Statistical assessment of an atmospheric mercury passive sampler at a regional site in South Africa

Abstract

South Africa has been ranked among the top ten mercury (Hg) emitters globally, with emissions from coal-fired power plants being the most significant contributor. The expansion of atmospheric Hg measurement networks in southern Africa is vital within the global context but is constrained by high costs and logistics. Passive air samplers were developed to address these issues and expand atmospheric monitoring networks. A commercially available passive sampler widely used for atmospheric Hg monitoring is the Mercury Passive Air Sampler (MerPAS®). Therefore, this study aimed to statistically evaluate the performance of these samplers in the unique South African environment by comparing Hg concentrations determined with MerPAS® with active in situ atmospheric Hg measurements conducted in this region. Measurements were conducted from June 2021 to September 2022 at the Welgegund atmospheric monitoring station, considered one of Africa's most comprehensively equipped atmospheric measurement sites. Hg concentrations measured with MerPAS® were derived for different sampling rates (SR), i.e. the original SR (OSR) provided by the supplier and an adjusted original SR (AOSR) derived using the OSR with adjustments for mean temperature and wind speed. Statistical analyses, including Kruskal–Wallis, Mann–Whitney U, Bland–Altman, and Spearman correlation tests, were used to assess the performance of MerPAS®. The OSR overestimated Hg concentrations by 16%, while the AOSR reduced this overestimation to 10%, improving alignment with active sampling data. The Mean Normalized Difference (MND) also decreased from 17.4% with OSR to 12.7% with AOSR, indicating greater accuracy. Spearman correlation analysis showed moderate agreement between passive and active sampling, with correlation coefficients of 0.39 for OSR and 0.58 for AOSR, supporting the improved comparability of AOSR. Seasonal patterns were consistent across both methods, with elevated Hg levels observed in winter due to atmospheric inversions and increased emissions. Despite a slight positive bias, the Bland–Altman analysis further confirmed good reliability between the AOSR and active measurements. This study demonstrates that MerPAS®, when calibrated for local environmental conditions, provides an accurate, cost-effective alternative for Hg monitoring, offering a feasible solution for expanding networks in regions with limited resources. By enabling broader and more accessible atmospheric Hg data collection, MerPAS® can support critical environmental policies, such as the Minamata Convention, and deepen scientific understanding of Hg dynamics in under-monitored areas like southern Africa. These findings lay the groundwork for enhancing global Hg monitoring, contributing essential insights into regional pollution and atmospheric processes across diverse environments.