In an urban site affected by fresh vehicle exhaust emissions, the ambient air number concentrations of particles coarser than 3 nm (N) was split into two components, N = N1 + N2. This was done using a method based on the high correlation between black-carbon (BQ and number (N) concentrations which is typically observed in ambient air and is the result of vehicle exhaust emissions. The component N I accounts for "those aerosol components directly emitted in the particle phase" and "those components nucleating immediately after emission". The component N2 accounts for the new particle formation enhancements during the "dilution and cooling of the vehicle exhaust" and is also influenced by "in situ new particle formation in ambient air". The contribution of NI to N exhibits a maximum of 55% during the morning rush hours (07:00-08:00). The contribution of N2 to N exhibits a daily evolution with a broad maximum during daylight (as solar radiation intensity), while for about 7 h (11:00-17:00) the N2 contribution to N is about 70%. During some "afternoon N2 events", N2 contributions exceeded 90%. Enhancements in the new particle formation processes may increase the N/BC concentrations ratio in one order of magnitude, from 4.82 x 10(6) particles/ng BC to 47 x 10(6) particles/ng BC and during some events up to 97 x 10(6) particles/ng BC. The results show evidence of the high potential of the vehicle exhausts and of the urban atmosphere to trigger new particle formation if the ambient air conditions are favourable. The method used in this study is useful in assessing future changes in the number to BC relationship due to forthcoming regulations in the vehicle exhaust emissions. (c) 2007 Elsevier Ltd. All rights reserved.