Oxidized mercury (Hg(II)) is chemically produced in the atmosphere by
oxidation of elemental mercury and is directly emitted by anthropogenic
activities. We use the GEOS-Chem global chemical transport model with gaseous
oxidation driven by Br atoms to quantify how surface deposition of Hg(II) is
influenced by Hg(II) production at different atmospheric heights. We tag
Hg(II) chemically produced in the lower (surface–750 hPa), middle
(750–400 hPa), and upper troposphere (400 hPa–tropopause), in the
stratosphere, as well as directly emitted Hg(II). We evaluate our 2-year
simulation (2013–2014) against observations of Hg(II) wet deposition as well
as surface and free-tropospheric observations of Hg(II), finding reasonable
agreement. We find that Hg(II) produced in the upper and middle troposphere
constitutes 91 % of the tropospheric mass of Hg(II) and 91 % of the
annual Hg(II) wet deposition flux. This large global influence from the upper
and middle troposphere is the result of strong chemical production coupled
with a long lifetime of Hg(II) in these regions. Annually, 77–84 % of
surface-level Hg(II) over the western US, South America, South Africa, and
Australia is produced in the upper and middle troposphere, whereas
26–66 % of surface Hg(II) over the eastern US, Europe, and East Asia,
and South Asia is directly emitted. The influence of directly emitted Hg(II)
near emission sources is likely higher but cannot be quantified by our
coarse-resolution global model (2
Atmospheric deposition of mercury (Hg) is the main source of Hg to most
aquatic ecosystems. Methylmercury concentrations in fish in an ecosystem are
strongly linked to the local Hg deposition rate (Hammerschmidt and
Fitzgerald, 2006; Harris et al., 2007). Dry deposition and wet deposition are
both significant contributors to the global deposition flux of Hg (e.g.,
Bergan et al., 1999; Seigneur et al., 2001; Dastoor and Larocque, 2004; Jung
et al., 2009; Amos et al., 2012). Models suggest that the global dry
deposition fluxes of gaseous elemental mercury (Hg(0)) and oxidized mercury
in the gas and particle phases (Hg(II)) are comparable (Seigneur et al.,
2001; Amos et al., 2012). Wet deposition of Hg occurs almost entirely through
precipitation scavenging of Hg(II). Hg(II) is co-emitted with Hg(0) from
several anthropogenic sources, but the predominant source of Hg(II) in the
atmosphere is in situ oxidation of Hg(0) (Pirrone et al., 2010; Selin and
Jacob, 2008; Holmes et al., 2010). Br is likely the main oxidant of Hg(0)
(Ebinghaus et al., 2002; Laurier et al., 2003; Donohoue et al., 2006; Obrist
et al., 2011; Gratz et al., 2015), but the importance of O
Hg(II) concentrations in the planetary boundary layer are typically about
50 pg m
The influence of free-tropospheric Hg on deposition has been evaluated with regional and global chemical transport models. Using the global GEOS-Chem model, Selin and Jacob (2008) estimated that 59 % of the annual Hg(II) wet deposition over the US is from Hg(II) scavenged from altitudes above 850 hPa. In another study (Myers et al., 2013), Hg present at the upper boundary (5.4 km) of the regional CMAQ model was found to contribute about 40 % to dry deposition and about 80 % to wet deposition in July over the US. Coburn et al. (2016) estimated that most of the surface Hg(II) over Florida in April 2010 was produced above 700 hPa. However, these model estimates are limited to specific regions and seasons.
In this study, we use the GEOS-Chem global chemical transport model to quantify the regional contributions of Hg(II) produced at different heights in the atmosphere to the annual deposition of Hg(II). We have added a tagging method to the GEOS-Chem model to track Hg(II) produced in the lower (LT; surface–750 hPa), middle (MT; 750–400 hPa), and upper troposphere (UT; 400 hPa–tropopause), Hg(II) produced in the stratosphere (STRAT), and Hg(II) emitted by anthropogenic activities. This simulation is described and evaluated with ground-based observations of Hg(II) concentrations and wet deposition (Sect. 2). In Sect. 3, we present the distribution of the tagged Hg(II) and calculate their contributions to wet and dry deposition fluxes in different regions of the world. We also examine the sensitivity of our results to different model assumptions for Hg chemistry and anthropogenic emission speciation. We use our simulation to examine the role of the subtropical anticyclones as global reservoirs of Hg(II)-rich air (Sect. 4) and evaluate the role of tagged Hg(II) tracers in explaining the observed variability of Hg(II) concentrations and wet deposition fluxes (Sect. 5). Finally, we discuss the implications of our study in Sect. 6 and present conclusions in Sect. 7.
