Abstract. The chemical composition and sources of ambient fine particulate matter (PM1) over a period of 2.5 years for a regional background site in the western Mediterranean are presented in this work. Furthermore, sub-micron particle number concentrations and the sources of these particles are also presented. The mean PM1 concentration for the measurement period was 8.9 μg m−3, with organic matter (OM) and sulphate comprising most of the mass (3.2 and 1.5 μg m−3 respectively). Six sources were identified in PM1 by Positive Matrix Factorisation (PMF): secondary organic aerosol, secondary nitrate, industrial, traffic + biomass burning, fuel oil combustion and secondary sulphate. Typically anthropogenic sources displayed elevated concentrations during the week with reductions at weekends. Nitrate levels were elevated in winter and negligible in summer, whereas secondary sulphate levels underwent a contrasting seasonal evolution with highest concentrations in summer, similar to the fuel oil combustion source. The SOA source was influenced by episodes of sustained pollution as a result of anticyclonic conditions occurring during winter, giving rise to thermal inversions and the accumulation of pollutants in the mixing layer. Increased levels in summer were owing to higher biogenic emissions and regional recirculation of air masses. The industrial source decreased in August due to decreased emissions during the vacation period. Increases in the traffic + biomass burning source were recorded in January, April and October, which were attributed to the occurrence of the aforementioned pollution episodes and local biomass burning emission sources, which include agriculture and domestic heating systems. Average particle number concentrations (N9-825 nm) from 5/11/2010 to 01/06/2011 and from 15/10/2011 to 18/12/2011 reached 3097 cm−3. Five emission sources of particle of sub-micron particles were determined by Principal Component Analysis (PCA); industrial + traffic + biomass burning, new particle formation + growth, secondary sulphate + fuel oil combustion, crustal material and secondary nitrate. The new particle formation + growth source dominated the particle number concentration (56% of total particle number concentration), especially for particles < 100 nm, followed by industrial + traffic + biomass burning (13%). Secondary sulphate + fuel oil combustion (8%), nitrate (9%) and crustal material (2%) were dominant for particles of larger diameter (> 100 nm) and thus did not influence the particle number concentration significantly.