The temporal variability of indoor Particle Number (PN) concentrations, their determinants and their relative contributions in an occupied workspace were investigated. The presented study is based on the receptor modeling approach, focusing on Non-negative Matrix Factorization (NMF) to provide new insights on the source time variability. Continuous size distribution from 0.3 μm to 20 μm were collected with a short time step sampling (1 min) over six months in 2015. The measurements were made inside and outside an open-plan office occupied by 6–8 persons. NMF distinguished five major patterns obtained from PN concentrations time series. The apportionment results were expressed as source diurnal profiles and strengths by relating the obtained source contributions to the source information provided by the office occupancy and natural ventilation (the effect of opening windows). Factor 2 contributes to 75% of the total contributions for finer size fraction (<0.5 μm). Combining NMF results with indoor occupancy and windows states successfully demarcated the main sources of fluctuation. The diurnal profiles of the third factor (F3) and PN0.9−1.8 concentrations time series are very similar (r = 0.95). The diurnal variation of factor 1 is very similar to that observed for CO2 variations and PN6.25−12.5 time series. Coarse particles (>17.5 μm) are associated with the 4th factor. The latter does not contribute to any of the other particle ranges. The NMF factors interpretation was supported by correlation analysis and statistical tests, as well as by temporal variation comparison. © 2021 Elsevier Ltd