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.

Keywords: atmospheric Hg; mass-independent fractionation (MIF); polar vortex; troposphere and stratosphere; wet deposition.