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.

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.

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.

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.

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.

Mercury data discussed in this paper are reported within the GMOS central database and are available upon request at http://sdi.iia.cnr.it/geoint/publicpage/GMOS.