Combined particle number size distributions and readily available gaseous concentration data were used to identify winter particle sources in the urban atmosphere of Prague, the capital of Czech Republic by applying bilinear Positive Matrix Factorization (PMF2). Ambient Particle Number Concentration (PNC) were obtained using a Scanning Mobility Particle Sizer (SMPS) in the size range between 18.8 and 723.5 nm (midpoint diameters) along with the ambient gaseous concentrations of CO, SO2, NOx (NO + NO2), O3, CH4, Non Methane Hydrocarbons (NMHC) and Total Hydrocarbons (THC) at the receptor site with 5-minute time resolution. Meteorological data including wind direction, wind speed, temperature, relative humidity and solar the UV-A,B radiation were also recorded in 5-minute intervals to assist in the interpretation and to support the use of Conditional Probability Function (CPF) in determining the directionality of the sources. For convenient data handling and matrix preparation, all of the data obtained were reduced to hourly average concentrations. This study has identified and apportioned four major sources. They are ozone-rich, (transported ozone/ozone precursors, mixed down from above boundary layer associated with high wind speed and temperature), NOx-rich (diesel emissions), traffic and local heating.