Mass independent fractionation of even and odd mercury isotopes during free tropospheric mercury oxidation

Abstract

<p>Even mercury (Hg) isotope mass independent fractionation (MIF), observed in rainfall globally, is used to quantify atmospheric Hg deposition pathways. The underlying reaction and MIF mechanism are unknown however. Here we investigate the Hg isotope composition of free tropospheric gaseous elemental Hg0 and HgII forms. We find that gaseous oxidized HgII has positive Δ199Hg, Δ201Hg, Δ200Hg, and negative Δ204Hg signature, similar to rainfall HgII, and we document rainfall HgII Δ196Hg to be near-zero. Cloud water and rainfall HgII show enhanced odd MIF compared to gaseous HgII, indicating in-cloud HgII photoreduction. Hg MIF observations of free tropospheric Hg0 dynamics show how net Hg0 oxidation leads to opposite MIF in Hg0 and HgII. A Δ200Hg mass balance for Hg0 and HgII forms suggests that measurements and models underestimate the tropospheric HgII pool.</p>

Mercury stable isotope composition of seawater suggests important net gaseous elemental mercury uptake

“…Modern, northern hemispheric Hg(II) wet deposition is characterized by a positive  200 Hg of 0.16 ‰ (0.11 ‰ to 0.22 ‰; median, IQR, n = 106), for background sites with Hg concentrations < 25 ng L -1 ; Table S1). A new study shows that gaseous atmospheric Hg(II) forms, that are the precursor to Hg(II) in wet deposition, have the same  200 Hg as Hg(II) wet deposition in the lower free and middle troposphere 27 . A recent modeling study evaluated the origin of global Hg(II) wet and dry deposition 37 , and found that Hg(II) sourced in the upper and middle troposphere constitutes 91% of the annual global Hg(II) wet deposition flux.…”

“…Atmospheric Hg(0) and Hg(II) in rainfall have different mass-dependent (MDF, 202 Hg) and mas-independent fractionation (MIF,  199 Hg and  200 Hg) signatures. Even-Hg MIF ( 200 Hg) is thought to be generated exclusively by upper tropospheric photochemical reactions 26,27 , and no Hg transformations at the Earth's surface have been shown to fractionate  200 Hg.  200 Hg is therefore considered as a conservative tracer for atmospheric Hg deposition pathways 24 .…”