Introduction
Mercury (Hg) is a persistent pollutant of global concern due to its toxicity
and its capacity to bioaccumulate in aquatic food chains with serious
consequences on human and wildlife health . Long-range
atmospheric transport is the main pathway for contamination of remote
ecosystems; therefore, atmospheric deposition is the primary indicator for the
understanding of its impact on aquatic and terrestrial ecosystems
. Hg exists in the atmosphere mainly in
three operationally defined forms: gaseous elemental mercury (GEM), oxidized
gaseous mercury (GOM), and particulate bound mercury (PBM). Globally, GEM is
the predominant form, whereas GOM and PBM are thought to be rapidly dry
deposited and wet scavenged by precipitation .
Currently, Hg dry deposition is often estimated by models, using measured
ambient concentrations of Hg and meteorological parameters, due to the lack
of existing direct and accurate measurement methodologies .
Therefore, the investigation of Hg fluxes to terrestrial and
aquatic surfaces in different parts of the world are often based on wet
deposition measurements . Hg wet deposition
represents the air-to-surface flux in precipitation .
Previous studies suggested that the magnitude of Hg wet deposition varies
geographically and seasonally due to climatic conditions, atmospheric
chemistry, and human influences, i.e., emissions of Hg from anthropogenic
sources . Current annual
atmospheric deposition of Hg has been estimated to be 3200 Mg yr-1
deposited on land and 3700 Mg yr-1 into oceans . The
preindustrial deposition rate has been estimated to be 1000 Mg yr-1
deposited on land and 2500 Mg yr-1 into oceans .
Developed countries in North America and Europe have reduced their
anthropogenic Hg use and emissions , but Hg use and
emissions are still occurring widely around the world .
In North America, seasonal patterns in THg concentrations in
precipitation and Hg wet deposition amounts have been observed with the
highest values in the summer and lowest values in the winter
.
Explanations for this observation include more effective Hg scavenging by
rain compared to snow , and a greater
availability of soluble Hg due to convective transport in summer events
. Geographic differences in Hg wet
deposition may be explained in part by the proximity to atmospheric sources.
Results from the National Atmospheric Deposition Program's (NADP) Mercury
Deposition Network (MDN) sites in the northeastern United States exhibit a
geographic trend, with southern and coastal sites receiving higher Hg
concentrations in precipitation and wet deposition fluxes
due to their location closer to the east
coast megalopolis and downwind of anthropogenic emission sources such as coal
burning power plants and waste incinerators. In addition, gaseous evasion of
Hg from marine waters is a significant global source of GEM which, throughout
active oxidation processes, may also contribute to elevated depositional
fluxes in coastal regions . A similar pattern exists in
northern Europe with a clear gradient in atmospheric concentrations and
deposition . Hg wet
deposition data are therefore important for verifying atmospheric models,
understanding the biogeochemical cycling of Hg on a regional/global scale,
and investigating ecosystem impacts. Regional monitoring networks with
properly chosen monitoring sites can provide accurate estimates of wet
deposition at regional scales. Long-term Hg wet deposition measurements exist
at many locations within already established regional network, such us in the
United States as part of the MDN or in Europe as part of the European
Monitoring and Evaluation Programme (EMEP); however, before the establishment
of the Hg network by the Global Mercury Observation System (GMOS) on a global
scale, long-term measurements of ambient Hg concentrations and measurements
of Hg wet deposition fluxes were lacking
in several regions of the world. Although a number of monitoring
stations have been in fact established to better understand the impact of Hg
wet deposition on ecosystems in many countries in the Northern Hemisphere
, the tropical zone and the
Southern Hemisphere were particularly lacking in wet deposition data
available, in terms of concentrations and deposition Hg fluxes.
To address this concern, seasonal and annual variations of Hg wet deposition
and concentration at 17 ground-based sites in the Northern and Southern
hemispheres were monitored as a part of GMOS
(http://www.gmos.eu). Here, an overview of the seasonal/annual Hg
wet deposition patterns across the 17 sites is presented, briefly examining
meteorological/climatological conditions, as well as indicators of
anthropogenic air mass sources and/or atmospheric chemical conditions in
relation to Hg wet deposition results observed. This study is the first
multiyear comparison of Hg wet deposition worldwide and provides insights
into annual and seasonal variations, as well as a spatial gradient in Hg deposition patterns.
Station locations that are part of the GMOS network and general
characteristics of the sites (i.e., code, name, country, latitude, longitude,
elevation), including the years of sampling as well as the type of monitoring
stations with respect to the Hg measurements carried out as speciated (M) or
not (S). M/S or S/M indicates change of the site from master to secondary or vice versa.
External GMOS partners are indicated in bold.
Code
Name
Country
Lat
Long
Elev.
Collector
Years of
Type*
(m a.s.l.)
type
sampling
Northern Hemisphere
1
NYA
Ny-Ålesund
Norway
78, 90
11, 88
12
IVL bulk
2012–2015
M
2
PAL
Pallas
Finland
68, 00
24, 24
340
IVL bulk
2011–2014
S
3
RAO
Råö
Sweden
57, 39
11, 91
5
IVL bulk
2011–2014
M
4
MHE
Mace Head
Ireland
53, 33
-9, 91
5
wet only
2012–2014
S
5
LIS
Listvyanka
Russia
51, 85
104, 89
670
wet only
2012–2013
S
6
CMA
Col Margherita
Italy
46, 37
11, 79
2545
IVL bulk
2014
S
7
ISK
Iskrba
Slovenia
45, 56
14, 86
520
wet only
2011–2015
M
8
MCH
Mt. Changbai
China
42, 40
128, 11
741
wet only
2011–2014
M/S
9
LON
Longobucco
Italy
39, 39
16, 61
1379
wet only
2012–2013
M
10
MWA
Mt. Waliguan
China
36, 29
100, 90
3816
wet only
2012–2014
M
11
MAL
Mt. Ailao
China
24, 54
101, 03
2503
wet only
2011–2014
S/M
Tropics
12
SIS
Sisal
Mexico
21, 16
-90, 05
7
wet only
2013–2014
S
13
CST
Celestún
Mexico
20, 86
-90, 38
3
wet only
2012–2013
S
Southern Hemisphere
14
AMS
Amsterdam Island
TAAF
-37, 80
77, 55
70
wet only
2013–2014
M
15
CPT
Cape Point
South Africa
-34, 35
18, 49
230
wet only
2011–2015
S
16
CGR
Cape Grim
Australia
-40, 68
144, 69
94
IVL bulk
2013–2015
S
17
BAR
Bariloche
Argentina
-41, 13
-71, 42
801
wet only
2014–2015
M
* M = master; S = secondary. TAAF indicates Terres Australes et Antarctiques Françaises.