Hg wet deposition fluxes over North America and Europe are measured by the
Mercury Deposition Network (MDN;
To calculate monthly means, we discard sites with fewer than 3 weeks of measurements in any given month. For annual means we require at least 8 months of valid measurements. The MDN network had 80 stations over the continental US that met the above data completeness criteria during 2013–2014, whereas the EMEP network had 9 stations over Europe (Table S1 in the Supplement).
The Atmospheric Mercury Network (AMNet;
Ground-based measurements of Hg wet deposition and Hg(II) surface concentration have been made as part of the Global Mercury Observations System (GMOS) network (Angot et al., 2014; Wängberg et al., 2016; Sprovieri et al., 2016, 2017; Travnikov et al., 2017) and at sites in Europe (Weigelt et al., 2013), Canada, and East Asia (Sheu et al., 2010; Sheu and Lin, 2013; Fu et al., 2015, 2016b). We use the 2013-2014 measurements wherever available but use all sites with 1 year or more of observations. We exclude sites in China classified as urban because of proximity to large Hg(II) sources. We include 14 sites with annual-mean measurements of Hg wet deposition (Table S1 in the Supplement) and 14 sites with annual-mean measurements of surface Hg(II) (Table S2 in the Supplement).
The NOMADSS aircraft campaign took place over the eastern US from 1 June to
15 July 2013. Total Hg and Hg(II) observations were made with the University
of Washington Detector of Oxidized Hg Species (DOHGS) instrument (Ambrose et
al., 2015; Swartzendruber et al., 2009; Lyman and Jaffe, 2012). The detection
limit of the DOHGS instrument for Hg(II) measurements during the campaign was
between 57 and 228 pg m
GEOS-Chem is a global chemical transport model that simulates the emissions,
transport, chemistry, and deposition of Hg(0), gas-phase Hg(II), and
particle-phase Hg(II) (Selin et al., 2007). The model is driven by
meteorological fields from the NASA Global Modeling and Assimilation Office
(GMAO) Goddard Earth Observing System Model Forward Processing (GEOS-FP)
modeling system. The GEOS-FP system consists of a general circulation model
coupled with a data assimilation system (Reinecker et al., 2008) and has a
native horizontal resolution of 0.25
The redox chemistry of Hg consists of oxidation of Hg(0) by Br, as described
below, and aqueous-phase reduction in the presence of sunlight (Holmes et
al., 2010). Gas–particle partitioning of Hg(II) on sea-salt aerosols is
simulated as a kinetic process (Holmes et al., 2010), while partitioning on
other aerosols is simulated as an equilibrium process (Amos et al., 2012).
The oxidation of Hg(0) to Hg(II) is simulated as follows (Goodsite et al.,
2004; Balabanov et al., 2005; Dibble et al., 2012):
The GEOS-Chem model includes wet deposition of Hg(II) and dry deposition of
Hg(0) and Hg(II). Wet deposition includes in-cloud scavenging (rainout) and
below-cloud scavenging (washout) in convective and large-scale precipitation
(Liu et al., 2001). Within clouds, the dissolution of gas-phase Hg(II) in
liquid droplets is modeled as an equilibrium process, while particle-phase
Hg(II) is assumed to be fully dissolved (Amos et al., 2012). We assume that
rainout of gas-phase Hg(II) does not occur during ice nucleation
(
Uncertainties in mercury modeling and chemistry have been recently reviewed by Gustin et al. (2015), Ariya et al. (2015), and Kwon and Selin (2016). Here we briefly discuss uncertainties which are pertinent to our study: uncertainties in the assumption of Br as the sole oxidant of Hg(0), in reduction kinetics of Hg(II), and in the assumed speciation of Hg(0) and Hg(II) in anthropogenic emissions.
