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. 2018 Nov 15;9(1):4796.
doi: 10.1038/s41467-018-07075-3.

Photoreduction of gaseous oxidized mercury changes global atmospheric mercury speciation, transport and deposition

Alfonso Saiz-Lopez  1 , Sebastian P Sitkiewicz  2   3 , Daniel Roca-Sanjuán  3 , Josep M Oliva-Enrich  4 , Juan Z Dávalos  4 , Rafael Notario  4 , Martin Jiskra  5 , Yang Xu  5 , Feiyue Wang  6 , Colin P Thackray  7 , Elsie M Sunderland  7 , Daniel J Jacob  7 , Oleg Travnikov  8 , Carlos A Cuevas  4 , A Ulises Acuña  4 , Daniel Rivero  4 , John M C Plane  9 , Douglas E Kinnison  10 , Jeroen E Sonke  5
Affiliations

Photoreduction of gaseous oxidized mercury changes global atmospheric mercury speciation, transport and deposition

Alfonso Saiz-Lopez et al. Nat Commun. .

Erratum in

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.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1
Current understanding of the formation of oxidized Hg(II) compounds from atmospheric gaseous elemental mercury initiated by different oxidant species. This figure also includes other secondary oxidation mechanisms involving single-step reactions with Cl2, O3, BrO, and ClO
Fig. 2
Fig. 2
Ball-and-stick representation and computed UV-VIS absorption spectra and cross sections (σ, cm2) of the Hg(II) compounds studied in the present work. The light-coloured areas correspond to the uncertainty of the cross section due to the statistical sampling. Note the different range of σ values for some of the spectra. Also note that only wavelengths >290 nm are relevant for ambient tropospheric conditions
Fig. 3
Fig. 3
Calculated and experimental cross section of gas-phase HgBr2. The calculated spectrum was obtained with the CASSCF/MS–CASPT2/SO RASSI methodological approach using the ANO-RCC-VTZP basis set. The light coloured areas correspond to the numerical error of absorption cross sections due to the statistical sampling
Fig. 4
Fig. 4
Annually- and globally-averaged photolysis rate (s−1) and lifetime (h) of Hg(II) compounds in the troposphere. Error bars correspond to one standard deviation
Fig. 5
Fig. 5
Global budget of Hg chemical cycling covering the troposphere and lower stratosphere (up to ca. 30 kms) for different tests in GLEMOS: a—Run #1; b—Run #2; c—Run #3; d—Run #4. The mass estimates are in Mg, the fluxes are in Mg a−1
Fig. 6
Fig. 6
Spatial distribution of Hg(0) surface concentration for different atmospheric Hg(II) reduction simulations in the GLEMOS model: a Run #1; b Run #2; c Run #3; d Run #4. Circles show observed values in the same colour scale. The measurement dataset is the same as in ref.
Fig. 7
Fig. 7
Comparison between simulations from the GLEMOS model with measurements for the year 2013: a Hg(0) air concentration; b Hg(II) wet deposition. The measurement dataset is the same as in ref.
Fig. 8
Fig. 8
Spatial distribution of total Hg (i.e. Hg(0) + Hg(II)) deposition for different tests in GLEMOS: a—Run #1; b—Run #2; c—Run #3; d—Run #4
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