Bulk characteristics and dynamics of atmospheric PM from urban and rural coastal sites in the Northwestern Mediterranean area

Atmospheric particulate matter (PM) significantly impacts Earth's climate, air quality, visibility, and public health. PM influences the carbon cycle, ocean-atmosphere interactions, and global dynamics. The Mediterranean Sea, a highly climate-sensitive ecosystem, faces growing vulnerability from rising demographic and economic pressures. As part of the FIRETRAC project, we monitored total suspended particles (TSP) for two years at two Northwestern Mediterranean sites: urban Marseille and rural Banyuls-sur-Mer, both in the Gulf of Lions. Chemical analyses (metals, ions, organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and water-soluble total nitrogen (WSTN)), and isotopic techniques (δ¹³C and Δ¹⁴C) along with statistical methods were used to detect TSP composition, sources, and dynamics.

TSP particles from Banyuls (-26.31 ± 0.52 ‰) and Marseille (-26.23 ± 0.51 ‰) showed comparable δ¹³C values, indicating a predominantly terrestrial origin. Apparent radiocarbon ages (Δ¹⁴C) ranged from 940 to 5000 years, pointing to a complex mixture of sources with varying reactivities. Additionally, HYSPLIT modeling revealed long-range transport from diverse regions, including the Sahara Desert, Eastern European industrial zones, and Northern Europe, contributing to the particulate matter in the Northwestern Mediterranean Sea. 

These findings suggest that TSP at both sites likely represent a mixture comprising both anthropogenic and natural sources which is further supported by the analysis of other chemical constituents.

Thus at the Marseille site, the Fe-Al correlation reached 0.94 (p < 0.0001, n=59), and during periods of higher concentrations, the HYSPLIT back-trajectory model confirmed its origin from natural source: Saharan dust. Conversely, at both sites, the strong correlation of Mn-Pb (r = 0.88 in Banyuls, 0.97 in Marseille, p < 0.0001, n=60), along with evidence from the HYSPLIT and PCA analysis, indicates shared sources, including industrial emissions and long-range pollution (via routes crossing the Atlantic Ocean, the industrial regions of southern Spain, and southern France). The OC and EC concentrations were relatively similar between the two sites (12.34 ± 5.56 µg·m^-³ in Banyuls versus 9.59 ± 4.91 µg·m^-³ in Marseille). Banyuls exhibited higher OC/EC ratios, indicating more secondary organic carbon, while Marseille showed stronger primary emissions. Seasonal variations in WSOC and WSTN concentrations suggest their dependence on local emissions and atmospheric seasonal processes. The higher WSTN/WSOC ratio observed in Marseille (0.51 ± 0.34 µg·m^-³ compared to 0.36 ± 0.18 µg·m^-³ in Banyuls) highlights greater nitrogen contributions, likely attributed to urban pollution.

Prncipal component analysis (PCA) showed that PC1 (41.9% variance) is dominated by OC, EC, WSOC, WSTN, and heavy metals (e.g., Pb, Zn, Cu), indicating anthropogenic influences like combustion and industrial emissions. In contrast, PC2 (15.4% variance) was driven by Na, Mg, Sr, and K, representing natural sources such as marine aerosols and soil dust. These findings suggest that regional pollution is largely driven by human activities but moderated by natural background sources, offering key insights for pollution source analysis.

Overall, this study reveals that TSP in the Northwestern Mediterranean Sea originates from a complex interplay of anthropogenic and natural sources, influenced by both local emissions and long-range transport and ongoing research in our lab focuses on specific BB tracers of TSP including anhydrosugars and PAHs.