While Br, O
The pathways for reduction of Hg(II) to Hg(0) in the atmosphere are poorly
characterized. Laboratory experiments suggest that photoreduction of Hg(II)
can occur in the aqueous phase in the presence of organic compounds or on dry
aerosol surfaces at atmospherically relevant rates (Si and Ariya, 2008; Tong
et al., 2013), and field studies have found some evidence for in situ
reduction of Hg(II) (Edgerton et al., 2006; Landis et al., 2014; de Foy et
al., 2016). Most global atmospheric mercury models include at least one
pathway of Hg(II) reduction in order to simulate realistic Hg(0)
concentrations (Ariya et al., 2015). The reduction rate of aqueous-phase
Hg(II) in GEOS-Chem is parameterized based on the simulated NO
We have assumed an emission speciation of 90 % Hg(0) and 10 % Hg(II)
for anthropogenic emissions from stacks, as opposed to the UNEP/AMAP
speciation of 55 % Hg
Same as Fig. 1 but for European Monitoring and Evaluation Programme (EMEP) sites.
We have added a tagging technique to the GEOS-Chem model to identify the production regions of Hg(II). We divide the atmosphere vertically into LT (surface–750 hPa), MT (750–400 hPa), UT (400 hPa–tropopause), and STRAT to track the Hg(II) produced in each of these regions as separate Hg(II) tracers. Hg(II) emitted directly to the atmosphere is also tagged separately (E–Hg(II)). Each of these tagged tracers undergo the same physical and chemical processes as the total Hg(II) tracer. Hg(II) loss by deposition or reduction in a model grid cell is divided among all tagged tracers present in the grid cell in proportion to their masses. We perform a simulation with the tagged tracers for the years 2013 and 2014 following a model spin-up period of 15 years.
We perform an additional simulation to quantify the role of the dry
subsidence regions of the subtropical anticyclones in the global transport of
Hg(II). We identify the dry subtropical subsidence areas as those that lie
between 45
In addition, we perform three 1-year (2013) sensitivity simulations with the
tagged tracers addressing uncertainties in mercury oxidation and
Modeled zonal mean
Figures 1–3 compare the modeled Hg(II) concentrations and wet deposition
fluxes to observations from the MDN, EMEP, and AMNet networks. The modeled
annual wet deposition flux at the MDN sites
(10.4
The model also captures the observed annual VWM concentrations (MDN:
10.0
Over Europe (Fig. 2a), the modeled wet deposition flux
(3.5
In Fig. 3 we present a comparison of modeled surface Hg(II) concentrations
with observations at AMNet sites. Modeled Hg(II) surface concentrations
(11.7
Supplement Figs. S1–S3 present further evaluation of the model with
observations at other ground-based sites as well as with aircraft
observations. The modeled Hg wet deposition fluxes and VWM concentrations are
in reasonable agreement with the observations (NMB: 48–52 %; FAC2:
64–78 %), and show a high correlation (
The annual zonal mean distribution of modeled Hg(II) concentrations is shown
in Fig. 4a. Hg(II) concentrations increase from 10 pg m
The lifetime of Hg(II) increases from less than 1 day in the lower troposphere to over 3 years in the tropical upper troposphere. Hg(II) in the lower troposphere is subject to dry deposition, as well as in-cloud reduction and scavenging by precipitation in the lower and middle troposphere. Thus, despite higher production rates, Hg(II) concentrations over the Arctic and the Southern Ocean are low. The long lifetimes of Hg(II) in the upper troposphere and in the descending branches of the Hadley circulation are due to infrequent occurrence of reduction within clouds and wet scavenging. As summarized in Table 1, we find that the tropospheric lifetime of Hg(II) decreases from 43 days for the STRAT tracer and 22 days for the UT tracer down to 0.6 days for the LT tracer. This is consistent with expectations, as most of the UT tracer, for example, resides in the upper troposphere, where deposition is slower.
