Northern–southern hemispheric gradients
A summary of descriptive statistics based on monthly and annual averages from
all GMOS sites is presented in Tables S1 and S2 in the Supplement. The 2013
and 2014 annual mean concentrations of 1.55 and 1.51 ng m-3,
respectively, for the sites located in the Northern Hemisphere were calculated
by averaging the 13 site means for both years. Similar calculations were made
for the Southern Hemisphere and the tropics (see Tables S1 and S2). Annual
mean concentrations of 1.23 and 1.22 ng m-3 for 2013 and 2014,
respectively, were obtained in the tropical zone and 0.93 and
0.97 ng m-3 for the Southern Hemisphere. Figure shows the
GEM yearly distribution for 2013 (blue) and 2014 (green). The sites have been
organized in the graphic as well as in the tables according to their latitude
from those in the Northern Hemisphere to those in the tropics and in the
Southern Hemisphere. The data so far do not cover a long enough timespan
to investigate temporal trends, but some attempts have been previously
made for the more established sites, such as Mace Head (MHD), Ireland
, and Cape Point (CPT), South Africa
. At MHD the annual baseline GEM means observed by
decreased from 1.82 ng m-3 at the start of the
record in 1996 to 1.4 ng m-3 in 2011, showing a downwards trend of
1.4–1.8 % per year. Both a downward trend of 1.6 % at MHD from 2013
and 2014 and the slight increase in Hg concentrations seen by
at CPT from 2007 to 2013 continued through the end of
2014. Some debate remains as to whether anthropogenic emissions are increasing
or decreasing . A clear
gradient of GEM concentrations between the Northern and Southern hemispheres
is seen in the data for both 2013 and 2014, in line with previous studies
.
The 13 northern sites had significantly higher median concentrations than
the southern sites did. The north–south gradient is clearly evident in Fig. where the probability density
functions (PDFs) of the data are reported. The datasets have been divided into three
principal groups related to the latitude: north samples, tropical samples and
south samples. The histograms, normalized to the unit area, have been
constructed following the Scott rule for the bin width ΔW: ΔW=3.5σ/n3, where σ represents the standard deviation
and n the number of samples. This choice is optimal when dealing with normal
distributed samples since it minimizes the integrated mean squared error of
the density estimate and is then fitted through a normal distribution (full line in
Fig. ), obtained through the classical maximum likelihood
estimation method. Since a clear overlap can be observed between the three
datasets presented in Fig. , in order to make the
distinction between the distributions clear we perform the standard Student t test
against the null hypothesis (h0) that the three distributions come from
the same mother distribution with the same mean (μ0) and unknown
standard deviation (σ0).
For every case the null hypothesis (h0) can be rejected, as the means of
the three distribution are significantly different, with a 99 % confidence
level. If XN, XS and XT are the mean of the experimental
measures,
respectively, for the northern, southern and tropical groups, the confidence
intervals evaluated from the t test are reported in Table . The
interpretations of the results clearly demonstrate that XN>XT>XS
(Table ), so that a significant gradient exists in the GEM
concentrations from the Northern Hemisphere to the Southern Hemisphere. Due to
the significant difference in the PDFs, the probability p (p value) of
observing a test statistic as extreme as, or more extreme than, the observed
value under the null hypothesis is close to zero. Thus the validity of the
null hypothesis should be rejected. The spatial gradient observed from
northern
to southern regions is highlighted in both Figs. and
, which also report the statistical monthly distribution of GEM
values obtained for 2013 and 2014, respectively, at all GMOS sites in the
Northern and Southern hemispheres as well as in the tropical area.
Probability density functions (PDFs) of the GEM data (ng m-3)
for the northern, southern and tropical sample groups (dash dotted lines).
Full lines the normal distribution fit of the samples.
![]()
The mean (X) of the experimental measures for the
northern (XN), southern (XS) and tropical
(XT) groups and the confidence intervals evaluated from the
Student t test among them.
Difference between
Minimum of the
Maximum of the
means
confidence interval
confidence interval
XN-XS
0.590
0.592
XN-XT
0.225
0.229
XT-XS
0.362
0.365
Monthly statistical distribution and spatial gradient for 2013 year
from Northern Hemisphere to Southern Hemisphere.
