Hg is one of the most toxic metals, and as such is regulated by the Industrial Emissions Directive (IED) 2010/75/EU, the Air Quality Directive 2004/107/EC, the Waste Incineration Directive 2000/76/EC and the Minamata Convention adopted in 2013; which is a global treaty to protect human health and the environment from the adverse effects of Hg. In addition to its elemental form, Hg also exists in oxidised forms (i.e. Hg(II)) that are reactive and can be transformed into organic Hg species such as methylmercury (MeHg), the most toxic Hg species and the one most prone to bioaccumulation in aquatic systems. Half of atmospheric Hg emissions are of natural origin while the rest is from anthropogenic sources.
The industrial sector is the main polluter of Hg, and it is in great need of traceable and comparable Hg measurement results. This is particularly true for fossil fuel combustion facilities, demonstrated by the United Nations Environment Programme (UNEP's) partnership programme on Hg emission from coal, which calls for the improved performance of measurements for effective implementation of the Minamata convention. Cement clinker production factories are also recognised as an important source of Hg due to high-temperature processes, and the European Association of Cement producers (CEMBUREAU) have also expressed their interest in the results of this project. In the area of waste management, the International Environmental Technology Centre of UNEP is currently responsible for waste management issues within the Minamata Convention and will promote the application of environmentally sound technologies (ESTs) such as the methodologies developed in this project in developing countries.
The atmosphere contains three forms of atmospheric Hg: gaseous elemental mercury (GEM), gaseous oxidised mercury (GOM), and particulate bound mercury (PBM). Through a series of photochemically initiated reactions in the atmosphere, involving halogens, GEM is converted to a more reactive species and is subsequently associated with particles in the air and deposited, particularly in polar environments. These phenomena are called the atmospheric mercury depletion events (AMDE), and so far, only one commercially available instrument has claimed to be able to measure these Hg forms, and it has been demonstrated that measurements made with this technology underestimate GOM concentrations by as much as a factor of 2 to 13. Further to this, sampling efficiency for GOM is affected by ozone and water vapour and underestimating GOM results in biased values that are too low for modelling dry deposition. The deposition of reactive mercury (RM = GOM + PBM) also produces inorganic Hg complexes that undergo abiotic and biological transformations on surfaces and in water.
However, traceable methods and calibration standards only exist for Hg(0), and even these are based on Hg vapour pressure equations that give differing results. UNEP 2013 "Global Mercury Assessment" claims that due to a lack of robust, reliable, traceable and validated methodologies for oxidised Hg measurements either in the air or anthropogenic emissions large uncertainties exist in global estimates of Hg emissions to air. Since Hg is emitted primarily in the vapour phase, as both elemental and oxidised Hg, it is essential that traceable measurements of both components can be made. Currently, there is no metrological infrastructure for traceable, validated and accurate measurements of oxidised Hg species in the atmosphere and emission sources. This infrastructure is of paramount importance for the implementation of the Minamata convention on Mercury and the Air Quality Directive 2004/107/EC and IED 2010/75/EU 3,4 that regulate atmospheric and industrial Hg emissions.
Many different efforts have been undertaken in the last decade, by NIST, the US EPA, the research community and instrument producers, to overcome the difficulties involved in oxidised Hg species determinations in the atmosphere and emission sources. However, these measurements are dependent on the availability of reliable Hg(II) gaseous reference standards and materials to assess and verify the quality of data and in most existing methods for Hg measurements, the different oxidised Hg species normally have to be reduced to the detectable elemental form, i.e. Hg(0) in order to be quantified. Therefore, reliable Hg(II) reference gases are needed to quantify this conversion and to assess the ability to quantitatively transfer in particular the reactive Hg(II), through the entire measurement system. So far none have proved fully successful thus highlighting the need for a collaborative approach, which will bring together expertise from NMIs and DIs, the research community and instrument providers, to work together to overcome these problems. To meet future global and European requirements (in relation to the Minamata Convention and European Directives), standardisation bodies have recognised the importance and need to standardise the method for measuring Hg in industrial flue gases and the atmosphere. This will be achieved by facilitating the transfer of the measurement infrastructure developed in the project to standards development organisations such as CEN/TC264/WG8 and the respective Articles of the Minamata Convention. Thus, in order to address these knowledge gaps comparable and traceable measurements are needed for GEM, GOM and PBM. This is also true for stable Hg isotope ratio measurements that until now have not been able to be sufficiently used in field conditions for stack gas emissions and atmospheric measurements. Knowledge of Hg speciation both in the air and in stack gas emissions is critical when validating models for predicting Hg emissions, transport, deposition and fate at the European level as well as on a global scale. Therefore, atmospheric Hg isotopic signatures that can be used to trace the origin and fate of atmospheric Hg also need metrological support and development.