Experimental
GMOS ground-based monitoring sites
The global Hg monitoring network has been established in the framework of the
GMOS and presented in . It has been developed by
integrating previously on-going ground-based Hg monitoring stations as part
of regional networks with those established as part of GMOS also in regions
of the world where atmospheric Hg measurements were previously limited. To
date, the GMOS network consists of 43 monitoring stations worldwide
distributed and located in climatically diverse regions, including polar
areas . In the present study, we refer the
discussion on Hg wet deposition to a representative number of 17 ground-based
sites distributed in the Northern and Southern hemispheres (Fig. S1 in the
Supplement). Table provides key information on the 17 monitoring sites such as
their location (country, coordinates, etc.), elevation (m a.s.l.), and type
of monitoring stations (master and secondary sites) with respect to the
atmospheric Hg measurements performed (Hg speciation and total gaseous mercury – TGM/GEM
measurements, respectively) along with THg wet deposition sampling.
Sample collection, analytical procedure, and QA/QC
Precipitation samples were collected across the sites primarily using
wet-only collectors, (i.e., N-CON MDN or the Eigenbrodt NSA 171 wet-only
samplers). Where necessary, due to site constraints or operator availability,
few GMOS sites (Table ) alternatively collected bulk precipitation
samples using the Swedish Environmental Research Institute (IVL) bulk sampler
. The detailed description of the
Swedish bulk sampler is reported elsewhere ,
where it also has been highlighted that utilizing clean handling and analysis
technique is equivalent to using a wet-only sampler in remote areas, and/or areas
not subject to large amounts of anthropogenic emission sources, as is the case
in most of the GMOS ground-based sites. Within GMOS, special attention was
paid with respect to protocol harmonization, data quality collection, and data
management in order to assure a full comparability of site-specific
observational data sets. During the implementation stage of the GMOS global
network, harmonized standard operating procedures (SOPs) as well as common
quality assurance/quality control (QA/QC) protocols have been addressed
in accordance with the measurement practice adopted in
well-established regional monitoring networks and based on the most recent
literature . For THg in
precipitation, an ad hoc standard operating procedure has been developed and
adopted within the network. Furthermore, the management of the measurement
program at most of the GMOS sites consisting of analysis of all precipitation
samples, cleaning procedures, and distribution of the sample bottles to all
sites, has been performed by three reference laboratories (IVL, Sweden;
CNR-IIA (CNR Institute of Atmospheric Pollution Research), Italy; and IJS (Jožef Stefan Institute), Slovenia), whereas the precipitation samples related
to some other GMOS sites, such as in Russia – Listvyanka (LIS); in China –
Mt. Waliguan (MWA), Mt. Ailao (MAL), and Mt. Changbai (MCH); and in South
Africa – Cape Point (CPT), have been analyzed by local laboratories. The
analytical performance and the QA/QC of the analysis carried out by the
reference laboratories as well as by the local laboratories were confirmed by
the results achieved during international intercomparison exercises for Hg
in water (i.e., Brooks Rand Instruments Inter-laboratory Comparison Study).
GMOS sites predominantly collected biweekly samples. However, considering
the spatial distribution and the diversity of meteorological parameters and
conditions characterizing the monitoring site locations, the sampling
frequency was sometimes different across the sites. THg concentrations in
precipitation samples, refrigerated and kept in the dark before the analysis
(to avoid photo-induced reduction of the Hg in the precipitation sample),
were determined according to the US EPA Method 1631 (version E)
: each sample was first oxidized by BrCl (0.5 mL/100 mL
sample), followed by neutralization with hydroxylamine hydrochloride
(NH2OH ⋅ HCl). Stannous chloride (SnCl2) was then added to the
sample to reduce Hg(aq)2+ to Hg(g)0 which was quantified by
cold vapor atomic fluorescence spectrometry (CVAFS) using a Tekran Mercury
Analysis System Model 2600 (Tekran Inc. Corporation, Canada). Working Hg
standards solutions were obtained from a standard reference material (SRM)
produced by accredited laboratory (ISO/IEC 17025). Calibration standards were
analyzed in the range from 0.2 to 100 ng L-1 (recovery 93–109 %). The standard
curve was used within the coefficient of determination (r2) greater
than 0.998 (linear). Initial (IPR) and ongoing precision and recovery (OPR)
solutions (5 ppt) were analyzed prior to the analysis of samples and again
after every 12 samples (recovery 91–103 %). These values were within the
quality control acceptance criteria for performance in the EPA Method 1631e.
The method detection limit (MDL; 40 CFR (Code of Federal Regulations) 136, Appendix B) for Hg has been
determined to be 0.02 ng L-1. The minimum level of quantification (ML) has been
established as 0.05 ng L-1 for THg. The QA/QC of the analysis were obtained
using replicates, method blanks, field blanks, initial/ongoing precision
recovery (IPR/OPR) standards, matrix spikes, and certified reference
materials (CRMs) with different certified Hg concentrations. Method and field blanks
were always below the respective MDL, indicating minimal contamination during
sampling, transport, and treatment for this study. Additionally, the sampling
train materials (fluorinated polyethylene – FLPE – bottles, cylindrical
glass funnels, Teflon adapters along with the glass capillary S-shaped tubes to
prevent loss of mercury from the sample, etc.) were thoroughly
acid-cleaned and rinsed with ultra-pure water in the Hg laboratory before and
after sampling steps, and randomly tested for Hg concentrations; they were
always below the MDL. All of these materials have been triple-bagged in
zip-type plastic bags to keep them clean prior to use in the field. The
results of “blanks” analysis allowed us to exclude possible contamination
of all samples during different steps.
Hg wet deposition flux calculation
Considering the geographical distribution of the 17 sites located at
different latitude and longitude, and therefore, under different
meteorological and climatological conditions, the precipitation was not
collected over an entire year at each station due to limited amount of
precipitation samples occurring during specific periods (i.e., dry seasons).
Therefore, Hg flux was necessarily estimated based on the volume-weighted
mean (VWM) concentration and the precipitation amount collected at each site
during the times that the Hg sampler was operating. This means that the THg
wet deposition flux has been approximately calculated by the following
equation:
FW=CHgx∑i=1i=nPi1/1000,
where FW is the THg wet deposition flux (µg m-2 yr-1)
and CHgx is the VWM concentration
of THg (ng L-1). Pi (mm; 1 mm = 1 L m-2) represents the
precipitation amount associated with each wet deposition sample.
