The contribution of carbon to particulate matter is larger in urban areas than outside those areas. The increment in urban EC and OC concentrations was found mainly to be caused by traffic emissions. The average annual increment was about 0.5 μg/m³. As a consequence, the potential to reduce urban background concentrations by abatement of traffic exhaust emissions, is small. However, from a health point of view, a reduction of carbon in PM is believed to be important.