![]()
Monthly statistical distribution and spatial gradient for 2014 from
Northern Hemisphere to Southern Hemisphere.
![]()
Seasonal pattern analysis in the Northern Hemisphere
Statistics describing the spatial and temporal distribution of GEM
concentrations at all GMOS sites for 2013 and 2014 are summarized in
Fig. whereas Figs. and show the monthly
statistical GEM distribution for both years considered. The GEM
concentrations highlight that the mean GEM values of most of the GMOS sites
were between 1.3 and 1.6 ng m-3, with a typical interquartile range of
about 0.25 ng m-3. Only a few sites have shown a mean values above
1.6 ng m-3, such as MCH, MIN and MAL, and only the EVK site, located
at 5050 m a.s.l. in the Eastern Himalaya of Nepal, reported mean
values below 1.3 ng m-3. This value is comparable with free
tropospheric concentrations measured in August 2013 over Europe
. The mean GEM concentration observed at EVK is less
than the reported background GEM concentration for the Northern Hemisphere
(1.5–1.7 ng m-3) and more similar to expected background levels of
GEM in the Southern Hemisphere (1.1–1.3 ng m-3) . The values between 1.3 and 1.6 ng m-3 observed at the
other GMOS sites in the Northern Hemisphere are comparable to the
concentrations measured at the long-term monitoring stations at Mace Head,
Ireland , and Zingst, Germany
. GEM concentration means are also in good agreement with the
overall mean concentrations observed at multiple sites in the Canadian
Atmospheric Mercury Measurement Network (CAMNet) (1.58 ng m-3)
reported by and those reported from Arctic stations in this
paper (VRS, PAL). Seasonal variations of GEM concentrations have also been
observed at all GMOS sites in the Northern Hemisphere. Most sites show higher
concentrations during the winter and spring and lower concentrations in
summer and fall seasons (Figs. and ). However, few
sites such as VRS, Station Nord (northeastern Greenland:
81∘36′ N, 16∘40′ W), show a slightly different seasonal
variation. In winter this high Arctic site (VRS) is sporadically impacted by
episodic transport of pollution mainly due to high atmospheric pressure
systems over Siberia and low pressure systems over the North Atlantic
. During the spring (April–May) and summer
(August–September) seasons GEM concentrations show a higher variability with
low concentrations near the instrumental detection limit due to episodic
atmospheric Hg depletion events (AMDEs) that occur in the spring
and high GEM concentrations (2 ng m-3) in June and
July, probably due to GEM emissions from snow and ice surfaces
and Hg evasion from the Arctic Ocean
. Models of the MBL
that simulate the temporal variations of Hg species show that photo-induced oxidation
of GEM by Br can reproduce the diurnal variation of GOM observed in the MBL
during cruise measurements better than other oxidation candidates
and also the seasonal variation
. Although Br is currently considered to be the
globally most important oxidant for determining the lifetime of GEM in the
atmosphere, there are also other possible candidates that can enhance Hg
oxidation . The lack of a
full understanding of the reaction kinetics and fate of atmospheric Hg
highlights the need to have a global observation system as presented here in
order to calibrate and constrain atmospheric box and global/regional-scale
models .
GMOS sites in Asia
As can be seen in Fig. , the group with the highest GEM median
variability and maximum concentrations is in Asia, which includes the following
sites: Mt. Ailao (MAL), Mt. Changbai (MCH), Mt. Waliguan (MWA) and Minamata
(MIN), where 95th percentile values ranged from 3.26 to 2.74 ng m-3 in
2013 (Table S2). These sites are often impacted by air masses that have
crossed emission source regions . GEM concentrations
recorded at all remote Chinese sites (MAL, MCH and MWA) are elevated
compared to that observed at background/remote areas in Europe and North
America, and at others sites in the Northern Hemisphere . A previous study by at MWA suggested that
long-range atmospheric transport of GEM from industrial and urbanized areas
in northwestern China and northwestern India contributed significantly to
the elevated GEM at MWA. MAL station is located in Southwest China, at
the summit of Ailao Mountain National Nature Reserve, in central Yunnan
province. It is a remote station, isolated from industrial sources and
populated regions in China. Kunming, one of the largest cities in
Southwest China, is located 180 km to the northeast of the MAL site. The
winds are dominated by the Indian summer monsoon (ISM) in warm seasons (May
to October), and the site is mainly impacted by Hg emission from eastern
Yunnan, western Guizhou and southern Sichuan of China and the northern part
of the Indochinese Peninsula. In cold seasons the impact of emissions from
India and northwestern part of the Indochinese Peninsula increased and
played an important role in elevated GEM observed at MAL .