It is also necessary point out that the “precipitation” and “rainfall” terms
throughout this work have been interchangeably used and, further, rainfall
included all forms of precipitation (i.e., rain, frozen). The frozen
precipitation has been considered as “liquid rain equivalent”. In addition,
to overcome the irregularity in time-sampling frequency of the rainy samples
collected at each site (see Tables S3 and S4) the rainfall amounts as well
as THg wet deposition flux weighted data have been normalized with respect to
the ideal time-sampling period, equal to 15 days, as previously established
in our GMOS standard operating procedures . In this way,
all data resulted comparable and harmonized.
Hg wet deposition patterns and interannual variability
The annual variations in THg concentration and wet deposition recorded at all
17 monitoring GMOS sites are summarized in Tables S1 and S2. Both tables list
the monitoring sites according to their latitude and rain
amounts collected, the number of the sampling days, as well as the annual wet
deposition flux calculated for each site for each year in the period 2011–2015. The
rainfall amounts as well as THg wet deposition flux weighted data have been
normalized with a 15-day sampling reference as described in the section above.
Annual THg wet deposition fluxes calculated at each site according to their
latitude are shown in Fig. . The Hg deposition at each site
tends to vary from year to year, but to a different degree at different
locations. It is well known that the magnitude of Hg wet deposition varies
geographically and seasonally due to different meteorological and climatic
conditions, atmospheric chemistry, and anthropogenic influences
. Therefore, considering the
11 sites distributed in the Northern Hemisphere, the discussion of the
results will be separately related to the 7 European sites (ruling out
the discussion on the data related to LIS site, due to the lower number of
samples collected over the sampling period and thus not representative enough
for such a conclusion) and the 3 Chinese sites (see Tables S1–S4)
as well as those located in the tropical area and the sites distributed
in the Southern Hemisphere. Considering the THg wet deposition from 2012
to 2014 at the European sites, there appears to be a geographical trend with an
increase in Hg deposition from north (Arctic area, i.e., Ny-Ålesund – NYA,
Norway; Pallas – PAL, Finland) to south in the Northern Hemisphere
(i.e., Råö – RAO, Sweden; Mace Head – MHE, Ireland; Listvyanka – LIS, Russia; Col Margherita – CMA,
Italy; Longobucco – LON, Italy). At the Chinese sites, as well as at
lower latitude (i.e., tropical area and Southern Hemisphere), no north–south
spatial trend has been observed. However, it is important to point out that
the sites in the Southern Hemisphere are limited in number compared to those
in the Northern Hemisphere and the data coverage is less complete for each
year considered. This makes detailed evaluation of spatial trends at the
southern sites difficult. In addition, apart from CPT, no historical records
of THg deposition exist for the new stations established in the GMOS project.
Scatterplot reporting the annual wet deposition fluxes versus latitude
observed at the 17 GMOS sites during 2011–2015.
Annual wet deposition fluxes and the corresponding cumulative
precipitation amounts (rainfall), observed at the 17 GMOS sites during
2011–2015.
The geographical trend observed at the European stations with higher
deposition of Hg in southern sites than in the north is in line with emission
patterns with the main source areas in central and eastern Europe. The
present data in combination with ground-based atmospheric Hg measurements
performed within the GMOS project during 2012–2015 period indicate that
these findings are in good agreement with the geographical distribution of
atmospheric Hg with a downward gradient from the Northern to the Southern
Hemisphere . Figures and
show, from 2012 to 2014 (the period with more data coverage), a general
increasing of THg wet deposition from the NYA station to Iskrba (ISK), Slovenia;
this finding is particularly evident during the 2013 and 2014 (Fig. )
event if the pattern is not apparent for the LON site in 2013 and
for MHE and CMA for 2014, indicating the influence of other parameters and/or
atmospheric transport pathways. In Fig. , the THg
wet deposition fluxes calculated on an annual basis are reported, taking into account the
annual precipitation amounts recorded at each site. It well known that wet
deposition of atmospheric Hg at any given location is influenced by factors
such as (a) atmospheric Hg concentration depending upon the local, regional,
and global sources; (b) site location in relation to the predominant wind
direction in relation to the source areas; (c) precipitation amount which
removes Hg from the atmosphere; (d) type of precipitation (rain or snow);
(e) length of precipitation events which affect Hg concentrations; (f) height and
thickness of the precipitating cloud layer in the atmosphere and the degree
of convection involved; and (g) at least (but not less important than the
others) the oxidizing capacity of the atmosphere, which can be the dominant
factor particularly in remote/polar areas. Hg concentrations appear to be
higher at the beginning of a precipitation event (i.e., rain or snow), and
lower at the end of a precipitation event .
This is most evident during periods of prolonged
precipitation (i.e., over a period of several days). It is obvious therefore
that the Hg deposition obtained at some sites should be strongly influenced
by the precipitation amounts. In particular, the annual deposition amounts
during the 2011–2015 period show the influence of the precipitation amount on
Hg deposition between, for example, the RAO site and the PAL site (Fig. ).
The THg wet deposition fluxes recorded during 2011–2014 were, respectively, 5.8,
6.5, 4.2, and 6.3 µg m-2 yr-1 at the RAO site.
This is more than 2 times higher than at Pallas during the same years
(2.9, 1.9, 1.3, and 2.3 µg m-2 yr-1), and since the precipitation amounts are also a
factor of 2 higher at the RAO site in comparison to PAL, the Hg deposition
results seem to be consistent with this increase in the south compared to the
northern sites. These findings also confirmed the results obtained by
during an assessment on available Hg data in precipitation
carried out from 1996 to 2002 at five Scandinavian EMEP monitoring stations,
and among them also at the RAO and PAL GMOS sites. highlights,
in fact, that the highest annual Hg wet deposition and yearly averaged THg
concentrations in precipitation have been recorded at the southern
Scandinavian coastal sites where the highest average annual deposition
amounts also occurred. The annually based THg wet deposition flux
(µg m-2 yr-1) calculated, conversely, at MCH, MWA, and MAL shows no
significant geographical trend with high variability and notable differences
in concentrations among the sites during the same period. These stations are
all remote sites in China, and considering the 2012–2014 period,
which is the most representative in terms of number of samples recorded, it
is possible to see that the averaged THg wet deposition fluxes
(ng m-2 day-1) in remote areas of China were not significantly higher
than the values observed at the rest of the GMOS sites (i.e., ISK, MHE, RAO)
(Fig. ).
