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

Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg0. Oxidation to water-soluble HgII controls Hg deposition to ecosystems. Here we implement a new mechanism for atmospheric Hg0 / HgII 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 Hg0 / HgII cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg0 oxidant, and that second-stage HgBr oxidation is mainly by the NO2 and HO2 radicals. The resulting lifetime of tropospheric Hg0 against oxidation is 2.7 months, shorter than in previous models. Fast HgII 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 ≡ Hg0 + HgII(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase HgII-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 mid-latitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but does not reproduce the lack of seasonality observed at southern hemisphere marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM-ozone relationship indicative of fast Hg0 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, including the maximum over the US Gulf Coast driven by HgBr oxidation by NO2 and HO2. Low Hg wet deposition observed over rural China is attributed to fast HgII reduction in the presence of high organic aerosol concentrations. We find that 80 % of global HgII deposition takes place over the oceans, reflecting the marine origin of Br and low concentrations of marine organics for HgII reduction, and most of HO2 and NO2 for second-stage HgBr oxidation.

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

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-6353-2017 ◽

2017 ◽

Vol 17 (10) ◽

pp. 6353-6371 ◽

Cited By ~ 126

Author(s):

Hannah M. Horowitz ◽

Daniel J. Jacob ◽

Yanxu Zhang ◽

Theodore S. Dibble ◽

Franz Slemr ◽

...

Keyword(s):

Wet Deposition ◽

General Circulation ◽

Rural China ◽

Deep Convection ◽

Circulation Model ◽

Redox Chemistry ◽

Ocean General Circulation Model ◽

Atmospheric Mercury ◽

Water Soluble ◽

Second Stage

Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg0. Oxidation to water-soluble HgII plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg0 ∕ HgII 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 Hg0 ∕ HgII cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg0 oxidant and that second-stage HgBr oxidation is mainly by the NO2 and HO2 radicals. The resulting chemical lifetime of tropospheric Hg0 against oxidation is 2.7 months, shorter than in previous models. Fast HgII 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  ≡  Hg0 + HgII(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase HgII–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 Hg0 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 NO2 and HO2 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 HgII reduction in the presence of high organic aerosol concentrations. We find that 80 % of HgII deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for HgII reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO2 and NO2 for second-stage HgBr oxidation.

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Climate variability of the mid- and high-latitudes of the Southern Hemisphere in ensemble simulations from 1500 to 2000 AD

Climate of the Past ◽

10.5194/cp-8-373-2012 ◽

2012 ◽

Vol 8 (1) ◽

pp. 373-390 ◽

Cited By ~ 15

Author(s):

S. B. Wilmes ◽

C. C. Raible ◽

T. F. Stocker

Keyword(s):

Climate Variability ◽

Southern Hemisphere ◽

General Circulation ◽

Circulation Model ◽

Volcanic Eruptions ◽

Ocean General Circulation Model ◽

Southern Annular Mode ◽

Significant Shift ◽

External Forcing ◽

Proxy Records

Abstract. To increase the sparse knowledge of long-term Southern Hemisphere (SH) climate variability, we assess an ensemble of 4 transient simulations over the last 500 yr performed with a state-of-the-art atmosphere ocean general circulation model. The model is forced with reconstructions of solar irradiance, greenhouse gas (GHG) and volcanic aerosol concentrations. A 1990 control simulation shows that the model is able to represent the Southern Annular Mode (SAM), and to some extent the South Pacific Dipole (SPD) and the Zonal Wave 3 (ZW3). During the past 500 yr we find that SPD and ZW3 variability remain stable, whereas SAM shows a significant shift towards its positive state during the 20th century. Regional temperatures over South America are strongly influenced by changing both GHG concentrations and volcanic eruptions, whereas precipitation shows no significant response to the varying external forcing. For temperature this stands in contrast to proxy records, suggesting that SH climate is dominated by internal variability rather than external forcing. The underlying dynamics of the temperature changes generally point to a combination of several modes, thus, hampering the possibilities of regional reconstructing the modes from proxy records. The linear imprint of the external forcing is as expected, i.e. a warming for increase in the combined solar and GHG forcing and a cooling after volcanic eruptions. Dynamically, only the increase in SAM with increased combined forcing is simulated.

