A new mechanism for atmospheric mercury redox chemistry: Implications for the global mercury budget
Other literature type, Article
Horowitz, Hannah M.
Jacob, Daniel J.
Dibble, Theodore S.
Amos, Helen M.
Schmidt, Johan A.
Corbitt, Elizabeth S.
Marais, Eloïse A.
Sunderland, Elsie M.
- Publisher: Copernicus Publications
(issn: 1680-7324, eissn: 1680-7324)
Chemistry | QD1-999 | Physics | QC1-999
Mercury (Hg) is emitted to the atmosphere mainly as
volatile elemental Hg<sup>0</sup>. Oxidation to water-soluble Hg<sup>II</sup> plays a
major role in Hg deposition to ecosystems. Here, we implement a new
mechanism for atmospheric Hg<sup>0</sup> ∕ Hg<sup>II</sup> redox chemistry in the
GEOS-Chem global model and examine the implications for the global
atmospheric Hg budget and deposition patterns. Our simulation includes a new
coupling of GEOS-Chem to an ocean general circulation model (MITgcm),
enabling a global 3-D representation of atmosphere–ocean Hg<sup>0</sup> ∕ Hg<sup>II</sup>
cycling. We find that atomic bromine (Br) of marine organobromine origin is
the main atmospheric Hg<sup>0</sup> oxidant and that second-stage HgBr oxidation
is mainly by the NO<sub>2</sub> and HO<sub>2</sub> radicals. The resulting chemical
lifetime of tropospheric Hg<sup>0</sup> against oxidation is 2.7 months, shorter
than in previous models. Fast Hg<sup>II</sup> atmospheric reduction must occur
in order to match the ∼ 6-month lifetime of Hg against
deposition implied by the observed atmospheric variability of total gaseous
mercury (TGM ≡ Hg<sup>0</sup> + Hg<sup>II</sup>(g)). We implement this reduction
in GEOS-Chem as photolysis of aqueous-phase Hg<sup>II</sup>–organic complexes in
aerosols and clouds, resulting in a TGM lifetime of 5.2 months against
deposition and matching both mean observed TGM and its variability. Model
sensitivity analysis shows that the interhemispheric gradient of TGM,
previously used to infer a longer Hg lifetime against deposition, is
misleading because Southern Hemisphere Hg mainly originates from oceanic
emissions rather than transport from the Northern Hemisphere. The model
reproduces the observed seasonal TGM variation at northern midlatitudes
(maximum in February, minimum in September) driven by chemistry and oceanic
evasion, but it does not reproduce the lack of seasonality observed at southern
hemispheric marine sites. Aircraft observations in the lowermost
stratosphere show a strong TGM–ozone relationship indicative of fast
Hg<sup>0</sup> oxidation, but we show that this relationship provides only a weak
test of Hg chemistry because it is also influenced by mixing. The model
reproduces observed Hg wet deposition fluxes over North America, Europe, and
China with little bias (0–30 %). It reproduces qualitatively the observed
maximum in US deposition around the Gulf of Mexico, reflecting a combination
of deep convection and availability of NO<sub>2</sub> and HO<sub>2</sub> radicals for
second-stage HgBr oxidation. However, the magnitude of this maximum is
underestimated. The relatively low observed Hg wet deposition over rural
China is attributed to fast Hg<sup>II</sup> reduction in the presence of high
organic aerosol concentrations. We find that 80 % of Hg<sup>II</sup> deposition
is to the global oceans, reflecting the marine origin of Br and low
concentrations of organic aerosols for Hg<sup>II</sup> reduction. Most of that
deposition takes place to the tropical oceans due to the availability of
HO<sub>2</sub> and NO<sub>2</sub> for second-stage HgBr oxidation.