At the sites located at lower latitude and Southern Hemisphere, the
relationship between precipitation amount and deposition was not as evident
as in the Northern Hemisphere. At the Sisal monitoring station (SIS), a
coastal site of the tropical area located on the Yucatán Peninsula (Gulf of
Mexico), the 2013 annual wet THg deposition flux was
7.4 µg m-2 yr-1, whereas the rainfall amount was 669.6 mm, which is lower than the rainfall
recorded at the remote southern sites, such as Amsterdam Island (AMS) (833.2 mm
rainfall), southern Indian Ocean, and Cape Grim (CGR), Australia (775.6 mm
rainfall), where the annual wet Hg deposition flux recorded was considerably
lower at 1.95 and 3.1 µg m-2 yr-1, respectively (see Tables S1 and S2).
The 2013 and 2014 annual wet deposition fluxes recorded at SIS are
comparable or higher than those observed at most GMOS sites in the Northern
and Southern hemispheres (Tables S1 and S2). Because of the Hg deposition at
any given location is dependent upon both THg concentrations (which have a
geographical component) in precipitation and precipitation amounts
, the results obtained across the sites located from the
tropical area to the Southern Hemisphere highlighted that, in this case, the
geographical component in terms of local meteorology and local emission
sources has had a higher influence on the THg results. During the sampling
period, SIS was typically influenced by air masses originating from the Atlantic
Ocean coming from east–southeast, but crossing the Caribbean islands and/or
Central/South America with occasional air masses coming from east–northeast
mostly during the winter period and crossing the south of Florida and Caribbean
archipelago prior to arriving at the monitoring site .
Very few Hg deposition measurements have been
performed at tropical latitudes .
, in a study over 7 years (2005–2012) on Hg wet deposition in Puerto Rico (Caribbean archipelago,
US), highlighted that despite receiving prevailing unpolluted air off the
Atlantic Ocean from northeasterly trade winds, wet Hg deposition recorded at
the site was about 30 % higher than that observed in Florida and the Gulf
Coast, which in turn are the highest deposition areas in the US, and thus
was greater than at all other MDN sites. The wet Hg deposition map from the MDN,
in fact, shows a general pattern of relatively low deposition over the
western US (∼ 2–5 µg m-2 yr-1) and higher in the eastern US
(6–15 µg m-2 yr-1) due to increasing precipitation and location of
important anthropogenic Hg sources. In addition, in the eastern US, a
north–south latitudinal gradient exists in wet Hg loading, with wet
deposition reaching a maximum in the southeastern US over Florida .
Despite its unpolluted tropical setting, Puerto Rico seems to
fit as a southern extension to a latitudinal gradient of increasing Hg
deposition from north to south in the eastern US . The
high wet Hg deposition at SIS can be directly linked to the meteoclimatic
conditions and pressure systems typical of the tropics. The higher THg wet
deposition observed at latitudes lower than south of Florida and or Mexico
(such as in Puerto Rico (27.9 µg m-2 yr-1), an unpolluted tropical site
crossed often by air masses detected at SIS prevalently in summer and fall
and less so in winter) also suggests that frequent high convective clouds in this
subtropical region likely access the reservoir of oxidized Hg species in the
upper free troposphere .
found that the high Hg deposition was not correlated with
GOM at ground level but with the maximum height of rain detected within clouds
(obtained from the echo tops using the NOA-NEXRAD radar station), suggesting
that droplets in high convective cloud tops scavenged GOM from above the
mixing layer ( and references therein). Numerous studies
suggest in fact that the upper free troposphere holds a large pool of GOM
that has been oxidized from the global Hg pool and that frequent high convective
clouds occurring in tropical regions, particularly closer to the Equator,
scavenge GOM by precipitation being readily soluble .
Closer to the Equator, the Hadley cell structure
indeed gives way to the intertropical convergence zone (ICT), and the
atmospheric circulation there may affect upper-atmosphere Hg levels. The few
measurements in the Northern Hemisphere tropics, such as at SIS, generally
indicate lower Hg fluxes than those measured at lower tropical latitude
probably due to fewer convective rain events with clouds that reach the upper
atmosphere ( and references therein). The higher annual
wet Hg deposition observed at SIS compared to the other GMOS sites could be
also due to a contribution of air masses crossing areas with discrete
anthropogenic emission sources, particularly in late spring and summer, such
as the metropolitan area of San Juan and/or minor industrial plants in
Fajardo and the Antilles islands, and/or from air masses crossing, particularly in
winter, several coal power plants and waste incinerations in the southern
United States and southern Florida . In addition, also
legal and/or illegal gold mining activities, which are widespread
in the southern regions of the
Yucatán Peninsula (Nicaragua, Guatemala, etc.), could contribute to the
Hg wet deposition at SIS.
Seasonal distribution of volume-weighted THg concentration in
precipitation at the European GMOS sites from 2011 to 2015. Each box includes
the median (midline), mean (▪), 25th and 75th percentiles (box
edges), 5th and 95th percentiles (whiskers), minimum (*), and
maximum (+).
The southern sites, AMS, CPT, CGR, and Bariloche (BAR, Argentina) are more
remote compared to SIS. AMS is a very small island located in the southern
Indian Ocean where atmospheric Hg concentrations recorded during the same
period were remarkably steady with an annual median of 1.03 ± 0.10 ng m-3 and
lower than those recorded at the tropical sites
but slightly higher than
annual averages and medians recorded at CGR in 2013 . Both
AMS and CGR, for most of the time, receive clean marine air masses
. Previous studies analyzed
atmospheric observations of GOM from Mediterranean, Pacific, and Atlantic
cruises in terms of Hg chemistry and deposition in the marine atmosphere, and
suggested that elevated levels of halogen atoms, and in particular of bromine (Br) in
the marine boundary layer (MBL) are an important source of GOM from oxidation
of GEM that are more readily deposited throughout sea-salt aerosols followed by
aerosol deposition. GEM evasion from marine waters therefore could represent
a significant source of atmospheric Hg which contributes to depositional
fluxes in marine regions , such as AMS and CGR. In 2013,
among the southern sites, the highest annual THg wet deposition fluxes has
been recorded at CPT (5.2 µg m-2 yr-1) which also showed both the lowest
precipitation amount (264.9 mm) and the number of sampling days
(Tables S1 and S2) compared to AMS (with an annual wet deposition flux of
1.95 µg m-2 yr-1, considering a rainfall of 833.2 mm) and CGR (with wet
deposition flux of 3.1 µg m-2 yr-1, considering a rainfall of
775.6 mm). These findings have not been observed at CPT in 2014 with the lowest
annual wet deposition flux (0.57 µg m-2 yr-1) and comparable
precipitation amounts and number of sampling days of the year before (see
Tables S1 and S2). CPT is situated on the southern tip of South Africa
, and during the wetter season (May–October)
normally precipitation increased due to the passage of cold
fronts moving from west to east . In a previous study by
, it was highlighted that CPT receives continental and
polluted air masses more frequently during the winter period with air masses
advected to the station from north to northwestern
regions where the Gauteng and Mpumalanga provinces are located.
