Mercury in arctic snow: factors affecting the kinetics of photoreactions

Mann, Erin (2015) Mercury in arctic snow: factors affecting the kinetics of photoreactions. Masters thesis, Memorial University of Newfoundland.

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Abstract

Mercury (Hg) is a ubiquitous and toxic environmental contaminant. Hg is problematic in Arctic regions, where organisms accumulate high contaminant levels, and changing climate conditions may alter Hg dynamics in unpredictable ways. This thesis quantified the effects of ultraviolet radiation (UV; 280 – 400 nm) intensity, snow temperature, and chloride (Cl⁻) concentration on Hg photoreaction kinetics in Arctic snow, using controlled laboratory experiments and an Arctic field flux campaign. In the first study, photoreduced Hg amounts (Hg(II)red) were found to increase with UV intensity for snow from three Arctic sites, while photoreduction rate constants (k) had a parabolic relationship with UV. Photooxidized Hg amounts (Hg(0)ₒₓ) differed at these three Arctic sites, with the greatest photooxidation occurring at the site with the highest Cl⁻ concentration. The second laboratory experiments determined that increasing temperatures (-20 to -2°C) with constant UV irradiation does not significantly alter Hg photoreduction kinetics, until snow melts (above -5 °C). From a field flux study and generalised additive model analysis, both temperature and snow age have statistically significant influences on Hg(0) flux from snow. Cross correlation analysis found that changes in Hg(0) flux lag temperature by ~30 min, implying that temperature does not have an immediate effect on Hg(0) flux, and so likely does not directly affect Hg photoreaction kinetics. In contrast, Hg(0) flux lags UV radiation changes by only ~5 min, implying that UV changes result in near immediate Hg(0) flux changes, and is directly influencing Hg reaction kinetics. The third laboratory study concluded that increasing Cl⁻ decreases Hg(II)red in melted Arctic snow, while k increases. When both UV and Cl⁻ vary, it is changing Cl⁻ that controls k, but both are important for determining Hg(II)red. Overall, this work shows that greater emphasis should be placed on examining cryospheric Hg dynamics in the changing Arctic environment. Changing conditions will mean greater Cl⁻ loading to coastal snow, with an earlier onset of snowmelt (exposure to different intensities of UV pre and post melt), and higher temperatures earlier in the season, and these factors will affect the rate (k) and extent (Hg(II)red) of Hg photoreactions in Arctic snow.

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