Ultrafine particles (UFP; <100 nm diameter) play a disproportionate role in human health and atmospheric processes, yet their sources and growth mechanisms in urban environments in different seasons remain poorly constrained. This study presents a comprehensive characterization of UFP number concentrations, size distributions, chemical drivers, and meteorological influences in downtown Munich across summer, winter, and spring. We combine high resolution particle number size measurements with source characterization of organic aerosol (OA), semi-volatile organic aerosol (SVOA) and volatile organic compounds (VOC) to disentangle primary emissions from secondary processes. Results show that UFP number concentrations are driven by traffic- and cooking-related emissions, consistently peaking during nighttime boundary layer collapse due to accumulation. In contrast, ultrafine particle growth (from 40 to 80 nm) arises predominantly from condensation of semi-volatile or low-volatility organic vapours, with distinct seasonal pathways: biogenic and biomass burning OA at night, and photochemically oxidized low-volatility vapours during daytime in summer; biomass-burning and combustion-related SVOA in winter; and a mixed regime involving both primary emissions and moderate photochemistry in spring. No evidence of classical new particle formation appears in these non-nucleation UFP growth events. These findings demonstrate that UFP evolution in Munich is governed by the interplay between boundary-layer dynamics and seasonally varying organic vapour sources, highlighting the need for season-specific mitigation strategies.