These South African areas represent the major anthropogenic Hg sources with
former mine dumps from gold mining and large coal-burning power stations
. Therefore, in the first instance, the highest annual
average THg wet deposition flux observed at CPT in 2013 compared to the other
southern sites, which received more precipitation amounts than the CPT site,
could be prevalently influenced by regional/large-scale emission sources
during the sampling period. Measurements of atmospheric Hg deposition in BAR
have been carried out for the first time from 2014 to 2015. The BAR site has
been established inside a well-protected natural reserve in northern
Patagonia, on the shore of the Gutierrez River, southeast of the Nahuel Huapi
lake. GEM records at the BAR station resemble background concentrations
comparable to levels found in Antarctica and other remote locations of the
Southern Hemisphere with annual mean GEM concentrations of 0.9 ± 0.14 ng m-3
. The annual THg wet
deposition flux calculated at BAR in 2014 was very low
(0.1 µg m-2 yr-1); however, it is necessary to point out that the number of samples
carried out during the year was scarce (n = 91) and therefore less
representative than that recorded in 2015 and calculated over a number of
sampling days of nearly 50 % of the year. The 2015 THg wet deposition flux
was 0.5 µg m-2 yr-1, which is lower than those recorded
at most of the other southern GMOS sites.
Seasonal distribution of sampling-weighted fluxes (by 15-day
reference) at the European GMOS sites from 2011 to 2015. Each box includes
the median (midline), mean (▪), 25th and 75th percentiles (box
edges), 5th and 95th percentiles (whiskers), minimum (*) and
maximum (+).
Seasonal mean values distribution of sampling-weighted (by 15-day
reference) fluxes and rainfall at the European GMOS sites from 2011
to 2015.
Seasonal patterns and influence of meteorological conditions on Hg wet deposition
European stations
In this study, seasons are delineated according to the meteorological
definition. Since THg wet deposition flux depends on the total precipitation
amount and the concentration of total Hg in that precipitation, the seasonal
cycles of both these parameters are shown along with the cycles of Hg wet
deposition in Figs. –.
In particular, Figs. and show that seasonal trends
of THg in precipitation are clearly evident at all sites, with increased Hg
concentrations and deposition observed during spring and summer months at
most of them, implying a significant dependence on meteorological conditions
throughout the years. The seasonal variability in Hg concentrations and Hg
deposition has been reported in previous studies in North America
and Europe .
The warm month maximum in seasonal THg wet deposition is predominant at most
European GMOS sites (Fig. ) except at MHE, where the maximum THg
wet deposition occurs during the winter. However, the patterns of THg
concentrations and precipitation amounts reveal that, at most of the sites,
the seasonal THg wet deposition maximum corresponds to the maximum in
precipitation amounts collected, except at NYA, ISK, and LON (Fig. ).
Therefore, the dominant factor in determining the Hg wet
deposition loading recorded at all the European sites was generally related
to the amounts of precipitation collected. Hg concentrations in rainfall at
NYA peaked in spring and decreased through the summer, fall, and winter
seasons (Fig. ). Rainfall was fairly equally distributed in all
seasons except the winter season (Fig. ). Thus, wet Hg loading
was highest in spring, intermediate in winter and summer, and lowest in fall
(Figs. and ). High levels of soluble species could
in general be due to direct enhanced atmospheric oxidation of GEM to GOM,
which occurs in regions with high concentrations of oxidants, such as in polar
regions during springtime (where atmospheric mercury depletion events – AMDEs – such as NYA occur). At PAL, Hg
concentrations in rainfall increased through the winter, peaking in spring,
and decreased through the summer and fall (Fig. ). Rainfall was
not fairly equally distributed in all seasons but lowest values were recorded
during winter and spring and highest rainfall was observed in summer
(Fig. ). Thus, wet Hg loading was highest in summer and lowest in winter
(Fig. ). Similar behavior was observed at RAO (Fig. ),
whereas at MHE, wet Hg loading was highest in winter, when also the
highest rainfall amounts have been recorded, and the lowest in fall
(Figs. and ). At ISK, Hg concentrations in rainfall and wet
Hg loading peaked in summer and decreased in fall and winter, respectively
(Figs. and ), whereas rainfall was highest in fall
and lowest in winter (Fig. ). LON shows highest seasonal THg wet
deposition in summer and the lowest during spring. In this latter case, it is
necessary to point out that these results are related to one year (2013) in
contrast to the other sites in which all precipitation samples were grouped
and analyzed season by season for a period of 3 to 5 years. Among the
European sites, the highest THg wet deposition has been recorded at the
remote RAO and PAL stations during the more photochemically active summer
months, whereas lower amounts were found to be deposited in the colder months.
In addition, rainfall amount during summer seems to be identified as the
overriding factor controlling wet Hg loading at these sites. The lowest
concentrations and total wet deposition were seen in winter months at most of
sites. The seasonal pattern in the atmospheric Hg, with highest precipitation
concentrations and wet deposition typically seen in summer and lowest
concentrations and wet deposition in winter, was believed partly to be the
result of increased convection and mixing during the warmer summer months,
which can increase the ability of the air to transport Hg over longer
distances, leading to greater precipitation amounts that remove Hg from the
atmosphere. This may also indicate the role of precipitation type in the
amount of Hg wet deposition, as rain may have a greater capacity to scavenge
and hold different forms of Hg than snow. Higher Hg deposition, typically
observed during the warmer months, could be the result of a mix of
meteorological source emission and atmospheric chemistry influences. For
example, it is widely known that the concentrations of oxidants such as
ozone, OH radicals, and acids that oxidize GEM to GOM are higher during
warmer months and would lead to elevated concentrations of oxidized species
. Scavenging of soluble oxidized Hg species has
also been considered to be more efficient in summertime precipitation events
than in winter due to differences in the cloud microphysical processing
between rain and frozen precipitation .
