Development of a Ground-Based Atmospheric Monitoring Network for the Global Mercury Observation System \(GMOS\)

Consistent, high-quality measurements of atmospheric mercury (Hg) are necessary in order to better understand Hg emissions, transport, and deposition on a global scale. Although the number of atmospheric Hg monitoring stations has increased in recent years, the available measurement database is limited and there are many regions of the world where measurements have not been extensively performed. Long-term atmospheric Hg monitoring and additional ground-based monitoring sites are needed in order to generate datasets that will offer new insight and information about the global scale trends of atmospheric Hg emissions and deposition. In the framework of the Global Mercury Observation System (GMOS) project, a coordinated global observational network for atmospheric Hg is being established. The overall research strategy of GMOS is to develop a state-of-the-art observation system able to provide information on the concentration of Hg species in ambient air and precipitation on the global scale. This network is being developed by integrating previously established ground-based atmospheric Hg monitoring stations with newly established GMOS sites that are located both at high altitude and sea level locations, as well as in climatically diverse regions. Through the collection of consistent, high-quality atmospheric Hg measurement data, we seek to create a comprehensive assessment of atmospheric Hg concentrations and their dependence on meteorology, long-range atmospheric transport and atmospheric emissions.


Introduction
Mercury (Hg) is ubiquitous in the atmospheric boundary layer, as well as in the free troposphere and stratosphere.It has ground-level background concentrations that are nearly constant over hemispheric scales, with southern hemisphere concentrations slightly lower than those in the northern hemisphere (Sprovieri et al., 2005a,b;Hedgecock et al., 2008;Dommergue et al., 2010).In the troposphere, atmospheric Hg exists predominantly as gaseous elemental mercury (Hg 0 ; GEM), gaseous oxidized mercury (GOM), and fine particle-bound Hg (PBM 2.5 ).Conversions between different Hg forms add complexity to the ability to understand Hg chemistry and transport on the local, regional, and global scales (Lindberg et al., 2007;Sprovieri et al., 2010).
Mercury cycling between environmental compartments depends on the rate of different chemical and physical mechanisms (e.g., wet and dry deposition) and meteorological conditions, as well as on anthropogenic emissions and atmospheric forcing (Pirrone et al., 2010).Consequently, a complex mixture of chemical, physical and meteorological parameters control the fate of atmospheric Hg, and it is challenging to understand the global impact of Hg emissions, transport, and deposition (Lindberg et al., 2007).
A wide range of monitoring activities have been carried out in different regions of the world in order to assess the levels of mercury (Hg) in ambient air and precipitation, as well as its variation over time and with changing meteorological conditions (Ebinghaus et al., 2002;Pirrone and Mason, 2009;Sprovieri et al., 2010).In the past two decades a number of Hg monitoring sites have been established.These sites are located primarily in Europe, Canada and the USA, and a few are located in Asia.In contrast, very few Hg monitoring sites have been established in South and Central America, central and north Africa and other regions of the southern hemisphere.
In November 2010, under the auspices of the Global Mercury Observation System (GMOS) project, we began developing a global scale ground-based network of atmospheric Hg monitoring sites.The objective of this monitoring network is to unify existing regional Hg measurement efforts with new monitoring stations in regions of the world where atmospheric Hg observational data is limited in order to improve our understanding of Hg transport and deposition the global scale.
A large volume of high-quality data has been generated over the past decade and beyond as part of on-going regional (i.e., EMEP, AMAP, CAMNet, NADP-MDN) and national (i.e., in Sweden, Italy, Japan, Korea, Taiwan, U.S.A. and Canada) programs and during focused EU projects (i.e., MAMCS, MOE, MERCYMS).The integration and synthesis of existing data sets will have high added value for the GMOS project.
Here, we provide a review of the current state of this new globally representative Hg monitoring network, including the principal characteristics and measurements being performed at the GMOS monitoring sites that are distributed across both the Northern and Southern Hemispheres.

Methods
At the GMOS monitoring sites, we aim to measure both ambient Hg and Hg in precipitation using high-quality, consistent measurement techniques.In this effort, a primary objective has been to harmonize the chosen measurement techniques with those being performed at existing monitoring stations around the world.At the start of the project, we performed a survey all new and existing GMOS sites to fully comprehend the measurements currently being collected and to determine an appropriate measurement scheme for atmospheric Hg measurements across this global scale network of sites.Based upon this information, we elected to measure speciated ambient Hg using the Tekran 2537/1130/1135 Hg speciation system.TGM measurements are being collected using either the Tekran 2537 or the Lumex RA-915-AM.Weekly precipitation samples are being collected primarily using wet-only collectors, such as the N-CON MDN or the Eigenbrodt NSA 171 wet-only samplers.Where necessary, due to site constraints or operator availability, some GMOS sites are alternatively collecting bulk precipitation samples.After selecting the measurement instrumentation, we developed Standard Operating Procedures, which are consistent with existing techniques in other regional networks, to be employed at all GMOS sites.

Results and Discussion
To date, there are 38 monitoring sites participating in the GMOS network.This includes existing sites that are already collecting atmospheric Hg measurements (ambient air and/or precipitation), new stations who are initiating Hg measurements for the first time through the GMOS project, and externally partnering sites who are contributing their measurement data to the GMOS database.Figure 1 shows the location of the ground-based monitoring sites that are part of GMOS.Additionally, Table 1 provides information on the location, type, and affiliation of each site.Master Stations are those where Gaseous Elemental Mercury (GEM), Gaseous Oxidized Mercury (GOM), and fine particulate-bound Hg (PBM 2.5 ) as well as Hg in precipitation are, or will be, continuously measured.Secondary Stations are those where only Total Gaseous Mercury (TGM) and Hg in precipitation are, or will be, continuously measured.Table 2 summarizes the state of measurements to date at GMOS sites.While most new sites have purchased instrumentation, this process is still ongoing.
In addition to advising site operators on the purchase and installation of equipment, we have performed several training programs with the GMOS participants in order to inform the scientists and technicians on how to operate the instrumentation according to the GMOS SOPs.This has included one-on-one training sessions at the CNR-IIA in Rende, as well as a training meeting in Rome on measurements of Hg speciation in ambient air.
Furthermore, in the first two years of the GMOS project we have assisted with the installation of Hg monitoring equipment at a number of critical and challenging monitoring sites, such as the EvK2CNR Pyramid Laboratory in Nepal and the Dome C Laboratory in Antarctica.Sites such as these are providing new and valuable Hg data in areas where atmospheric Hg has not been thoroughly measured in the past.We have also established agreements with a number of external partners who are managing sites and networks outside of GMOS.Their support, collaboration, and data will greatly enhance the GMOS project and data record.

Conclusion
Within the GMOS project, we are continuing to establish new measurements of ambient Hg and Hg in precipitation at the GMOS sites to develop a full global 17007-p.2scale network of atmospheric Hg observations.This is an ongoing task, which will ultimately result in a global-scale network of monitoring sites where consistent and comparable atmospheric Hg measurements are being collected.
We anticipate that by the end of 2012, all instrumentation will be operational at the monitoring sites.We continue to provide support to site operators to manage the consistent collection of data, and to manage the quality assurance and quality control of the data being collected.Ultimately, we intend to provide this data to regional and global atmospheric modelers so that they can validate and improve their model results.This collaborative effort will advance our understanding of the global scale emissions, transport, chemistry and deposition of atmospheric Hg.

Fig. 1 .
Fig. 1.Global map showing the locations of GMOS ground-based monitoring stations.Those in red are managed by external GMOS partners.