However, most of the important Chinese anthropogenic sources of Hg and other
air pollutants are located to the north and east of the station, whereas
anthropogenic emissions from southern and western Yunnan province are fairly
low . Average atmospheric GEM
concentrations during this study calculated for MWA and MAL during 2013 and
2014 are in good agreement with those observed during previous measurements
at both sites from October 2007 to September 2009 at MWA and from
September 2011 to March 2013 at MAL . Also the
overall mean GEM concentration observed in 2013 and 2014 at MCH background
air pollution site (1.66 ± 0.48 ng m-3 in 2013 and
1.48 ± 0.42 ng m-3 in 2014, respectively) is in good agreement
with the overall mean value recorded earlier from 24 October 2008 to
31 October 2010 (1.60 ± 0.51 ng m-3, Fu et al.,
2012b). highlighted a higher mean
TGM concentration of 3.58 ± 1.78 ng m-3 observed from
August 2005 to July 2006 that was probably due to surface winds circulation with
effect of regional emission sources, such as the large iron mining district
in the
northern part of North Korea and two large power plants and urban areas to
the southwest of the sampling site.
In summary, the observed concentrations are a function of site location
relative to both natural and anthropogenic sources, elevation and local
conditions (i.e., meteorological parameters), often showing links to the
patterns of regional air movements and long-range transport. Seasonal
variations at ground-based remote sites in China have been observed. At MCH
GEM was significantly higher during cold seasons compared to that recorded in
warm seasons (from April to September) whereas the reverse has been observed
at the other two Chinese GMOS sites.
In order to statistically check the difference of GEM concentrations among
the three Chinese sites an alternative statistical test has been performed,
since in this case the distributions are strongly non-normal.
As in the previous case we construct the unit-area histogram, then we fit
with a log-normal distribution. It is worth noting that in this case the
histograms has been constructed by manually setting the bin width ΔW.
With this choice the total number of bins can be evaluated as
n=(Xmax-Xmin)/ΔW=61.
By looking at Fig. , is easy to notice that the skewness
(μ3/σ3∼2 where μ3 is third-order moment of the
distribution and σ is the standard deviation) and the kurtosis
(μ4/μ22∼10 where μi is the ith-order moment of the
distribution) are far from being zero. In the following the alternative is
briefly described. Let us consider a pair of our three time series, namely
Xi (i=1,2), which corresponds to independent random samples described
by the log-normal distributions. Then the random variables Yi=ln(Xi)
are close to normal distribution with
means μi and variances σi2, namely Yi∼N(μi,σi2).
Probability density functions (PDFs) of the GEM data (ng m-3)
for the Chinese sample groups (dash dotted lines). Full lines the log-normal
distribution fit of the samples.
![]()
Since ηi=exp(μi+0.5σi2) is the expectation value for
Xi, the problem of our interest is then to test the null hypothesis on
η2-η1. More formally, we test H0:θ≤0, where θ=η2-η1. In other words, we test the null hypothesis to see if there is a
significant difference in the sample means. Using the algorithm described in
and , specifically designed to
perform the inference on difference of means of two log-normal distributions, we obtain the estimates for the p-values which are close to 1 and the
confidence intervals, calculated at a confidence level of 95 % (reported in Table ).
From the statistical results we can conclude that a clear distinction exists
between the MWA site and the other two (MCH, MAL) as shown from the values in
Table . However, despite the large overlap in the samples
distributions of MCH and MAL the difference in their ηi (ηMCH
and ηMAL, respectively) is also significant, with a smaller confidence
interval.