Seasonal distribution of volume-weighted THg concentration in
precipitation at the three Chinese GMOS sites from 2011 to 2014. Each box
includes the median (midline), mean (▪), 25th and
75th percentiles (box edges), 5th and 95th percentiles (whiskers),
minimum (*), and maximum (+).
Chinese stations
China has been regarded as one of the largest atmospheric Hg emission sources
region in the world . However, limited monitoring
sites and data are available to understand Hg deposition patterns in China.
Few previous measurements of THg deposition in China have been conducted in
remote areas like Mt. Fanjing , Mt. Leigong ,
Wujiang River basin , and Mt. Gongga
in southwestern China, as well as at MCH in northeastern
China. In order to evaluate the spatial and temporal distribution of THg at
the three GMOS Asian stations, all measurements performed from 2011 to 2014
at MCH, MWA, and MAL were grouped by season and by site
(Figs. –). Seasonal variations of THg in precipitation
were observed at the three Chinese sites (Fig. ). The results
obtained during the sampling period were similar to the seasonal variations
of THg in precipitation in other Chinese regions, such as in the Wujiang River
basin, Guizhou, China, but in contrast to the observations in North America
, the Adirondacks , and the Great Lakes region
, which found increased THg concentration during summer
months . Geographic differences in Hg wet deposition
worldwide may be explained in part by the proximity to atmospheric sources
and regional difference in anthropogenic emission sources.
Seasonal distribution of sampling-weighted (by 15-day reference)
fluxes at the three Chinese GMOS sites from 2011 to 2014. Each box includes
the median (midline), mean (▪), 25th and 75th percentiles (box
edges), 5th and 95th percentiles (whiskers), minimum (*), and
maximum (+).
Seasonal mean values distribution of sampling-weighted (by 15-day
reference) fluxes and rainfall at the three Chinese GMOS sites from 2011
to 2014.
Atmospheric Hg species, in particular GEM and PBM, have been found to be
substantially increased over recent years in both remote and urban areas of
China, especially in central and eastern China, compared to those observed in
North America and Europe which reported opposite long-term trends
. The increasing trend in China is possibly caused by the
increase in anthropogenic Hg emissions in the past decade and indicates that
the influence of regional emissions on Hg levels in China exceed global
emission influence ( and references therein). The
seasonal variation of weighted THg concentration, observed in precipitation
with the highest value in winter and lowest in summer (see Fig. ),
could be attributed in a first instance to lower rain amounts collected in
winter (Fig. ). The results obtained at the three Chinese sites
show in fact that the THg concentrations varied with rain amount. In
particular, at MCH, THg concentrations slightly increased in autumn, peaked
during the winter season, and decreased during spring and summer when the
lowest values were recorded. The reverse trend has been observed in the
precipitation amount, with the highest value observed in summer and the lowest
in winter (Fig. ). THg wet deposition trend is comparable to
that of the precipitation amount, with values of THg flux increasing from
winter and peaking in summer (Fig. ). Ruling out the winter
season at MWA during which very few rainy samples have been collected (and is thus
not representative for the present discussion) weighted THg concentrations
peaked in fall with lowest values in spring. Therefore, on average, wet Hg
loading was highest in spring and lowest in summer. At MAL, the rainy samples
show a fairly seasonal variability during all seasons, with the lowest rainfall in
winter and the highest in summer (Fig. ), while THg
concentrations showed high values in winter and lowest in fall, and wet Hg
loading was highest in summer, and lowest values were recorded in winter.
highlight significant positive correlations between rainwater
THg concentrations and PBM and GOM concentrations, resulting in positive
correlations between wet deposition fluxes and PBM and GOM concentrations.
This has been explained by the authors with the washout process of PBM and
GOM during rain events which could contribute to enhancing Hg wet deposition in
China, particularly in urban areas where PBM and GOM concentrations are much
higher. Wet deposition is, in fact, commonly distinguished in terms of
in-cloud and below-cloud washout and involves oxidized mercury forms (GOM,
PBM). Gaseous Hg0 does not undergo direct scavenging by precipitation
because of its low solubility, but it can be washed out indirectly through
dissolution and oxidation in cloud water. In remote areas of China, however,
washout of elevated atmospheric PBM does not seem to drive a notable increase
in Hg wet deposition flux, probably due to the low washout rate of PBM during
rain events at high-altitude monitoring sites, such as MAL and MWA where
low-level clouds reduced the contribution of Hg washout .
, in a previous study in Guizhou on Hg in
precipitation, also pointed out that maximum THg concentrations in rainy
samples during cold seasons may be related to coal burning in domestic
activities. Similar conclusions have also been reported in a study performed
by at three Chinese sites (urban, residential and
near-remote sites) in the Chongqing province from 2010 to 2011, where they also
found a high correlation between THg and particulate Hg (PBM) concentrations,
suggesting that THg concentration in precipitation may be influenced by the
PBM concentration. Additionally, comparable seasonal behavior of Hg
concentrations in precipitation with our results have been also observed, but
with annual mean THg concentrations (ng L-1) significantly higher than those
observed at the MCH, MWA, and MAL sites which are located in remote Chinese
areas. The seasonal pattern in deposition flux observed at the remote MCH,
MAL, and MWA are comparable with those observed at remote sites in Europe and
North America ,
with maximum values during warmer months. It was suggested by
and that this annual maximum was mainly
due to more effective scavenging by rain in summer than by snow in the cold
season . Hg is not
incorporated into cold cloud precipitation as efficiently as it is into warm cloud
precipitation . Other explanations for this observation
have also been addressed by the authors, including a greater availability of
soluble Hg due to convective transport in summer events
and a summer increase in Hg-containing soil-derived particles in the atmosphere .
Seasonal distribution of volume-weighted THg concentration in
precipitation at the tropical GMOS site (Sisal, Mexico) in 2013 and 2014.
Each box includes the median (midline), mean (▪), 25th and
75th percentiles (box edges), 5th and 95th percentiles (whiskers),
minimum (*), and maximum (+).