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Evaluation of PMIP2 and PMIP3 simulations of mid-Holocene climate in the Indo-Pacific, Australasian and Southern Ocean regions

Climate of the Past ◽

10.5194/cp-13-1661-2017 ◽

2017 ◽

Vol 13 (11) ◽

pp. 1661-1684 ◽

Cited By ~ 2

Author(s):

Duncan Ackerley ◽

Jessica Reeves ◽

Cameron Barr ◽

Helen Bostock ◽

Kathryn Fitzsimmons ◽

...

Keyword(s):

Southern Hemisphere ◽

General Circulation ◽

Ice Core ◽

Circulation Model ◽

Ocean General Circulation Model ◽

Warm Pool ◽

Primary Objective ◽

Convective Precipitation ◽

Proxy Records ◽

Proxy Reconstructions

Abstract. This study uses the simplified patterns of temperature and effective precipitation approach from the Australian component of the international palaeoclimate synthesis effort (INTegration of Ice core, MArine and TErrestrial records – OZ-INTIMATE) to compare atmosphere–ocean general circulation model (AOGCM) simulations and proxy reconstructions. The approach is used in order to identify important properties (e.g. circulation and precipitation) of past climatic states from the models and proxies, which is a primary objective of the Southern Hemisphere Assessment of PalaeoEnvironment (SHAPE) initiative. The AOGCM data are taken from the Paleoclimate Modelling Intercomparison Project (PMIP) mid-Holocene (ca. 6000 years before present, 6 ka) and pre-industrial control (ca. 1750 CE, 0 ka) experiments. The synthesis presented here shows that the models and proxies agree on the differences in climate state for 6 ka relative to 0 ka, when they are insolation driven. The largest uncertainty between the models and the proxies occurs over the Indo-Pacific Warm Pool (IPWP). The analysis shows that the lower temperatures in the Pacific at around 6 ka in the models may be the result of an enhancement of an existing systematic error. It is therefore difficult to decipher which one of the proxies and/or the models is correct. This study also shows that a reduction in the Equator-to-pole temperature difference in the Southern Hemisphere causes the mid-latitude westerly wind strength to reduce in the models; however, the simulated rainfall actually increases over the southern temperate zone of Australia as a result of higher convective precipitation. Such a mechanism (increased convection) may be useful for resolving disparities between different regional proxy records and model simulations. Finally, after assessing the available datasets (model and proxy), opportunities for better model–proxy integrated research are discussed.

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Climate variability of the mid- and high-latitudes of the Southern Hemisphere in ensemble simulations from 1500 to 2000 AD

Climate of the Past Discussions ◽

10.5194/cpd-7-3091-2011 ◽

2011 ◽

Vol 7 (5) ◽

pp. 3091-3129 ◽

Cited By ~ 1

Author(s):

S. B. Wilmes ◽

C. C. Raible ◽

T. F. Stocker

Keyword(s):

