International audience A new highly sensitive cavity ring-down spectrometer (CRDS) system was designed and assembled to determine the aerosol extinction coefficient (bext) at 532 nm. The performance of the CRDS was tested by the monodisperse polystyrene latex spheres (PSL) particles with diameters between 200 and 500 nm. By comparing the tested results with Mie theory curve, the uncertainty of the newly developed CRDS system was determined to be < 3%. Simultaneous measurements of the size distribution and extinction coefficient of ambient aerosols were conducted in Orleans, France, from 26th October to 21st December 2012 by using a scanning mobility particle sizer (SMPS) coupled to the CRDS system. For the non-dehydrated aerosols measured from 26th October to 4th November, the average bext has been found to be 41 ± 35 Mm-1. For the dehydrated aerosols measured from 7th November to 21st December, bext expresses a good agreement with the particle number (N) and volume (V) concentration, the average values of bext, N and V are 36 ± 31 Mm-1, 3300 ± 2700 cm-3 and 3.1 ± 2.8 × 109 nm3/cm3, respectively. Further analysis of the particle size distribution reveals that car and boiler emissions maybe the main aerosol sources in Orleans. In addition, back trajectory results indicate that the air parcel transported from Atlantic Ocean may play a role in cleaning up the ambient air in Orleans.