Tropical station: Sisal, Mexico
Hg deposition measurements are rare in tropical latitudes, with very few
scientific publications in the past decade ( and
references therein). The tropics are a particularly important region
with regard to global atmospheric chemistry. Due to intense ultraviolet radiation
and high water-vapor concentrations, high OH concentrations oxidize inorganic
and organic gases, and induce an efficient removal from the atmosphere of the
oxidized products ( and references therein). Strong
convective events in the tropical regions lead to huge volumes of air being
drawn out of the subcloud layer with the resultant chemical composition of
the precipitation coming from the capture of gases and small particles by the
liquid phases of cloud and rain. Hg deposition measurements started in Mexico
at Celestùn station (CST) in 2012 (see Table ), but after a short
time period of sampling, the monitoring station changed the location with
SIS; therefore, we refer the discussion to the SIS data related to both
2013 and 2014 during which sufficient precipitation samples have been
recorded (Figs. –). Despite
receiving unpolluted air off the Atlantic Ocean from northeasterly and
southeasterly trade winds, during most of the years , the
site recorded higher wet Hg deposition fluxes during summer and fall compared
to those observed during the other seasons (Fig. ). The SIS
high Hg deposition rates, comparable to other sites in the Northern
Hemisphere, such as the Chinese sites (i.e., MCH) or European sites (i.e.,
ISK) that sometimes are also impacted by anthropogenic emissions, are driven
in part by high rainfall events more intensely during summer and fall, and less
during winter and spring periods. The high wet Hg deposition flux at this site
suggests that other tropical areas may be hotspots for Hg deposition as well.
A number of studies have suggested that this could be due to higher
precipitation and the scavenging ratios from the global pool in the
subtropical free troposphere where high concentrations of oxidized Hg
species exist . These findings
were also highlighted in previous studies in the south of Florida and the Gulf of
Mexico coastal areas, confirming that local and regional Hg emissions play
only a minor role in wet Hg deposition
and suggesting that the primary source of scavenged oxidized Hg could be the global pool.
Seasonal distribution of sampling-weighted (by 15-day reference)
fluxes at the tropical GMOS site (Sisal, Mexico) in 2013 and 2014. Each box
includes the median (midline), mean (▪), 25th and
75th percentiles (box edges), 5th and 95th percentiles (whiskers),
minimum (*), and maximum (+).
Seasonal mean values distribution of sampling-weighted (by 15-day
reference) fluxes and rainfall at the tropical GMOS site (Sisal, Mexico)
in 2013 and 2014.
Seasonal distribution of volume-weighted THg concentration in
precipitation at the four GMOS sites in the Southern Hemisphere from 2012
to 2015. Each box includes the median (midline), mean (▪),
25th and 75th percentiles (box edges), 5th and 95th percentiles (whiskers),
minimum (*), and maximum (+).
Weather patterns in SIS exhibit a seasonality in annual rainfall, with
highest rainfall from June/July through October/November. Summer tropical
waves and systems characterized by deep convection and low pressure produced
greater rainfall. During summer and fall, the site indeed receives rainfall
from deep convection associated with tropical waves embedded in the
prevailing easterly airflow. THg concentrations were higher in low volume
samples. With larger storms, Hg concentrations were diluted; this means that
rainout of Hg was maximum (the decreasing of Hg concentrations with the
increasing of the rainfall depth). Weighted THg concentrations in rainfall
(ng L-1) peaked in winter and decreased through the spring and summer
(Fig. ). THg in wet deposition was highest in summer and lowest
in spring and winter (Fig. ). The higher summer Hg deposition
flux is not driven by higher Hg concentrations in rainfall since the highest
Hg concentrations in rain samples occurred in winter. Different mechanisms
leading to enhanced Hg concentrations in rain during the winter, including
greater anthropogenic emissions, are probably associated with higher use of
fossil fuels in power plants during the cold season. As reported in
Sect. , relating to the annual wet deposition patterns, the
THg wet deposition observed at SIS could also be influenced by air masses
crossing particularly in winter the southern United States and southern
Florida where several coal power plants and waste incinerators
are located. The high wet deposition of Hg during the
rainy seasons (May/June to October/November), in contrast, could be due to
more efficient scavenging processes of reactive gaseous mercury from the free
troposphere by tall convective thunderstorms, and the concentration of GOM by
the sea breeze effect, where the diurnal alternation of onshore and offshore
winds can lead to a buildup of pollutants in the air mass. Greater
information on Hg deposition and cycling is needed in tropical regions, where
populations are more likely to be exposed to Hg through fish consumption and
artisanal gold mining activity.
Seasonal distribution of sampling-weighted (by 15-day reference)
fluxes at the four GMOS sites in the Southern Hemisphere from 2012 to 2015.
Each box includes the median (midline), mean (▪), 25th and
75th percentiles (box edges), 5th and 95th percentiles (whiskers),
minimum (*), and maximum (+).
Seasonal mean values distribution of sampling-weighted (by 15-day
reference) fluxes and rainfall at the four GMOS sites in the Southern
Hemisphere from 2012 to 2015.
Southern Hemisphere stations
In remote areas far from any local sources, atmospheric deposition has been
recognized as the main source of Hg to the ocean .
Hg can then be re-emitted back to the atmosphere via gas
exchange, and modeling studies suggest that re-emission from oceans is a major
contributor to atmospheric concentrations of GEM, particularly in the
Southern Hemisphere where oceans were shown to contribute more than half of
the surface atmospheric concentration ( and references
therein). In the Southern Hemisphere, we considered the four monitoring sites,
AMS, CPT, CGR, and BAR, which recorded a
representative number of samples over the 2012–2015 period. Figures
and show the box plots related to rainfall, THg
concentrations in precipitation, as well as wet deposition flux of Hg
recorded, whereas Fig. shows the mean values of rainfall amounts with
the corresponding mean values of Hg fluxes at the four southern sites. An
NSA-171 (Eigenbrodt) collector was set up at AMS at the beginning of the 2013.
The GMOS site experienced a mild oceanic climate with monthly median
air temperature ranging from 11 ∘C in austral winter to 17 ∘C in
austral summer and frequent presence of clouds . In 2013
and 2014, AMS displayed a seasonal variation of the precipitation amounts, with
the highest values collected during the winter season (Fig. ).
On the contrary, the THg wet deposition flux patterns did not show a similar
variation throughout the seasons as well as the THg concentrations in
precipitation samples (Figs. and ). At CPT, the Hg
concentrations in precipitation, Hg wet deposition fluxes as well as the
precipitation amounts, followed the same trend during the rainy season (May–October),
with a maximum in wintertime for all the parameters recorded.
CPT experiences a Mediterranean-type climate that is characterized by rather
dry summers comprising moderate temperatures. The austral autumn to spring
season normally experiences increased precipitation due to the passage of cold
fronts moving from west to east; therefore, CPT generally receives clean
marine air from the Atlantic Ocean, whereas continental and polluted air
masses are observed at the site more frequently, mainly during the winter
period , due to the prevailing air masses from
the north to northwestern sector . The
highest THg concentrations and wet deposition fluxes recorded during the
winter season could be due also to the contribution of polluted air masses
crossing the Cape Town metropolitan area before arriving at the stations.
