dutycycles from a number of heavy duty vehicles run on a full vehicle transient chassis dynamometer. The emphasis has been on urban buses, as in many urban street canyons buses are a major source of NOx emissions.