Differences between the ηi obtained for MCH, MWA and MAL,
confidence intervals and associated p values.
Difference of
Minimum of the
Maximum of the
P
ηi
confidence interval
confidence interval
ηMCH-ηMWA
0.285
0.286
1
ηMCH-ηMAL
0.043
0.043
1
ηMWA-ηMAL
0.328
0.329
1
Several hypothesis have been made to explain the seasonal variations of GEM
in China, including seasonal changes in anthropogenic GEM emissions and
natural emissions. The seasonal emission changes mainly resulted from coal
combustion for urban and residential heating during cold seasons. This source
lacks emission control devices and releases large amounts of Hg, leading to
elevated GEM concentrations in the area and thus at MCH . Conversely, GEM at MAL and MWA was higher in warm
seasons than in cold seasons. These findings highlight that emissions from
domestic heating during the winter could not explain the lower winter GEM
concentrations observed at MWA and MAL, but there might be other
not-yet-understood factors that played a key role in the observed GEM
seasonal variations at these sites, such as the monsoonal winds influence
which can change the source–receptor relationship at observational sites and
subsequently the seasonal GEM trends . Among the remote
Chinese sites, MAL started as secondary site and in 2014 was upgraded to a
master site; conversely, MWA started as a master site and then became a
secondary site whereas MCH operated continuously as a master site. Therefore,
PBM and GOM concentrations have been measured during the years 2013 and 2014
at all Chinese sites even if not continuously (see Fig. for Hg
speciation data coverage). The GOM and PBM concentrations measured at these
sites were substantially elevated compared to the background values in the
Northern Hemisphere, from 1.8 to 42.8 pg m-3 and from 40.4 to 167.4 pg m-3 at the MCH and MWA, respectively, in 2013. The 2014 PBM maxima
were 44.2 and 45.0 pg m-3 at MCH and MAL, respectively. Regional
anthropogenic emissions and long-range transport from domestic source regions
are likely to be the primary causes of these elevated values
. Seasonal variations of PBM observed at the Chinese master
sites mostly showed lower concentrations in summer and higher concentrations
(up to 1 order of magnitude higher) in winter and fall . The higher PBM in
winter was likely caused by direct PBM emissions, formation of secondary
particulate Hg via gas–particle partitioning and a lack of wet scavenging
processes . PBM has an atmospheric
residence time ranging from a few hours to several days and can therefore be
transported to the remote sites when conditions are favorable
. Atmospheric PM pollution is of special
concern in China due to the spatial distribution of anthropogenic emission
concentrations of PM2.5 in heavily populated areas of eastern and northern
China, which are among the highest in the world . The GOM
concentrations observed at both master sites show high variability and
several episodes with high GOM values were probably due to local emission
sources (such as domestic heating in small settlements) rather than to
long-range transport from industrial and urbanized areas . GOM
has a shorter atmospheric residence time that limits long-range transport
. However, with low RH and high winds, the
possibility of regional transport of GOM cannot be ruled out. For example,
the observations at MWA exhibit a number of high GOM events related to air
plumes originating from industrial and urbanized centers that are about 90 km
east of the sampling site . MWA is a remote
site situated at the edge of the northeastern part of the Qinghai–Xizang
(Tibet) plateau. The monitoring station is relatively isolated from
industrial point sources and there are no known local Hg sources around the
site. Most of the Chinese industrial and populated regions associated with
anthropogenic Hg emissions are situated to the east of MWA. Predominantly
winds are from the west to southwest in cold seasons and the east in warm
seasons . East Asia is, in fact, the largest Hg source
region in the world, contributing to nearly 50 % of the global anthropogenic
Hg emissions to the atmosphere .