However, a more recent study on GEM concentrations and THg in precipitation
carried out by over a period of 7 years (2007–2013)
highlighted that GEM, THg, CO, and 222Rn levels within the urban–marine events
observed at CPT did not substantially differ from those seen in the marine
rain episodes, concluding that no significant local anthropogenic influences
were detected on THg concentrations. Conversely, a significant positive
correlation was found at CPT between GEM and THg concentrations, and with the
Southern Oscillation Index (SOI), suggesting that both GEM and THg
concentrations are primarily influenced by large-scale meteorology which in
turn controls Hg emission sources in terms, for example, of enhanced sea
surface temperature that could increase large-scale droughts leading to
raised biomass burning .
Measurements of atmospheric Hg deposition in Australia have never been
reported before . From 2013 to 2015, at the CGR GAW station,
located on the northwestern coast of Tasmania, Australia, highest value in
rainfall have been observed during winter an lowest in summer, whereas Hg
concentrations peaked in summer and dropped to lowest values in winter (see
Figs. and ). Indeed, an increase in precipitation
volume results in a decrease in Hg concentrations in rain, probably due to
the dilution of the washout loading . This means that any
change in meteorological conditions, especially precipitation, complicates the
interpretation of GMOS observations at different latitudes and might mask any
trends due to changes in Hg emissions. The trend of Hg wet deposition fluxes
shows a seasonal variability with highest values in spring and lowest in cold
seasons. At BAR, the highest precipitation amounts in 2014 and 2015 were
collected during the fall and winter seasons and decreased in spring when the
highest THg concentrations occurred (see Figs. and ).
Therefore, the seasonal THg wet deposition peaked in spring
and decreased during the cold seasons (Fig. ). It is necessary
to point out, however, that in both 2014 and 2015 at BAR very few samples
have been recorded in fall and summer (Tables S1–S4). This means
that further measurements and studies are needed to draw any conclusion and
improve our understanding of deposition processes and oxidation mechanisms in
this region. There are very few previous observations of Hg wet deposition in
the Southern Hemisphere, and this makes difficult any comparison of data
recorded during GMOS. The results observed at the four southern GMOS sites
highlighted that the magnitude of wet deposition is affected by two main
factors: amount of precipitation and the THg concentration in precipitation
influenced by soluble Hg species (oxidized Hg) in the atmosphere. High levels
of soluble species could in general be due to direct anthropogenic emissions
of Hg oxidized species or by enhanced atmospheric oxidation of GEM to GOM,
which occurs in regions with high concentrations of oxidants such as southern
locations (where more solar radiation occurs) or polar regions during
springtime (where AMDEs occur).
Conclusions
Mercury deposition measurements are critical for constructing an accurate
global Hg budget and modeling the benefits or consequences of changes in Hg
emissions, for example, as prescribed by the Minamata Convention. The scarce
availability of long-term wet Hg deposition data for calibration or
validation of models could cause uncertainties in modeling applications to
assess the influence of local emission sources. A synthesis of available Hg
measurements in precipitation from selected GMOS ground-based sites is
presented, including trends and seasonal cycles. Wet deposition samples were
collected for approximately 5 years, from 2011 to 2015, at 17 selected
GMOS monitoring sites located in the Northern and Southern hemispheres, as
well as in the tropical area. In the Northern Hemisphere, and specifically at
the European stations, a geographical trend with an increase in THg wet
deposition from north to south has been observed. These findings are in good
agreement with the geographical distribution of atmospheric Hg data obtained
during the same period within the GMOS network with a downward gradient from
the Northern to the Southern Hemisphere. At the other GMOS monitoring sites
in the Northern Hemisphere (i.e., Chinese sites), as well as those at lower latitude
(i.e., tropical area and Southern Hemisphere) no north–south spatial trend
has conversely been observed. Annual and seasonal patterns in Hg wet
deposition are clearly evident at all GMOS sites, implying a significant
dependence on meteorological conditions throughout the years. Most of the
ground-based sites report, in particular, Hg deposition strongly influenced
by the precipitation amounts. In the Northern Hemisphere, interannual
differences in THg wet deposition are mostly linked with precipitation
volume, with the greatest deposition flux occurring in the wettest years,
whereas at the sites located at lower latitude and in the Southern Hemisphere the
relationship between precipitation amount and deposition was not as evident
as in the north. It is however necessary to point out the need to expand the global
network particularly in the tropics and Southern Hemisphere regions in order
to provide more information throughout long-term monitoring activities. As a
starting point of a global network, these results provide a set of data for
modeling applications to fully understand THg wet deposition patterns as well
as the transformation and deposition mechanisms of atmospheric Hg. With broad
geographic coverage, including background and remote sites as well as local or
regional sources, GMOS's observation network gives important insights to
modeling applications to evaluate future Hg trends and their fate and transport
on a global scale. The results of THg wet deposition carried out in this study
open the way for new avenues in future modeling studies as well as highlight
the need for additional and integrated measurements in ambient air and
rainwater samples to improve our understanding of deposition processes and
oxidation mechanisms. These new observations, in fact, give scientists and
modelers some insight into baseline concentrations of THg concentrations in
precipitation and depositional fluxes especially in the tropical area, and in
the Southern Hemisphere where wet deposition as well as atmospheric Hg
species were not investigated before. Greater information on Hg deposition
and cycling is obviously needed in these regions. Moving forward, in addition
to continued monitoring GMOS sites, integration with other ground-based
monitoring sites at strategic locations, along with integrations with
atmospheric Hg species and other key oxidants, and identification of the
compounds making up GOM and PBM2.5 continue to be needed. Knowledge of
these exact chemical species would also lead to improved understanding of the
chemistry and wet and dry deposition processes of oxidized Hg species in
different air masses. These and other uncertainties are the subject of
ongoing research. The magnitude of Hg dry deposition is to date uncertain,
especially dry deposition of GEM, and few measurements are available to
constrain model estimates. Further measurements of dry deposition, especially
in locations where wet deposition measurements are available, would
dramatically improve scientific understanding of the Hg cycle. Wet deposition
measurements worldwide would assist modelers in constraining the atmospheric
Hg budget on a global scale, as would additional direct measurements of dry
deposition across the GMOS network.