Tracing aerosol sources and transport dynamics across altitudinal gradients in the Western Italian Alps using multi-site Positive Matrix Factorization (PMF)

High-mountain regions are key observatories for investigating atmospheric composition and dynamics, providing access to background conditions while remaining sensitive to regional and long-range atmospheric transport influences. Despite their distance from major emission sources, high-altitude sites are frequently affected by pollutant transport driven by synoptic scale circulation, mesoscale thermally driven flows, and episodic long-range transport events. Notably, in the Italian Alps, aerosol advection from the Po Valley, one of the most polluted regions in Europe, is particularly relevant due to its proximity.

In this study, we apply a data-driven receptor modeling approach to characterize aerosol sources and transport pathways in the Aosta Valley region (Western Italian Alps) using multi-site Positive Matrix Factorization (PMF). Aerosol samples were collected during a summer field campaign (July-September 2024) at three sites with altitudes ranging from 318 to 3480 m a.s.l., allowing investigation of vertical gradients and source contributions from valley to mountain peaks. Sampling was carried out using PM₁₀ high-volume samplers with 24-h resolution and 12-h resolution (00:00-12:00 and 12:00-00:00), to capture mesoscale circulation patterns.

The PMF analysis relied on a comprehensive dataset of chemical markers,  including major ions, major and trace elements, carbon fractions, sugars and two classes of emerging contaminants (benzothiazoles and organophosphate flame retardants). This extensive  characterization allowed detailed resolution and quantification of aerosol types and sources,  such as mineral dust, secondary inorganic and organic aerosol, biogenic emissions, traffic-related and industrial sources, and long-range transported components. The inclusion of novel tracers improved the identification of specific anthropogenic contributions in remote alpine environments.

To support the interpretation of PMF results, aerosol optical properties and size distribution were analyzed to examine the temporal variability and physical characteristics of the identified sources. These complementary high-resolution data provided independent evidence of transport dynamics, capturing both diurnal patterns associated with valley-mountain circulation and long-range transport events, including Saharan dust intrusions and wildfire smoke from North America. Vertically-resolved remote sensing observations combined with meteorological analyses confirmed the role of medium- to long-range transport in the observed variations in surface aerosol concentrations and properties.

Overall, this study underlines the role of mountain regions as sensitive receptors of pollution from both nearby and distant sources and highlights the importance of integrated, multi-site approaches for understanding aerosol dynamics over complex topography. These findings offer valuable insights for improving atmospheric modeling and air quality assessment in remote alpine environments.