The Low Emission Zone in Leipzig was established in March 2011, allowing only access of Diesel vehicles of Euro4 and higher with few exceptions. The ban of older vehicles and subsequent modernization of the car fleet resulted in slightly reduced PM10 and PM2.5 mass concentrations. However, the mass concentration of black carbon (soot particles) emitted mainly from Diesel vehicles decreased by 60% at the street site. These particles are believed to be most dangerous due to their carcinogenic trace compounds such as polycyclic aromatic hydrocarbons. Furthermore, also the number concentration of ultrafine particles, which can penetrate deep into the lungs, decreased by approximately 70%. Despite modernized Diesel vehicles, nitrogen oxides concentrations did not follow these trends and remained nearly constant. The main achievement of the Low Emission Zone was the improvement of air quality by the reduction of the most dangerous particles.