Sources of PM2.5-bound water soluble ions at EMEP’s Auchencorth Moss (UK) supersite revealed by 3D-Concentration Weighted Trajectory (CWT) model

The Concentration Weighted Trajectory (CWT) model is a well-known tool which combines the residence time (trajectory points) of air masses over specific regions with ambient concentrations of air pollutants, aiming to identify potential long range transport impacts. An upgraded 3D-version of CWT model (3D-CWT), investigating not only the geographical origin of the exogenous emissions but also the altitudinal layers in which the transport occurs, was developed and coupled with PM2.5-bound concentrations of water soluble ions (nss- SO4−2 (non-sea salt sulfates), NO3−, Cl−, NH4+, Na+, Mg+2, Ca+2 and K+) for the years 2017–2018, derived by the Auchencorth Moss supersite in Southeast Scotland, United Kingdom (UK). The 3D-CWT model was implemented in two distinct altitudinal layers above ground level (0 m ≤ Layer 1