Size-resolved aerosol Mass Concentrations, Elemental Composition, and Long-Range Transport Effects in Four Moroccan Coastal Cities

Morocco has experienced notable urban and industrial growth in recent years, resulting in increasing air pollution concerns. This study investigates the characteristics of size-resolved aerosols (PM₁₀ and PM_2.5) and their sources across four distinct locations: two Mediterranean coastal cities (Tetouan and Nador) and two Atlantic coastal cities (Kenitra and Salé). Aerosol samples were collected and analyzed for elemental composition (Al, Fe, Ni, V, Cu, Cr, Zn, and Pb) using a range of analytical techniques such as Total X-ray Fluorescence (TXRF), Atomic Absorption Spectroscopy (AAS), Microwave Plasma Atomic Emission Spectrometry (MP-AES), and Instrumental Neutron Activation Analysis (INAA). Gravimetric analysis was performed to determine daily PM mass concentrations. To evaluate the effects of long-range transport, air mass back-trajectories were generated using the HYSPLIT™ model. Source apportionment was conducted using inter-elemental ratios, Positive Matrix Factorization (PMF) receptor modeling, and air mass back-trajectory statistics. The analysis of inter-elemental ratios highlighted urban emissions, largely attributable to traffic and construction activities, as the primary anthropogenic source. Contributions from long-range transport were identified by linking PM mass concentrations with air mass flow directions, demonstrating the significant impact of emissions from the Mediterranean Basin and the Atlantic Ocean on air quality in Moroccan cities. Moreover, the PMF source apportionment indicated that the contributing sources vary between PM fractions. For PM_2.5, major sources were identified as vehicle exhaust/non-exhaust emissions, regional secondary aerosols, and local anthropogenic activities. In contrast, PM₁₀ was predominantly associated with soil dust, re-suspended road dust, and fresh/aged sea salt emissions. Finally, the study highlighted that the PM_2.5/PM₁₀ ratio is site-dependent. Mediterranean coast cities presented higher PM_2.5/PM10 ratios (>0.5), indicating a significant contribution from fine anthropogenic particles. Conversely, Atlantic coastal cities displayed ratios below 0.5, suggesting a predominance of coarse particles, likely emanating from local pollution sources characteristic of those areas.