Atmospheric mercury (Hg) exists as gaseous elemental, oxidised, and particulate bound mercury (GEM, GOM, and PBM). Due to their ultra-trace concentration levels at environmentally relevant conditions (especially GOM and PBM), preconcentration is required in the sampling step. KCl-coated denuders are used as a standard method for sampling and preconcentration of ambient air GOM. Potential sampling losses of GOM during the analysis period could result in measurement biases leading to under/over-estimation of Hg species in the ambient air. In the present study, we evaluated the performance of KCl-coated denuders for better estimation of biases in real-time atmospheric Hg speciation measurements. Laboratory experiments were performed at ultra-trace level concentrations relevant to ambient air Hg concentrations using 197Hg radiotracer as a novel method. The efficiency of KCl denuder is debatable since HgII losses (cumulative, 0.5h × 4 intervals) were higher when exposed to ambient air, which leads to speciation biases (GOM biased low). Such losses were very low when exposed to synthetic air and N2 flow. Moreover, losses were different for different denuders and varied for the same denuders depending upon whether they were freshly prepared or not. Reusing KCl denuders for longer periods reduced their efficiency at retaining HgII. It was observed that losses from KCl denuders were in the form of Hg0. This implies that HgII loaded in the denuder is unstable when exposed to ambient air for longer period, and complex Hg transformations occur in the denuder after sampling under constant flow. In ambient air, HgII losses in the absence of UV light (dark condition) were only 5-7% compared to losses (upto 79%) in the presence of UV light. Obtained results indicate the importance of gas-phase photoreduction in sampling losses from KCl denuders observed in laboratory experiments.
Authors: Sreekanth Vijayakumaran Nair, Jan Gačnik, Igor Živković, Milena Horvat