Statistical assessment of atmospheric mercury passive samplers

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 sampling conducted in this region. Measurements were conducted 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, an adjusted original SR (AOSR), a recalibrated SR (RSR) derived from active in situ measurements, and an adjusted recalibrated SR (ARSR). Hg levels calculated utilising the OSR and AOSR were generally higher than the active in situ measurements, while Hg concentrations determined with the RSR and ARSR were lower and closer to the atmospheric Hg levels determined with the active in situ samplers. ANOVA, t-test, linear regression and Bland-Altman analysis revealed, in general, good agreement between active in situ measurements and Hg levels derived from MerPAS® utilising the different SRs. Overall, Hg concentrations derived from the recalibrated SRs indicated better comparisons between active in situ measurements and MerPAS®. However, considering that these samplers were developed as a low-cost alternative to be deployed at sites that are logistically difficult to access, it is recommended that the AOSR be used at sites in southern Africa where MerPAS® is deployed. Similar seasonal trends were observed for atmospheric Hg concentrations determined with MerPAS® utilising the AOSR and active in situ measurements, with the highest concentrations determined in winter. Finally, a brief assessment of a manual active sampler also revealed good agreement with active in situ measurements.