Tropospheric budgets of Hg(II) and individual tagged Hg(II) tracers.
The large production rates of Hg(II) in the upper and middle troposphere combined with a longer lifetime result in the large contributions of the UT and MT tracers to the tropospheric mass and deposition of Hg(II). Overall, the tropospheric burden of Hg(II) is dominated by Hg(II) produced in the UT (84 %), with contribution of 8 % from the stratosphere and the MT and less than 1 % from the LT and direct emissions (Table 1). The UT and MT tracers each contribute 35–40 % of the Hg(II) burden in the lower troposphere (Table 1 and Fig. 4d and e). The contribution of the LT tracer increases to > 50 % near the surface over the Arctic and the Southern Ocean (Fig. 4f), where local production of Hg(II) in the polar and marine boundary layers is larger. We also find that most of the Hg(II) in the lowermost stratosphere is comprised of the UT tracer (Fig. 4d), because Hg(0) is rapidly oxidized in the upper troposphere and is almost completely depleted before reaching the stratosphere, as shown by observations (Talbot et al., 2007; Lyman and Jaffe, 2012). The E-Hg(II) tracer accounts for 5 % of the Hg(II) burden in the lower troposphere (Table 1), but its contribution increases to > 10 % over the northern midlatitudes (Fig. 4h).
81 % of the tropospheric Hg(II) production happens in the upper and middle troposphere (Table 1). Together, the UT and MT tracers account to 91 % of global surface wet deposition (60 % from UT and 31 % from MT) and 52 % of dry deposition (24 % from UT and 28 % from MT). Their higher contributions to wet deposition are because precipitation scavenging can directly remove these tracers from higher altitudes, while dry deposition requires the transport of these tracers to the planetary boundary layer.
As shown in Fig. 5a, the highest surface Hg(II) concentrations
(> 50 pg m
The global distribution of the Hg(II) wet deposition flux (Fig. 5b) largely follows the spatial distribution of precipitation, with high wet deposition along the Intertropical Convergence Zone (ITCZ) and in the midlatitude storm tracks. Globally, the UT tracer accounts for 60 % of Hg(II) in wet deposition, but in some areas over South America, Africa, and Asia it exceeds 70 % (Fig. 5f). The MT tracer makes up most of the remaining fraction of wet deposition, with a global average contribution of 31 % (Table 1). The contribution from the LT tracer is significant only at high latitudes, while the contribution from E-Hg(II) reaches values greater than 10 % mainly over East Asia. The relative wet deposition contributions of the tagged Hg(II) tracers remain fairly uniform across the 10 regions summarized in Fig. 6c.
The Hg(II) dry deposition flux (Fig. 5c) maximizes in the subtropical anticyclones, where subsidence provides a source of free-tropospheric Hg(II) to the planetary boundary layer. In addition, local maxima occur downwind of the emission regions of the eastern US and East Asia, over high-elevation regions in western US and the Himalayas, and over the Southern Ocean. In terms of the tagged tracers, their spatial contribution to dry deposition (Fig. 5g) is similar to their contribution to surface Hg(II) concentrations (Fig. 5e). We find that 79–82 % of the Hg(II) dry deposition over western US, South America, Africa, and Australia is from the UT and MT tracers. The E-Hg(II) tracer contributes 21–62 % to dry deposition over eastern US, Europe, and South and East Asia (Fig. 6d). Over the Pacific and North Atlantic oceans, the UT, MT, and LT tracers each contribute about 30 % to the dry deposition flux (Fig. 6d).
In Sect. 2.2.3 we saw that the model overestimated observed wet deposition of
Hg(II) over southeastern US during winter and spring. As a result, our estimate
of the contribution of UT and MT tracers is likely an overestimate for this
region and season. From our model evaluation, we had also concluded that our
free-tropospheric Hg(II) production was too slow over Europe and, possibly,
other regions north of 45
Contribution of tagged Hg(II) tracers to the tropospheric mass and total deposition of Hg(II) for the base case and the sensitivity simulations.
