Assessment of Urban Air Quality in Salé, Morocco: An In-Depth Analysis of PM2.5 Seasonal Variations, Elemental Composition, and Enrichment Factors

Sale, situated along the Atlantic Ocean, ranks as the third-largest city in Morocco, characterized by its high demographic density and significant air pollution challenges. The Bouregreg River delineates Sale from Rabat, the capital of Morocco. Key sources of air pollution in Sale include traffic emissions, maritime activities, potteries, fishing, domestic heating, and artisanal operations. From July 2018 to July 2019, we conducted a comprehensive study on ambient PM2.5 concentrations in an urban area of Sale. Using a Dichotomous sampler, we collected PM2.5 samples and employed Total X-Ray Fluorescence to analyze the elemental composition of the collected filters. The results indicated that PM2.5 mass concentrations fluctuated between 3.08 μg/m³ and 49.48 μg/m³, with an average of 17.30 μg/m³. Notably, peak concentrations occurred during winter months, contrasting with lower levels observed in summer. To investigate the influence of atmospheric transport on PM2.5 levels, we applied the HYSPLIT™ model for air mass back-trajectories analysis, identifying four primary transport clusters arriving at the monitoring site in Sale: Iberian Coast (24%), Near Atlantic Ocean (33%), local sources (28%), and Oceanic influences (15%). Seasonal analysis revealed variations in metal concentrations. Elements such as K, Ca, V, Mn, Fe, Ni, Cu, Zn, As, Sr, Ba, and Pb showed elevated levels during winter, while Ti and Cr peaked in autumn. Enrichment Factor (EF) assessments indicated that Ti, Sr, Mn, K, Ca, and Ba predominantly originated from airborne dust (EF < 10). Meanwhile, V, Cu, and Cr exhibited both natural and anthropogenic contributions (10 < EF < 100), while Ni, Zn, and Pb were largely anthropogenic (EF > 100). According to the Morocco National Ambient Air Quality Standard, the annual mean limit for PM2.5 is set at 35 µg/m³, indicating that the average PM2.5 concentration of 17.30 µg/m³ observed in Sale is below this limit, suggesting that while air quality is better than the standard, seasonal spikes could still pose health risks.