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Insufficient data on the chemical composition of PM_2.5 and its emission sources in the southwestern (SW) Mediterranean area has been identified. Ambient PM_2.5 samples were collected in an urban area of Tetouan city, northern Morocco. Chemical mass closure calculations and positive matrix factorization were performed for the comprehensive dataset of PM_2.5 chemical analyses. Mass closure improved when multipliers (1.2 and 0.23, respectively) were used for the conversion of organic carbon (OC) and calcium ion (Ca²⁺) into particulate organic matter (POM) and mineral dust masses, respectively. The mass closure model performed well in this SW Mediterranean region, with a significant correlation (r² = 0.97) obtained between gravimetrically measured and chemically determined PM_2.5 mass. The one-year average concentration of PM_2.5 was 17.96 μg/m³, and the major chemical components were POM (34%), secondary inorganic aerosol (SIA) (28%), and black carbon (18%), while unidentified mass was 4%. The mass concentration and most of the chemical components of PM_2.5 showed clear seasonal variations, with a summer-high and winter-low pattern for SIA, dust, and BC. In the winter months, POM was the dominant component. Source apportionment analysis revealed that PM_2.5 emission sources, regarding their typical tracers, were ammonium sulfate (SO₄²⁻, NH₄⁺, K⁺, NO₃⁻), road traffic and biomass burning emissions (OC, BC), fresh sea salt (Cl⁻, K⁺, NO₃⁻), aged sea salt (Mg²⁺, Na⁺, Ca²⁺), and oxalate-rich (oxalate, NO₃⁻) factors. Further, it is hoped that these findings help to improve the scientific understanding of SW Mediterranean aerosols.