In every major European cities, aerosol pollution is of great scientific, economic, health and social concern. Despite geographical disparities, the particulate phase in urban areas is complex by virtue of its chemical composition and the multitude of emission sources. Long-term and detailed chemical speciation studies are key variables to constrain atmospheric processes and local versus continental imports leading to urban pollution episodes; European standards and legislations however mainly focus on indiscriminate mass concentrations (PM2.5 and PM10). While Aerosol Mass Spectrometers (AMS) may provide valuable 'real-time' data for the chemical characterization of submicron particles, their cost and implementation as networking devices make them improper to be used for long-term measurements. Developed by Aerodyne Research Inc. (Billerica, MA), the Aerosol Chemical Speciation Monitor (ACSM) measures quantitative chemical composition for non-refractory sub-micron aerosol particles. Smaller, lower cost and more robust, the ACSM is designed for long-term unattended deployment and routine monitoring applications. In this context, the infrastructure EU-FP7- ACTRIS program has been promoting the use of ACSM for long-term observations at background stations, among which the SIRTA/LSCE site. This station, representative of PM pollution in the region of Paris, has been equipped with an ACSM since June 2011. Co-located multi-wavelength absorption measurements (AE-31, Magee Scientific) allow for Black Carbon source apportionment (Sciare et al. 2011). Reliable PM1 data (TEOM-FDMS 1405F, Thermo) are also available since February 2012. The ACSM has been providing continuous data since June 2011. This large dataset highlight raising important issues on data processing. Nevertheless, the two datasets (ACSM + BC versus PM1) show very good temporal agreement, with a slope close to 1 (slope=1.08, r2=0.87), pointing out the very good consistency and long term stability of ACSM measurements. First results enlighten the major role played by organics and ammonium nitrate on PM loadings in the region of Paris (Fig.1), especially during severe winter pollution episodes. Statistical tools, such as Potential Source Contribution Function, reveal the strong influence of continental import on secondary species (nitrate and sulphate) highest concentrations (Fig.2). Such methodologies applied to Positive Matrix Factorization outputs will also help to better understand the sources and atmospheric processes of organic aerosols. For time-limited intensive campaigns, a substantial analytical fleet is engaged in order to investigate specific pollution sources. Since early February 2012, PILS-IC, PILS-TOC and PILS-MS/MS (for the anionic fraction, water-soluble organic carbon and levoglucosan, respectively) have been implemented to constrain wood burning emissions. Fertilizer spreading emissions will be investigated this spring, emphasizing on gaseous precursors of aerosol.