Our estimate that 92 % of Hg(II) wet deposition and 73 % of dry
deposition over the US is contributed by production in the upper and middle
troposphere is qualitatively consistent with the estimates of Selin and Jacob
(2008). They calculated that 59 % of the Hg(II) wet deposited over the US
was scavenged above 1.5 km, and that 70 % of the Hg(II) below 1.5 km
was transported from elsewhere. For comparison, with our simulation we find
that 85 % of the Hg(II) wet deposited over the US is scavenged above
1.5 km (note that to be consistent with Selin and Jacob (2008), we are
comparing here the contribution of Hg(II)
We now assess the sensitivity of our results to our assumptions about mercury
oxidation and
When we use the original GEOS-Chem Br concentration, the contribution of the
UT and MT tracers to the tropospheric Hg(II) burden decreases to 78 %
(base: 92 %), while the contribution to total deposition decreases to
64 % (base: 77 %; see Table 2). In the O
Mean and anomaly (maximum deviation from the mean) of the
contributions of dry-air Hg(II) to
In this section, we focus on the specific role of subtropical anticyclones as
a global reservoir of Hg(II). The large-scale sinking motion in the
subtropical anticyclones transports Hg(II) produced in the upper and middle
troposphere downwards and suppresses cloud formation and precipitation,
thereby inhibiting Hg(II) loss of by reduction and wet deposition. The
subtropical anticyclones, therefore, act as global reservoirs of Hg(II), as
we presented in Shah et al. (2016). Here, we further quantify how much Hg(II)
is transported from the subtropical anticyclones (dry-air Hg(II) tracer) with a
simulation where we artificially set to zero the Hg(II) present in the
subtropical dry areas (defined as RH < 20 % and
latitude < 45
We see from Fig. 7a that dry-air Hg(II) exerts a disproportionate influence on
surface Hg(II) concentrations between 40
90 % of the mass of Hg(II) present at 500 hPa in the 40
During the 2013 NOMADSS aircraft campaign, high Hg(II) concentrations were
observed and simulated above 5 km altitude (observations: 189
Our finding is consistent with ground-based Hg(II) observations in western
US, an area heavily influenced (> 60 %) by Hg(II) present in
the dry subtropical regions (Fig. 7). Weiss-Penzias et al. (2009) reported
that occurrence of higher (
Our tagged simulation show that the upper and middle troposphere are the
predominant regions of production of Hg(II). Thus, areas where wet deposition
is strongly influenced by Hg(II) produced in these regions can be expected to
have higher wet deposition flux of Hg(II). We now examine whether such an
enhancement in Hg wet deposition flux is indeed observed at MDN sites. Figure
9 shows the relationship between observed MDN annual Hg wet deposition fluxes
to precipitation and modeled contribution of the UT and MT tracers to the wet
deposition flux at the site locations. As expected, we see that Hg wet
deposition fluxes increase with increasing precipitation (e.g., Prestbo and
Gay, 2009; Selin and Jacob, 2008). In addition, we find that the Hg wet
deposition fluxes increase with increasing contribution of the UT and MT
tracers to the wet deposition flux. Using multiple linear regression, we
derive the following relationship between the observed Hg flux
(
Modeled contribution of the dry-air Hg(II) tracer to observed Hg(II) concentrations during the NOMADSS aircraft campaign. The number of 2.5 min observations points in each concentration bin is shown on top of the bars.
Relationship of observed MDN Hg wet deposition flux (in units of
AMNet sites in the eastern US are close to regional Hg(II) emission sources
and are thus more likely to be influenced by Hg(II) directly emitted rather
than by Hg(II) produced aloft. Figure 10 shows that the 2009–2012 median
Hg(II) concentrations observed at the AMNet sites in the eastern US are
higher at sites where the contribution of E-Hg(II) tracer is higher. For
example, the surface Hg(II) concentrations at sites NY06, WV99, and MD08 are
Our modeling study indicates that even in areas with large anthropogenic sources of Hg(II) most of the mercury wet deposition flux consists of Hg(II) produced in the upper and middle troposphere. This implies that regional decreases in anthropogenic Hg emissions do not lead to a proportional regional decrease in wet deposition. Indeed, numerous studies have demonstrated the importance of intercontinental transport to mercury wet deposition (see Pirrone and Keating, 2010, and references therein). For example, Jaeglé et al. (2009) found that a 20 % decrease in regional anthropogenic mercury emissions in the GEOS-Chem model leads to between 3 % (North America) and 12 % (East Asia) decrease in mercury deposition. Moreover, observed long-term temporal trends in mercury wet deposition reflect trends in the global emissions of Hg(0) (Zhang et al., 2016; Weiss-Penzias et al., 2016; Zhang and Jaeglé, 2013). Our study shows that oxidation of Hg(0) in the upper and middle troposphere is the key to linking global emissions to deposition of mercury.
