Aerosols in Northern Morocco (Part 4): Seasonal Chemical Signatures of PM2.5 and PM10

Atmospheric aerosols are recognized as a major air pollutant with significant impacts on human health, air quality, and climate. Yet, the chemical composition and seasonal variability of aerosols remain underexplored in several Western Mediterranean regions. This study presents a year-long investigation of PM2.5 and PM10 in Tetouan, Northern Morocco, where both local emissions and regional transport influence air quality. PM2.5 and PM10 samples were collected and analysed for total mass and comprehensive chemical characterization, including organic carbon (OC), elemental carbon (EC), water-soluble ions (WSIs), and sugar tracers (levoglucosan, arabitol, and glucose). Concentration-weighted trajectory (CWT) modelling and air mass back-trajectory analyses were used to assess potential source regions and transport pathways. PM2.5 concentrations ranged from 4.2 to 41.8 µg m−3 (annual mean: 18.0 ± 6.4 µg m−3), while PM10 ranged from 11.9 to 66.3 µg m−3 (annual mean: 30.8 ± 9.7 µg m−3), with peaks in winter and minima in spring. The PM2.5-to-PM10 ratio averaged 0.59, indicating a substantial accumulation of particle mass within the fine fraction, especially during the cold season. Carbonaceous aerosols dominated the fine fraction, with total carbonaceous aerosol (TCA) contributing ~52% to PM2.5 and ~34% to PM10. Secondary organic carbon (SOC) accounted for up to 90% of OC in PM2.5, reaching 7.3 ± 3.4 µg m−3 in winter. WSIs comprised ~39% of PM2.5 mass, with sulfate, nitrate, and ammonium as major components, peaking in summer. Sugar tracers exhibited coarse-mode dominance, reflecting biomass burning and biogenic activity. Concentration-weighted trajectory and back-trajectory analyses identified the Mediterranean Basin and Iberian Peninsula as dominant source regions, in addition to local urban emissions. Overall, this study attempts to fill a critical knowledge gap in Southwestern Mediterranean aerosol research by providing a comprehensive characterization of PM2.5 and PM10 chemical composition and their seasonal dynamics in Tetouan. It further offers new insights into how a combination of local emissions and regional transport shapes the aerosol composition in this North African urban environment.