Seasonal pattern analysis in the Southern Hemisphere
For the sites located in the Southern Hemisphere, the GEM concentrations
highlight that the mean GEM values ranged between 0.84 and
1.09 ng m-3, in both 2013 and 2014, with a typical interquartile range
of about 0.25 ng m-3 (see Figs. , and
). The mean GEM concentrations observed at the southern sites are
lower than those reported in the Northern Hemisphere but in good agreement
with the southern hemispherical background (1.1 ng m-3)
and the expected range for remote
sites in the Southern Hemisphere. As in the Northern Hemisphere, a seasonal
variation of GEM concentrations was observed in the Southern Hemisphere. In
particular, GEM concentrations from the coastal Global Atmosphere Watch
station, Cape Point (CPT), South Africa, show seasonal variations with maxima
during austral winter and minima in summer. The site is located in a nature
reserve at the southernmost tip of the Cape Peninsula on a hill,
230 m a.s.l. It is characterized by dry summers with moderate temperatures
and increased precipitation (cold fronts) during austral winter. During the
summer months, biomass burning events sometimes occur within the
southwestern Cape region, affecting GEM levels. The dominant wind direction
at CPT is from the southeastern sector, advecting clean maritime air from the
South Atlantic Ocean which occurs primarily
during austral summer (December till February). Furthermore, the station is
also at times subjected to air from the northern sector, mainly during
austral winter. During such continental airflow events, anthropogenic
emissions from the industrialized area in Gauteng, 1500 km to the northeast
of CPT, can sometimes be observed . The GEM
seasonal variability at CPT is hence in good agreement with the prevailing
climatology at the site. Also GEM data at Amsterdam Island followed a similar
trend, with slightly but significantly higher concentrations in winter
(July–September) than in summer (December–February). Amsterdam Island is a
remote and very small island of 55 km2 with a population of about
30 residents, located in the southern Indian Ocean at 3400 and 5000 km
downwind from the nearest lands, Madagascar and South Africa, respectively
. GEM concentrations at AMS were remarkably steady with an
average hourly mean concentration of 1.03 ± 0.08 ng m-3 and a
range of 0.72–1.55 ng m-3. A small seasonal cycle has been observed
by and despite the remoteness of the island, wind sector
analysis, air mass back trajectories and satellite observations suggest the
presence of a long-range contribution from the southern African continent to
the GEM regional/global budget from July to September during the biomass
burning season extended from May to October . The higher GEM
concentrations at AMS are comparable with those recorded at Calhau (Cabo
Verde), Nieuw Nickerie (Paramaribo) and Sisal (Mexico) in the tropical zone,
whereas the lower concentrations of GEM observed, less than 1 ng m-3,
were associated with air masses coming from southern Indian Ocean and the
Antarctic continent. Bariloche (BAR) master site in North Patagonia also
shows higher concentrations during the austral winter (from end of May to
September) and lower concentrations in other seasons .
The Patagonian site has been established inside Nahuel Huapi National Park, a
well-protected natural reserve, located east of the Patagonian Andes.
The area is included in the Southern Volcanic Zone (SVZ) of the Andes, under
the influence of at least three active volcanoes with high eruption frequency
located at the west of the Andes cordillera . The climate of
the region is influenced by the year-round strong westerly winds blowing from
the Pacific which discharge the humidity in a markedly seasonal way
(fall–winter) in the western area of the park. GEM records at BAR station
show background concentrations comparable to those found in Antarctica and
other remote locations of the South Hemisphere with concentrations ranging
between 0.2 and 1.3 ng m-3, with an annual mean of
0.89 ± 0.15 ng m-3. Previous records of GEM concentrations from
a short-term survey in 2007 along a longitudinal transect across the Andes
with Bariloche as the eastern endpoint reported concentrations below
2 ng m-3 close to BAR . In this survey, the
highest GEM concentrations were recorded in the proximity and downwind from
the volcanic area, reaching concentrations up to 10 ng m-3
. Similarly to the seasonal trends at other GMOS sites in
the Southern Hemisphere, GEM concentrations were at their lowest level in
summer on the Antarctic Plateau at Concordia Station (DMC, altitude 3220 m)
but at their highest level in fall . GEM concentrations
reached levels of 1.2 ng m-3 from mid-February to May (fall) likely
due to a low boundary layer oxidative capacity under low solar radiation
limiting GEM oxidation and/or a shallow boundary layer (∼ 50 m in
average) limiting the dilution. In summer (November to mid-February), the DMC
GEM data showed a high variability with a concentration range varying from
