The influence of mixed layer depth along the course of incoming air masses to the transport of PM10 components at three rural sampling sites in Spain

The main objective of this research, was to incorporate Mixed Layer Depth (MLD) estimations across backward air mass trajectories to the broadly used Trajectory Sector Analysis (TSA) and Concentration Weighted Trajectory (CWT) methods, in order to attain a three-dimensional (3D) identification of aerosol transport pathways and to reveal the role of the Mixed Layer (ML) on the transferring of particulates. The developed 3D-TSA and 3D-CWT tools, were combined with daily concentrations of PM₁₀-bound ${\mathrm{SO}}_4^{2-}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{Na}}^{+}$, ${\mathrm{Mg}}^{2+}$, ${\mathrm{Ca}}^{2+}$ and $K^{+}$ measured at three rural sampling sites in Spain during the years 2019–2020. Vertically extended Saharan dust intrusions from North Africa were associated with air masses travelling both inside and outside the ML and were related to increases of PM₁₀-bound ${\mathrm{SO}}_4^{2-}$ and ${\mathrm{Ca}}^{2+}$ at all stations, attributed to the reactions of mineral dust with gaseous precursors of ${\mathrm{SO}}_4^{2-}$ such as SO₂. The advection of sea salt particles, marked by high levels of ${\mathrm{Na}}^{+}$ and ${\mathrm{Mg}}^{2+}$, was associated with marine air masses from the Mediterranean and Atlantic Ocean, moving mainly within the ML. Enhanced levels of PM₁₀ constituents emitted by anthropogenic sources, such as ${\mathrm{NO}}_3^{-}$ (traffic and industrial emissions) and $K^{+}$ (biomass burning), were clearly related to air masses originating from Iberian Peninsula, Central Europe and North African coastline, whilst in most cases the strongest contributions were transferred by air masses moving above the ML. Therefore, the implemented 3D version of TSA and CWT methods, revealed new information regarding the altitudinal characteristics of air masses affecting PM₁₀ levels in Spain.