Assessing the sources of submicron airborne elements at two sites in the Fos-Marseille basin through rolling positive matrix factorization

Abstract. The contributions and evolution of fine elemental particulate matter (PM) sources were investigated in the Marseille–Fos basin (south of France) based on a 1-year-long (January–December 2023) study using online X-ray fluorescence (Xact) PM1 measurements. The region's intense anthropogenic activity and complex meteorological conditions make it an ideal case study for fine aerosol characterization. Given the limited information available on fine elemental sources in the area, a dual-site approach was implemented, combining an urban background site (MRS-LCP) and an industrial site (FOS) to distinguish between regional and local emission influences. Source apportionment was conducted using a rolling positive matrix factorization (PMF) method, implemented via the Source Finder Professional (SoFi) toolkit. Several tests were carried out to determine optimal rolling PMF parameters. Eventually, a 21 d rolling-window configuration was selected, resolving nine factors at FOS and eight at MRS-LCP, with seven similar factors detected at both sites. Among them, three were attributed to secondary aerosols, including sulfur photooxidation leading to sulfate-rich aerosols (S-rich factor) and the formation of halogenated reactive particulate species (Cl-rich and Br-rich factors). Additionally, biomass-burning-, shipping-, and dust-related factors were identified at both locations. In contrast, three industrial factors (steel industry, Zn-industrial, Pb-industrial) were detected at FOS, while only the steel industry factor appeared at MRS-LCP, suggesting downwind transport of industrial plumes from Fos-sur-Mer to Marseille under mistral- and thermal-breeze regimes. Furthermore, the comparison of the dynamic rolling PMF approach to static PMF analysis demonstrated higher dissimilarities across factors profiles, reflecting an enhanced ability of rolling PMF to capture seasonal variability in aerosol sources. Overall, this study highlights the dominant anthropogenic imprint on submicron PM elements and the effectiveness of dynamic source apportionment in complex coastal–industrial environments.