below the detection limit to levels comparable to those recorded at
midlatitude background Southern Hemisphere stations due to an intense
chemical exchange at the air/snow interface. Additionally, the mean
summertime GEM concentration at DMC was ∼ 25 % lower than at other
Antarctic stations in the same period of the year, suggesting a continuous
oxidation of GEM as a result of the high oxidative capacity of the Antarctic
plateau boundary layer in summer. GEM depletion events occurred each year in
summer (January–February 2012 and 2013) with GEM concentrations remaining
low (∼ 0.40 ng m-3) for several weeks. These depletion events
did not resemble the ones observed in the Arctic. They were not associated
with depletion of ozone and occurred as air masses stagnated over the
Plateau,
which could favor an accumulation of oxidants within the shallow boundary
layer. These observations suggest that the inland atmospheric reservoir in
Antarctica is depleted in GEM and enriched in GOM in summer. Measurements at
DDU on the East Antarctic coast were dramatically influenced by air masses
exported from the Antarctic Plateau by strong katabatic winds
. These results, along with observations from earlier
studies, demonstrate that, in Antarctica, the inland atmospheric reservoir
can influence the cycle of atmospheric Hg at a continental scale
. Observations at DDU also highlighted that the Austral Ocean
is a net source of GEM in summer and a net sink in spring, likely due to
enhanced oxidation by halogens over sea-ice-covered areas.
Seasonal pattern analysis in the tropical zone
Relatively few observations of atmospheric Hg had been carried out in the
tropics, before the start of GMOS. Until recently atmospheric Hg data for the
tropics were only available from short-term measurement campaigns. To date,
therefore, there is no information in the tropical area that can be used to
establish long-term trends. Observations in this region may provide a
valuable input to our understanding of key exchange processes that take place
in the Hg cycle considering that the Intertropical Convergence Zone (ITCZ)
passes twice each year over this region and the northern and southern
hemispheric air masses may well influence the evolution of Hg concentrations
observed in this region. As can be seen in Fig. , five GMOS sites
are located in the tropics: Sisal (SIS) in Mexico, Nieuw Nickerie
(NIK) in Suriname, Manaus (MAN) in Brazil, Calhau (CAL) in Cabo Verde and
southern Kodaikanal (KOD) in southern India. GEM concentrations observed in
2013 and 2014 at all sites are comparable with Hg levels recorded at remote
sites in the Southern Hemisphere (1.1 to 1.3 ng m-3; Lindberg et al.,
2007). Among these sites, the Kodaikanal site (KOD) shows the highest monthly
mean GEM concentrations (see Figs. and as well as
Tables S1 and S2) ranging between 1.25 ng m-3 (5th percentile) to
1.87 ng m-3 (95th percentile) during 2013 with an annually based
statistic mean of 1.54 ± 0.20 ng m-3 and between
1.20 ng m-3 (5th percentile) to 2.03 ng m-3 (95th percentile)
during 2014 with an annually average of 1.48 ± 0.26 ng m-3. KOD
is a Global Atmospheric Watch (GAW) regional site which is operated by the
Indian Meteorological Department. It is worthwhile to point out that the other
tropical GMOS sites are close to sea level and on the coast, whereas KOD is a
high-altitude site (2333 m a.s.l.). Therefore different meteo-climatic
conditions influence the long-range transport of air masses to this site.
This site is also influenced by anthropogenic sources such as the well-known,
but not close, Hg thermometer plant, 2150 m far away from the monitoring
station at Kodaikanal . Due to this anthropogenic
influence, atmospheric Hg concentrations from 3 to 8 ng m-3 for the
years 2000 and 2001 have been reported . India is the
third largest hard coal producer in the world after the People's Republic of
China and the USA . For the past
3 decades, India has increased the production of metals, cement,
fertilizers and electricity through burning of coal, natural gas and oil,
becoming one of the most rapidly growing economies . Relatively little attention has been paid to potential Hg
pollution problems due to mining operations, metal smelting, energy and fuel
consumption, which could impact on ecosystem health . Hg
concentrations are in fact enhanced in India due to industrial emissions of
Hg mostly from coal combustion (the major source category (48 %),
followed by waste disposal (31 %), the iron and steel industry,
chloralkali plants, the cement industry and other minor sources (i.e.,
clinical thermometers) . Unfortunately,
details of Hg emissions from these facilities and atmospheric Hg data in
general are scarce. Therefore it is necessary for India as well as for the
other place in the world where Hg measurement are yet lacking to generate
continuous data, which can then be used by scientists for modeling applications
to improve emission inventories in order to prevent inaccurate assessments of
Hg emission and deposition.
