Particulate Matter Pollution Episodes in a Multisource Coastal Environment: Insights from Single-Particle Analysis and atmospheric dynamic

Coastal industrial and urban regions in Europe host a substantial fraction of the population and economic activity, yet they remain highly vulnerable to particulate matter (PM) pollution episodes. Despite the implementation of air quality regulations, exceedances of PM₁₀ and PM_2.5 concentration thresholds persist, driven by the coexistence of dense emission sources and complex coastal atmospheric dynamics. In this study, pollution days (PDs) were analyzed over a four-year period (2018–2021) in the Greater Dunkirk Area, a coastal region influenced by multiple anthropogenic and marine sources. Spatial analyses indicate that PM_2.5 pollution episodes are predominantly associated with regionally extended plumes, whereas PM₁₀ episodes are more frequently linked to locally confined plumes, exhibiting marked seasonal variability. Detailed aerosol chemical characterization was conducted using SEM–EDX analysis on more than 23,000 individual particles collected during a one-year field campaign in 2021. The results reveal a highly heterogeneous particle population, largely dominated by sea-salt and carbonaceous aerosols, with fine particles enriched in secondary sulfur-containing species and coarse particles characterized by calcium-rich components. The particle mixing state index (χ) spans a wide range (0.5–0.9), reflecting a continuum between externally and internally mixed aerosols, strongly modulated by atmospheric ageing processes, pollutant recirculation, and turbulent mixing. Our findings demonstrate that neither local wind direction nor plume spatial extent alone adequately explains the observed chemical variability. Instead, the evolution of aerosol composition and mixing state is governed by fine-scale meteorological processes, including sea-breeze circulations and recirculation events, which critically influence pollutant dispersion and ageing. These results underscore the importance of integrating high-resolution single-particle chemistry with urban-scale meteorological dynamics in air quality assessments, particularly in complex coastal environments subject to multiple emission sources.