This paper presents results derived from ultra-fine particle observations conducted in urban and semi-rural areas of the Athens basin in Greece. Aerosol particle size spectra were collected at 2.5-min time resolution with a Scanning Mobility Particle Sizer for approximately two months, along with standard meteorological, NO, NO2 and O-3 measurements. The results obtained showed that a 10-fold less maximum number concentration of the mean spectrum was noticed at the semi-rural site compared with that at the urban site. The corresponding maximum for the median spectrum of urban site was only 20% less, indicating that extreme cases had little effect, at least in maximum mean concentration of 9.8 nm particles. The largest number concentration maximum in the mean size distribution spectrum for each area occurred at a modal diameter <= 9.8 nm. For particles >= 13.4 nm, the mean spectrum for Patision contained prominent accumulation particle populations at number median diameters of 13.4, 29.7, and 43.7 nm. In contrast, the major features of the accumulation region of the mean semi-rural spectrum were fit with 5 distribution functions of nearly the same amplitude. The most dominant feature occurred at a number median diameter of 70.1 nm, and thus substantially greater than the 43.7 nm most-dominant in the mean urban spectrum. High ultra-fine particle concentrations at the urban site generally coincided with periods of high NO concentrations and were well correlated with benzene, signifying emissions from motor vehicles. (C) 2012 Elsevier Ltd. All rights reserved.