GEM levels observed at Sisal (SIS), Mexico, were below the expected global
average concentration (∼ 1.5 ng m-3). Monthly mean GEM
concentrations ranged between 1.0 and 1.47 ng m-3 in 2013 with an
annual average of 1.20 ± 0.24 ng m-3 (5th and 95th percentiles: 0.8
and 1.58 ng m-3), whereas in 2014 the range varied from 0.82 to 1.45 ng m-3, with an annual average of 1.11 ± 0.37 ng m-3 (5th
and 95th percentiles: 0.82 and 1.45 ng m-3). GEM measurements at SIS
showed, in addition, very little variability over the sampling period,
indicating that this relatively remote site on the Yucatán Peninsula was not
subject to any significant anthropogenic sources of Hg at all. During 2013
and 2014, the SIS site was typically influenced by the marine air
originating from the Atlantic Ocean before entering the Gulf of Mexico
. Average GEM concentrations reported at SIS are lower than
those recorded in other rural places in Mexico, such as Puerto Angel (on the
Pacific coast in Oaxaca state) and Huejutla (a rural area in the state of
Hidalgo), where average values of 1.46 and 1.32 ng m-3 were
determined, respectively . Low GEM concentrations were
recorded in 2013 during the later part of the wet season (July/October).
Those values may indicate a slight decrease, probably due to deposition
processes since the site is a coastal station and subject to frequent
episodes with high humidity caused by rain . These
findings have also been confirmed through wind roses and backward
trajectories that show the predominant wind direction from east-southeast
most of the time and sometimes from east-northeast (Atlantic Ocean)
. In addition, the ITCZ moves north of the Equator
passing over the Yucatán peninsula during the northern hemispheric summer,
causing tropical rain events which could contribute to the slight decrease of
Hg concentrations. Highest GEM levels were observed during the winter period
(December–January) in 2013, whereas 2014 had the lowest GEM concentration in January
and higher GEM levels during spring and summer. The background Hg
concentrations measured at Sisal are closely comparable to those recorded at
Nieuw Nickerie (NIK), Paramaribo, Suriname, located on the northeastern
coast of the South American continent, the first long-term measurement site
in the tropics which has been in operation since 2007 .
Analysis of data shows that the annual mean GEM for 2013 and 2014 at NIK are
a little lower than those at SIS: 1.13 ± 0.42 and 1.28 ± 0.46 ng m-3, respectively (see Tables S1 and S2). NIK is also a
background site because most of the time the air masses arriving at the site
come from the clean marine air of the Atlantic Ocean and the influence of
possible local anthropogenic sources and continental air is minimal. As the
ITCZ crosses Suriname twice each year, the NIK site samples both northern and
southern hemispheric air masses. Occasionally higher values are seen: 1.57 ng m-3 in February/March 2013 and 1.51 in August/September 2014 (see Figs. S1 and
S2). Manaus (MAN) in Amazonia (Brazil) is a GMOS master site located in the
Amazon region, an area with a history of important land use change and
significant artisanal and small-scale gold mining activities since the 1980s.
