Atmospheric aerosols are defined as relatively stable suspensions of solid or liquid particles in a gas. Despite their low concentration, both in terms of number and mass, as well as their size distribution, physical properties, and chemical composition, they play a significant role in atmospheric processes. The formation of new particles can significantly increase the concentration of CCN in the atmosphere in as little as a day, with some locations experiencing more than a twofold increase. Therefore, new particle formation (NPF) is crucial to our understanding of global climate and should be incorporated into climate models. A systematic study was conducted at the urban site of Ljubljana using physical and chemical measurements (offline and online) to gain a better understanding of urban NPFFor insight into NPF physical growth rates a setup of online instrumentation was used; Scanning Mobility Particle Sizer (SMPS) and Aethalometer (AEE33). The offline setup consisting of a cascade impactor and PM sampler was used for the chemical characterization of the nucleation mode particles. The chemical analysis included different chromatographic methods (ionic chromatography, High-performance liquid chromatography with UV/VIS or MS detection, and gas chromatography with MS detection), SEM-EDS analysis and electroanalytical approaches. Special attention was given to surface-active substances (SAS) and their role in urban NPF and identification, physical and chemical characterization of 6 characteristic days with different events. The analysis indicated the key role of SO2, NOx, VOC, O3 in the process of particle formation. Although the role of SO2 is significant, particle analyzes pointed out that carbon, oxygen and nitrogen predominated in the smallest particles (up to 30 nm in diameter), highlighting the key role of nitrogen compounds and VOCs for the given days.