Isotopic Composition and Fractionation of Mercury in Great Lakes Precipitation and Ambient Air

Abstract

Atmospheric deposition is a primary pathway by which mercury (Hg) enters terrestrial and aquatic ecosystems; however, the chemical and meteorological processes that Hg undergoes from emission to deposition are not well understood. Hg stable isotope geochemistry is a growing field used to better understand Hg biogeochemical cycling. To examine the atmospheric Hg isotopic composition in the Great Lakes, precipitation and ambient vapor-phase Hg samples were collected in Chicago, IL, Holland, MI, and Dexter, MI, between April 2007 and September 2009. Precipitation samples were characterized by negative mass-dependent fractionation (MDF) (δ(202)Hg = -0.79‰ to 0.18‰), while most vapor-phase samples displayed positive MDF (δ(202)Hg = -0.59‰ to 0.43‰). Positive mass-independent fractionation (MIF) (Δ(199)Hg = 0.04‰ to 0.52‰) was observed in precipitation, whereas MIF was slightly negative in vapor-phase samples (Δ(199)Hg = -0.21‰ to 0.06‰). Significant positive MIF of (200)Hg up to 0.25‰ was also measured in precipitation. Such MIF of an even-mass Hg isotope has not been previously reported in natural samples. These results contrast with recent predictions of the isotopic composition of atmospheric Hg and suggest that, in addition to aqueous photoreduction, other atmospheric redox reactions and source-related processes may contribute to isotopic fractionation of atmospheric Hg.

Historical (1850–2010) mercury stable isotope inventory from anthropogenic sources to the atmosphere

“…Further experimental and observational work on Hg isotope MDF during coal combustion, metal refining and liquid Hg 0 use is needed to confirm the presented conceptual models. High-precision Hg isotope measurement of total gaseous Hg (TGM, GEM+GOM) and precipitation Hg has been reported for several N-American and European regions (Chen et al, 2012;Demers et al, 2013Demers et al, , 2015Donovan et al, 2013;Fu et al, 2014;Gratz et al, 2010;Sherman et al, 2010Sherman et al, , 2012Sherman et al, , 2015Wang et al, 2015). Global TGM is enriched in the heavy isotopes (d 202 Hg=0.51 ± 0.41 ‰, 1SD) (Demers et al, 2013(Demers et al, , 2015Fu et al, 2014;Gratz et al, 2010;Sherman et al, 2010) compared to our model estimated GEM emissions (Table 1).…”

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

“…Bash et al (2014) found that including incloud aqueous photoreduction via organic acids based on the mechanism proposed by Si and Ariya (2008) improved the simulation of Hg wet deposition in their regional air quality model. Observed photochemically driven shifts in the abundance of naturally occurring Hg isotopes in the atmosphere and precipitation support the occurrence of aqueous-phase photoreduction involving Hg II -organic complexes (Gratz et al, 2010;Sonke, 2011;Sonke et al, 2015).…”

Sedimentary records of mercury stable isotopes in Lake Michigan

“…Precipitation scavenging oxidized Hg species (e.g., Hg II g and Hg p ) in the atmosphere is most likely the primary carrier for atmospheric Hg signals in offshore lake sediments (Shanley et al, 2015). Previous studies have reported mean d Hg (mean: ∼0.25‰)in precipitation collected in the Great Lakes region (Gratz et al, 2010;Chen et al, 2012;Demers et al, 2013;Sherman et al, 2011Sherman et al, , 2015. Atmospheric Hg(II) is susceptible to photoreduction in precipitation water droplets (Gratz et al, 2010), which may at least partially explain the positive pre-industrial D 199 Hg in deep layer sediments of MI-112 and other coastal cores.…”

“…202 Hg and D 199 Hg values have been reported in environmentally relevant matrices such as rocks (Smith et al, 2008), mineral deposits (Hintelmann and Lu, 2003;Smith et al, 2005Smith et al, , 2008Stetson et al, 2009;Sonke et al, 2010), coals (Biswas et al, 2008;Lefticariu et al, 2011;Sun et al, 2014;Yin et al, 2014b), soils (Biswas et al, 2008;Zhang et al, 2013;Feng et al, 2013), sediments (Gehrke et al, 2011;Foucher and Hintelmann, 2009;Feng et al, 2010;Liu et al, 2011;Bartov et al, 2013;Donovan et al, 2013;Ma et al, 2013;Yin et al, 2013aYin et al, , 2015, water (Štrok et al, 2014), air (Demers et al, 2013;Rolison et al, 2013), precipitation (Gratz et al, 2010;Chen et al, 2012;Demers et al, 2013;Sherman et al, 2011Sherman et al, , 2015 and biological samples Blum et al, 2013;Demers et al, 2013;Yin et al, 2013b). With well-defined source signatures, researchers may estimate the relative contribution of sources of Hg through the utilization of binary and/or triple mixing models (Foucher and Hintelmann, 2009;Feng et al, 2010;Liu et al, 2011;Bartov et al, 2013;Donovan et al, 2013;Ma et al, 2013;…”