Burning of natural vegetation to produce agriculture lands or pastures
represents an important diffuse source of Hg to the atmosphere in Brazil
. The analysis of atmospheric Hg species at
this site is thus important for the determination of the dynamics of
atmospheric Hg. Annual mean Hg concentrations in 2013 and 2014 at MAN are
slightly lower than those at both SIS and NIK, with little variability
between the two years (see Tables S1 and S2). The measurements from MAN
station may therefore suggest that although the Hg emissions from regional
biomass burning and artisanal and small-scale gold mining represent the major emission sources in the Amazon
basin as reported in a study performed by , they may not
have a significant impact locally but contribute to the global Hg background
(concerning Hg from biomass burning see . Unfortunately
the emissions from both these sources are associated with large uncertainties
and vary over time. Quantifying their impact in South America is extremely
important and there is a strong case for expanding the number of GMOS
measurement sites in the region. MAN is in fact a very remote site, inside
the campus of the Embrapa Amazonia Oriental and upwind from the three main
gold mining areas in the Amazon basin, which are located in Rondonia, Mato
Grosso and in the south of the Parà states . Previous Hg
measurements performed by during an aircraft experiment
over different sites in the Amazon basin highlighted Hg concentrations
between 0.5 and 2 ng m-3 at pristine sites (and among them also MAN)
not impacted by air masses enriched with emissions from gold mining areas
and/or biomass burning. Those data collected from August to September 1995
are comparable to ours observed in 2013 and 2014 at MAN during the same
period, whereas other sites over areas with intense biomass burning and
near areas with strong Hg emissions (Alta Floresta and Rondonia, for example)
reported very high Hg levels (5–14 ng m-3). These
high Hg concentrations were never observed at MAN during the 2013 and 2014
period. Monthly mean GEM concentrations at MAN ranged between 1.01
and 1.18 ng m-3 in 2013 and between 0.94 and 1.10 ng m-3 in 2014.
Also PBM and GOM recorded during 2013 show little variation and varied
between 1.35 and 12.70 pg m-3 (5th and 95th percentile,
respectively) with a median value of 3.17 pg m-3. In 2014, the range
was from 0.53 to 5.24 pg m-3 (5th and 95th percentile,
respectively) with a median value of 1.48 pg m-3. The MAN Hg
concentrations therefore seem not to be influenced by regional emissions.
However, a number of parameters, such as the intense air mass convection
occurring in the Amazon basin and meteorological condition in general, clearly
contribute to the observed Hg concentrations, and they do not necessarily
reflect only regional emissions . Most of the
air masses that reach the site in 2013 and 2014 come from tropical Atlantic
and travel for about 1500 km over pristine forest before reaching the site
; the prevailing winds during the wet seasons (from
January–March) were from north-northeast, northeast and east-northeast,
whereas during the dry seasons (from August to October) they were from north and
north-northeast as well as north-northwest .
The Cape Verde Atmospheric Observatory's Calhau Station (CAL) contributes data
from the eastern tropical Atlantic Ocean, where GMOS provides the only
existing dataset. CAL is an important GAW station located on Sao Vicente
Island, approximately 50 m from the coastline. GEM measurements from 2012 to
2014 were broadly consistent with previously published oceanographic campaign
measurements in the region, with typical Hg values between 1.1 and 1.4 ng m-3. The prevailing wind was from the northeast open ocean, bringing air
masses from the tropical Atlantic and from the African continent
. Due to its relatively long residence time in the
atmosphere, the ground-level background GEM concentration tends to be
relatively constant over the year in tropical regions, unlike midlatitude
and polar regions where a more noticeable seasonal variation has been
observed. When compared with measurements from cruise campaigns from North to
South Atlantic, we can see that the GEM data at CAL are similar to previously
reported South Atlantic data, where Hg concentrations are lower than the
northern part of the Atlantic. Monthly mean GEM concentrations in 2013 ranged
from 1.12 to 1.38 ng m-3, with an annually based mean of 1.22 ± 0.14 ng m-3 (5th and 95th percentile equal to 1.04
and to 1.46 ng m-3, respectively), whereas in 2014 the monthly mean
observed varied from 1.12 to 1.33 ng m-3, with an
annually based mean of 1.20 ± 0.09 ng m-3 (5th and 95th percentile
equal to 1.08 and to 1.36 ng m-3, respectively). The
highest GEM concentrations in air originating from central Africa have been
recorded at CAL when the relative humidity was lowest (occasionally during
dust events) . All tropical GMOS sites show little
atmospheric Hg variability through both the years (2013 and 2014) with small
GEM fluctuations during the months, which agrees well with a relatively long
atmospheric lifetime of Hg in the background troposphere and small variations
in the source strength . However, clear diurnal cycles
of Hg have been conversely observed.