Atmospheric mercury speciation at the Monte Curcio GAW/GMOS high-altitude observatory: A multi-year (2015-2023) assessment of long-range transport and regional dynamics in the Central Mediterranean basin.

High-altitude stations provide unique insights into background chemistry, although decoupling long-range transport from local mountain dynamics remains challenging. This study presents a comprehensive, high-resolution, multi-year (2015-2023) record of TGM/GEM, GOM, and PBM at the Monte Curcio observatory. The station's strategic position is highly significant as it is located within the Mediterranean basin, a well-recognized climate and mercury hotspot, allowing for the observation of the free troposphere and regional boundary layer dynamics. Atmospheric transport and chemical dynamics were thoroughly evaluated in terms of both seasonality and diurnal cycles. To identify the primary air mass transport pathways, cluster analysis combined with Concentration-Weighted Trajectory (CWT) modeling successfully pinpointed transboundary source regions. TGM/GEM concentrations remained stable around a background mean of 1.3±0.2ng m-3, peaking during winter (1.43±0.24ng m-3) due to a significant continental footprint from Central-Eastern Europe driven by cold-season boundary layer compression trapping surface emissions. A similar behavior was observed for PBM, whose CWT maps displayed a massive winter continental hotspot. Conversely, despite acknowledged measurement uncertainties, GOM levels peaked during spring and summer, with CWT maps revealing a pristine marine signature driven by active free-tropospheric photochemical oxidation. Multi-parameter seasonal correlations revealed a robust coupling between GOM and PBM (Rs=0.52-0.76), indicating a strong dynamic gas-particle partitioning equilibrium. In summer, intense solar radiation sustained a persistent noontime GEM surface re-emission while concurrently accelerating the photochemical depletion of GEM via oxidation into GOM and PBM, whose diurnal peaks were registered in the late afternoon. This enhanced oxidative capacity during the warm season was evidenced by a positive Spearman correlation between GOM and ozone (Rs=0.28) alongside a strong negative correlation with relative humidity (Rs=-0.43). Concurrently, positive correlations with the mixing ratio during warmer seasons provided evidence of a convective planetary boundary layer transporting moisture and mercury-enriched air masses from lower elevations up to the station altitude. While sub-period trend analysis revealed localized decreases for TGM/GEM during the initial years (2015-2018), a significant overall increasing trend of 0.02ng m-3yr-1 was detected across the full 2015-2023 record. This multi-year upward trend might suggest a growing relative contribution from natural evasion and legacy emissions overriding regional anthropogenic mitigation efforts, by effectively filling a critical knowledge gap in the central Mediterranean basin.