Continuous measurements of aerosol number size distributions from 3 nm to 10 ¿m have been first performed within the city area of Beijing since March 2004. Size distributions of the first 45 measurement days (March 05 to April 18, 2004) were investigated in terms of their high variability. Two dust storm events were observed indicated by high number concentrations greater than 1 ¿m and mass concentrations around 1 mg m-3. Continental highly polluted air was observed during 12 days indicated by a number peak in the accumulation mode range, and submicrometer volume concentrations above 150 ¿m3 cm-3 were observed. Newly formed particles with more than 100,000 cm-3 were observed on 25 days when the particle surface area concentration drops below a critical value (100--2000 ¿m2 cm-3) because of clean air from the north. Measurements show only a slight growth (~1 nm h-1) of the particles indicating that they are produced within the city area of Beijing.