Event-Based Atmospheric Precipitation Uncovers Significant Even and Odd Hg Isotope Anomalies Associated with the Circumpolar Vortex.

Abstract

The determination of the mass-independent fractionation of even Hg isotopes (even-MIF, Δ200Hg) in atmospheric samples adds another intriguing feature to the Hg isotope system. Despite our lack of sufficient experimental verification and the momentary absence of a valid mechanism to explain its occurrence, even-MIF could be instrumental in understanding the cycle and deposition of atmospheric Hg. In contrast to slightly positive Δ200Hg values (<0.30‰) frequently observed in most atmospheric samples, large Δ200Hg values (up to 1.24‰) determined in precipitation from Peterborough (Ontario, Canada) stand out and could provide valuable information for the origin of the even-MIF mystery. We now report a systematic analysis of high-resolution rainfall and snowfall samples collected in winter during cold weather at Peterborough, Canada. Dissolved and particulate Hg both displayed large variations of odd-MIF (from -0.93‰ to 2.02‰ for Δ199Hg), which may result from long-range transportation, as the negative odd-MIF in particulate Hg is likely a result of long-distance transport of arctic atmospheric Hg(II). Dissolved Hg revealed significant even-MIF values (from 0.25‰ to 1.19‰ for Δ200Hg) and a negative relationship between Δ200Hg and Δ204Hg, which provide further evidence for the previously proposed conceptual model of Δ200Hg. Disconnected odd-MIF and even-MIF trends were detected in sequentially collected precipitation samples, which further suggests different sources or mechanisms for Δ199Hg and Δ200Hg. Particularly, the high Δ200Hg values highlight the transport of stratospheric Hg through a polar vortex to the sampling region, stimulating further systematic investigation. The new Δ200Hg data for particulate Hg add to existing information on atmospheric Hg(II) worldwide, suggesting a global distribution of Hg characterized by even-MIF in the atmosphere, and further constrain the model of atmospheric Hg deposition.