The capital city of Mongolia, Ulaanbaatar, suffers from high levels of pollution due to excessive airborne particulate matter (APM). A lack of systematic data from the region results in great interest for investigations on the kind and origin of the contributing pollution sources, seasonal variations, the effect of meteorological conditions or even the time-dependence of anthropogenic sources. This work reports source apportionment results from a large data set of 184 fine (PM2.5) and 184 coarse (PM2.5-10) fraction atmospheric PM samples, collected over a three-year period (2014-2016) in Ulaanbaatar, Mongolia. The concentrations of 16 elements measured by an energy dispersive X-ray fluorescence spectrometer together with the black-carbon content measured by the reflectometry method were used as input data for the application of the Positive Matrix Factorization (PMF) method. The PMF results revealed that the contribution of mixed factors dominates the coarse fraction. Interestingly, soil and traffic sources were identified as principle contributors of the fine fraction. Examination of the source profiles and the seasonal variation of their contributions indicates that fly ash emanating from coal combustion mixes with traffic emissions and soil re-suspensions resulting in variable chemical source profiles. Four sources were identified for both fractions, namely soil, coal combustion, traffic and oil combustion. The contribution of these sources to the coarse fraction were 35%, 16%, 41% and 8% respectively, whereas for the fine fraction they were 31%, 27%, 31% and 11%. Additionally, the concentration-weighted trajectory analysis technique was used to assess the probable source contributions from long range transport events.