We also find that a large fraction of the upper- and mid-tropospheric Hg(II) over the US is transported from the subsiding subtropical anticyclone over the Pacific Ocean. Thus, we expect that variability in the location of the Pacific anticyclone, the synoptic wind patterns transporting Hg(II) to the US, and the heights of the precipitating clouds, in addition to the amount and type of precipitation, can affect Hg wet deposition flux over a particular area. These meteorological conditions vary in response to natural variability associated with multiyear phenomena, such as the El Niño–La Niña cycle (Gratz et al., 2009), and can confound the interpretation of spatial and temporal trends in wet deposition at MDN sites.
Our results support the idea of a global pool of Hg(II) in the free troposphere. We find that this global pool of Hg(II) is concentrated in the upper troposphere (above 7 km) and extends to lower altitudes in the subsidence areas of the subtropical anticyclones. These regions of the atmosphere are where most of the production of Hg(II) takes place and where the lifetime of Hg(II) against reduction and deposition is the longest, making them ideal target regions for future aircraft-based campaigns to understand the chemistry of mercury in the atmosphere.
We have added to the GEOS-Chem mercury model a Hg(II) tagging method
following regions where Hg(II) is produced. We have performed a 2-year
simulation (2013–2014) with the tagged Hg(II) tracers and have found that
Hg(II) produced in the upper and middle troposphere constitutes 91 % of
the tropospheric mass of Hg(II), 91 % of the annual Hg(II) wet deposition
flux, and 52 % of the annual Hg(II) dry deposition flux. The
disproportionately high contribution of the Hg(II) produced in these regions
is the result of higher production of Hg(II) in the upper and middle
troposphere combined with a longer lifetime of Hg(II) and the large-scale
subsidence of Hg(II) in the troposphere. Hg(II) produced in the upper and
middle troposphere contributes 63 % to surface Hg(II) over the
continents and 74–82 % over western US South America, Africa, and
Australia. Over the oceans, however, surface Hg(II) is formed locally in the
marine boundary layer because of Br released from sea-salt aerosols. Directly
emitted anthropogenic Hg(II) makes up a significant fraction (27–69 %)
of surface Hg(II) concentrations near source regions in eastern US, Europe,
and South and East Asia. However, the wet deposition flux in these regions is
largely (
We examined the consistency of our modeling results with measurements at the
MDN, EMEP, and AMNet sites. We found reasonable agreement between the modeled
and observed Hg wet deposition flux at the MDN sites (NMB:
We quantified the role of Hg(II) in dry subtropical anticyclones and found it
exerts a strong influence on Hg(II) concentrations at the surface (44 %
contribution) and 500 hPa (90 % contribution)
between 40
The GEOS-Chem model results are available from the corresponding author upon
request. The measurement data are available in the cited literature or at the
network's web sites. MDN:
The authors declare that they have no conflict of interest.
This work was supported by funding from the National Science Foundation under award number 1217010. We thank Dan Jaffe for helpful feedback on the paper. We thank Dan Jaffe, Jesse Ambrose, and Lynne E. Gratz for the NOMADSS measurements, the National Atmospheric Deposition Program for the MDN and AMNet measurements, the European Monitoring and Evaluation Programme for the wet deposition measurements over Europe, and Xinrong Ren and Steve Brooks for aircraft measurements over Tullahoma, TN. We appreciate support from the GEOS-Chem user community. We thank the three referees for reviewing the manuscript.Edited by: Aurélien Dommergue Reviewed by: three anonymous referees