Climate Variability ◽

Southern Hemisphere ◽

General Circulation ◽

Circulation Model ◽

Volcanic Eruptions ◽

Ocean General Circulation Model ◽

Southern Annular Mode ◽

Significant Shift ◽

External Forcing ◽

Proxy Records

Abstract. To increase the sparse knowledge of long-term Southern Hemisphere (SH) climate variability we assess an ensemble of 4 transient simulations over the last 500 yr performed with a state-of-the-art atmosphere ocean general circulation model. The model is forced with reconstructions of solar irradiance, greenhouse gas (GHG) and volcanic aerosol concentrations. A 1990 control simulation shows that the model is able to represent the Southern Annular Mode (SAM), and to some extent the South Pacific Dipole (SPD) and the Zonal Wave 3 (ZW3). During the past 500 yr we find that SPD and ZW3 variability remain stable, whereas SAM shows a significant shift towards its positive state during the 20th century. Regional temperatures over South America are strongly influenced by changing both GHG concentrations and volcanic eruptions whereas precipitation shows no significant response to the varying external forcing. For temperature this stands in contrast to proxy records suggesting that SH climate is dominated by internal variability rather than external forcing. The underlying dynamics of the temperature changes generally point to a combination of several modes, thus, hampering the possibilities regional reconstructions of the modes from proxy records. The linear imprint of the external forcing is as expected, i.e. a warming for increase in the combined solar and GHG forcing and a cooling after volcanic eruptions. Dynamically only the increase in SAM with increased combined forcing is simulated.

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Transport of 137Cs to the Southern Hemisphere in an ocean general circulation model

Progress In Oceanography ◽

10.1016/j.pocean.2010.12.006 ◽

2011 ◽

Vol 89 (1-4) ◽

pp. 38-48 ◽

Cited By ~ 31

Author(s):

Daisuke Tsumune ◽

Michio Aoyama ◽

Katsumi Hirose ◽

Frank O. Bryan ◽

Keith Lindsay ◽

...

Keyword(s):

Southern Hemisphere ◽

General Circulation Model ◽

General Circulation ◽

Circulation Model ◽

Ocean General Circulation Model ◽

Ocean General Circulation

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Performance evaluation for an ocean general circulation model: vectorization andmultitasking

Proceedings of the 2nd international conference on Supercomputing - ICS '88 ◽

10.1145/55364.55393 ◽

1988 ◽

Cited By ~ 1

Author(s):

P. Andrich ◽

G. Madec

Keyword(s):

Performance Evaluation ◽

General Circulation Model ◽

General Circulation ◽

Circulation Model ◽

Ocean General Circulation Model ◽

Ocean General Circulation

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The South China Sea Throughflow Retrieved from Climatological Data*

Journal of Physical Oceanography ◽

10.1175/2008jpo3955.1 ◽

2009 ◽

Vol 39 (3) ◽

pp. 753-767 ◽

Cited By ~ 40

Author(s):

Max Yaremchuk ◽

Julian McCreary ◽

Zuojun Yu ◽

Ryo Furue

Keyword(s):

South China Sea ◽

General Circulation Model ◽

South China ◽

General Circulation ◽

Circulation Model ◽

Ocean General Circulation Model ◽

Luzon Strait ◽

The South China Sea ◽

China Sea ◽

Ocean General Circulation

Abstract The salinity distribution in the South China Sea (SCS) has a pronounced subsurface maximum from 150–220 m throughout the year. This feature can only be maintained by the existence of a mean flow through the SCS, consisting of a net inflow of salty North Pacific tropical water through the Luzon Strait and outflow through the Mindoro, Karimata, and Taiwan Straits. Using an inverse modeling approach, the authors show that the magnitude and space–time variations of the SCS thermohaline structure, particularly for the salinity maximum, allow a quantitative estimate of the SCS throughflow and its distribution among the three outflow straits. Results from the inversion are compared with available observations and output from a 50-yr simulation of a highly resolved ocean general circulation model. The annual-mean Luzon Strait transport is found to be 2.4 ± 0.6 Sv (Sv ≡ 106 m3 s−1). This inflow is balanced by the outflows from the Karimata (0.3 ± 0.5 Sv), Mindoro (1.5 ± 0.4), and Taiwan (0.6 ± 0.5 Sv) Straits. Results of the inversion suggest that the Karimata transport tends to be overestimated in numerical models. The Mindoro Strait provides the only passage from the SCS deeper than 100 m, and half of the SCS throughflow (1.2 ± 0.3 Sv) exits the basin below 100 m in the Mindoro Strait, a result that is consistent with a climatological run of a 0.1° global ocean general circulation model.

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