The effect of a post-industrial megacity on local and regional air quality was assessed via a month-long field measurement campaign in the Paris metropolitan area during winter 2010. Here we present source apportionment results from three aerosol mass spectrometers and two aethalome-ters deployed at three measurement stations within the Paris region. Submicron aerosol composition is dominated by the organic fraction (30–36 %) and nitrate (28–29 %), with lower contributions from sulfate (14–16 %), ammonium (12–14 %) and black carbon (7–13 %). Organic source apportionment was performed using positive matrix factorization, resulting in a set of organic factors corresponding both to primary emission sources and secondary production. The dominant primary sources are traffic (11–15 % of organic mass), biomass burning (13–15 %) and cooking (up to 35 % during meal hours). Secondary organic aerosol contributes more than 50 % to the total organic mass and includes a highly oxidized factor from indeterminate and/or diverse sources and a less oxidized factor related to wood burning emissions. Black carbon was apportioned to traffic and wood burning sources using a model based on Published by Copernicus Publications on behalf of the European Geosciences Union. 962 M. Crippa et al.: The organic fraction in the metropolitan area of Paris wavelength-dependent light absorption of these two combustion sources. The time series of organic and black carbon factors from related sources were strongly correlated. The similarities in aerosol composition, total mass and temporal variation between the three sites suggest that particulate pollution in Paris is dominated by regional factors, and that the emissions from Paris itself have a relatively low impact on its surroundings.