Abstract
Anthropogenic mercury (Hg(0)) emissions oxidize to gaseous Hg(II) compounds, before deposition to Earth surface ecosystems. Atmospheric reduction of Hg(II) competes with deposition, thereby modifying the magnitude and pattern of Hg deposition. Global Hg models have postulated that Hg(II) reduction in the atmosphere occurs through aqueous-phase photoreduction that may take place in clouds. Here we report that experimental rainfall Hg(II) photoreduction rates are much slower than modelled rates. We compute absorption cross sections of Hg(II) compounds and show that fast gas-phase Hg(II) photolysis can dominate atmospheric mercury reduction and lead to a substantial increase in the modelled, global atmospheric Hg lifetime by a factor two. Models with Hg(II) photolysis show enhanced Hg(0) deposition to land, which may prolong recovery of aquatic ecosystems long after Hg emissions are lowered, due to the longer residence time of Hg in soils compared with the ocean. Fast Hg(II) photolysis substantially changes atmospheric Hg dynamics and requires further assessment at regional and local scales.
Highlights
Anthropogenic mercury (Hg(0)) emissions oxidize to gaseous Hg(II) compounds, before deposition to Earth surface ecosystems
The absorption cross sections of syn-HgBrONO, HgBrOOH, HgBrOH, HgBr2, HgBrOCl, and HgBrOBr were implemented into the GEOS-Chem[6] and GLEMOS33,34 global Hg chemistry and transport models (Methods), since these Hg(II) species are the most likely to be formed in the atmosphere[6,11,27]
No Hg(II) reduction Hg(II) reduction in aqueous phase using the experimentally derived rate constant (0.15 h−1) in this study Gas phase Hg(II) photoreduction to Hg(0) Gas phase Hg(II) photoreduction to HgBr not include the highly uncertain reduction reaction[6] HgBr + NO2 → Hg(0) + BrNO2. Omitting this reaction in GEOS-Chem lowers the Hg lifetime from 13 to 8 months in model Run#4 (The different model simulated scenarios for atmospheric Hg(II) reduction are shown in Table 1). syn-HgBrONO and HgBrOOH generally dominate the production of Hg(II) in both models (Supplementary Figure 3), whilst HgBr2 becomes the prevalent Hg(II) species in the troposphere (Supplementary Figure 4) due to its longer lifetime against photolysis
Summary
Anthropogenic mercury (Hg(0)) emissions oxidize to gaseous Hg(II) compounds, before deposition to Earth surface ecosystems. Global Hg models have postulated that Hg(II) reduction in the atmosphere occurs through aqueous-phase photoreduction that may take place in clouds. We compute absorption cross sections of Hg(II) compounds and show that fast gas-phase Hg(II) photolysis can dominate atmospheric mercury reduction and lead to a substantial increase in the modelled, global atmospheric Hg lifetime by a factor two. Models with Hg(II) photolysis show enhanced Hg (0) deposition to land, which may prolong recovery of aquatic ecosystems long after Hg emissions are lowered, due to the longer residence time of Hg in soils compared with the ocean. The long lifetime of Hg(0) leads to Hg deposition far from its emission sources to remote ecosystems, including the open oceans and polar regions. The dominant reaction to produce gaseous oxidized Hg(II)
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