Similarities and differences in the submicron atmospheric aerosol chemical composition are analyzed from a unique set of measurements performed at 21 sites across Europe for at least one year. These sites are located between 35 and 62°N and 10° W - 26°E, and represent various types of settings (remote, coastal, rural, industrial, urban). Measurements were all carried out on-line with a 30-min time resolution using mass spectroscopy based instruments known as Aerosol Chemical Speciation Monitors (ACSM) and Aerosol Mass Spectrometers (AMS) and following common measurement guidelines. Data regarding organics, sulfate, nitrate and ammonium concentrations, as well as the sum of them called non-refractory submicron aerosol mass concentration ([NR-PM₁]) are discussed. NR-PM₁ concentrations generally increase from remote to urban sites. They are mostly larger in the mid-latitude band than in southern and northern Europe. On average, organics account for the major part (36-64%) of NR-PM₁ followed by sulfate (12-44%) and nitrate (6-35%). The annual mean chemical composition of NR-PM₁ at rural (or regional background) sites and urban background sites are very similar. Considering rural and regional background sites only, nitrate contribution is higher and sulfate contribution is lower in mid-latitude Europe compared to northern and southern Europe. Large seasonal variations in concentrations (μg/m³) of one or more components of NR-PM₁ can be observed at all sites, as well as in the chemical composition of NR-PM₁ (%) at most sites. Significant diel cycles in the contribution to [NR-PM₁] of organics, sulfate, and nitrate can be observed at a majority of sites both in winter and summer. Early morning minima in organics in concomitance with maxima in nitrate are common features at regional and urban background sites. Daily variations are much smaller at a number of coastal and rural sites. Looking at NR-PM₁ chemical composition as a function of NR-PM₁ mass concentration reveals that although organics account for the major fraction of NR-PM₁ at all concentration levels at most sites, nitrate contribution generally increases with NR-PM₁ mass concentration and predominates when NR-PM₁ mass concentrations exceed 40 μg/m³ at half of the sites.