Assessment of multi-year mercury concentration measurements over the South African highveld: a case study of 2016–2023 selected dataset

Mercury (Hg) is a pollutant that adversely affects all regions worldwide. This study investigates how Gaseous Elemental Mercury (GEM), the predominant form of Hg, has changed over the Southern African Highveld Priority Area (SAHPA) since 2009. The SAHPA is one of the regions of interest regarding South African air quality, given its frequent and recurrent occurrence of pollutant concentrations exceeding the country's prescribed thresholds. The present study measured GEM concentrations and other South African criteria pollutants and meteorological parameters between 2016 and 2023 at Elandsfontein, an industrially/anthropogenically influenced monitoring site on the SAHPA. Following quality control procedures, major jumps and shifts were noticed in some of the data for 2016 and 2019, as well as all of the 2017 and 2018 data, these data were excluded from the analysis. The mean hourly GEM concentrations during the monitoring period at Elandsfontein ranged from 0.10 to 71.22 ng/m³ with a mean concentration of 1.74 ± 1.62 ng/m³. The observed mean GEM concentrations exhibit characteristic seasonal patterns, with the highest levels in winter (1.94 ± 2.00 ng/m³), spring (1.83 ± 1.85 ng/m³), autumn (1.79 ± 1.1.55 ng/m³), and summer (1.48 ± 1.08 ng/m³). The diurnal variation of GEM at Elandsfontein shows a distinct pattern, with concentrations peaking before sunrise and dipping in the late afternoon. High GEM concentrations (>6 ng/m³) are linked to air masses from the western interior, indicating significant local emissions. Pollution events peak between July and October 2016–2023, likely due to increased domestic fuel combustion and biomass burning during the dry season, contributing to elevated atmospheric pollutant levels. An Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) analysis showed that eastern air masses were predominantly ‘clean’ and ocean-derived, emphasising the significant influence of local and regional emissions of GEM surrounding the site. These findings highlight the complex interaction of meteorology and pollutant emissions in shaping GEM levels. Further research is needed to understand better how GEM variations are influenced by interactions with other pollutants, meteorology, halogens, and ozone, particularly in the southern hemisphere.