An Electrical Low Pressure Impactor (ELPI) was used during spring and autumn 2003 in the centre of Strasbourg for the measurement of atmospheric aerosols size distribution. The concentration of NO x and SO 2 in air was simultaneously measured with specific analysers. Samples were collected in the range 0.007–10 μm in equivalent aerodynamic diameter size. Number distributions are representative of a pollution originating from urban traffic with a particle size distribution exhibiting a nucleation mode below 29 nm and an accumulation mode around 80 nm in size. A mean particle density equal to 39000±35000 total particles per cm 3 with a size ranging from 7 to 10 μm was obtained after a sampling period of 2 weeks in spring. About 86.9% of the number of particles have an aerodynamic diameter below 0.1 μm and 13.1% between 0.1 and 1 μm. Correlation coefficients between the number of particles impacted on each ELPI plate and gas concentrations (SO 2 and NO x ) showed that the numbers of particles with diameter between 0.10 and 0.62 μm are highly related to the NO x concentration. This result indicates that particles are traffic induced since NO x is mainly emitted by cars as shown by measurements on various sites. Particles are less clearly correlated to the SO 2 concentration. Particle analysis on different ELPI plates for a sampling period of 2 weeks in autumn showed high level of soluble NO 3 −, SO 4 2− and NH 4 + ions. Indeed, up to 90% b.w. of these three species were found in the particle range 0.1–1 μm. The formation of particulate NH 4NO 3 is favoured by high NO x concentration, which induces the formation of gaseous HNO 3.