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

The determination of the mass-independent fractionation of even Hg isotopes (even-MIF, ΔHg) 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 ΔHg values (<0.30‰) frequently observed in most atmospheric samples, large ΔHg 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 ΔHg), 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 ΔHg) and a negative relationship between ΔHg and ΔHg, which provide further evidence for the previously proposed conceptual model of ΔHg. Disconnected odd-MIF and even-MIF trends were detected in sequentially collected precipitation samples, which further suggests different sources or mechanisms for ΔHg and ΔHg. Particularly, the high ΔHg values highlight the transport of stratospheric Hg through a polar vortex to the sampling region, stimulating further systematic investigation. The new ΔHg 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.