1a-1: Atmospheric mercury cycling and transformations: Insights from measurements and models

We use earth system and atmospheric chemistry modeling to investigate the implications of climate variability and change on strategies for monitoring the effectiveness of global mercury policy. As the Minamata Convention moves towards the implementation phase, developing strategies for monitoring treaty effectiveness is a science-policy priority. Given that the atmosphere is likely to respond relatively rapidly to policy-related emissions changes compared to other environmental media, atmospheric monitoring strategies are critical. However, atmospheric concentrations and fluxes (wet and dry) are not only influenced by anthropogenic mercury forcing; meteorological variables such as temperature, precipitation, and circulation patterns have implications for mercury chemistry and transport, and resulting observed concentrations and fluxes at monitoring sites. In this work, we assess the potential influence of climate variability and change on ability to detect policy-related anthropogenic changes in mercury emissions.

We simulate atmospheric concentrations and fluxes of mercury using the GEOS-Chem chemical transport model, driven by an ensemble of meteorological fields from the MIT Integrated Global System Model that encompasses a range of initial conditions, climate stabilization scenarios, and climate sensitivities. Using this ensemble, we estimate the distribution of atmospheric concentrations and fluxes consistent with the implementation Minamata Convention, under possible climate futures (mid-century and end of century). We find that more than 10 years of data are required to detect anthropogenic signals in mercury, given the magnitude of current and future climate variability when relying on a single monitoring metric (e.g., wet deposition); however, combining co-located flux and concentration measurements can reduce this time of emergence. To better understand what drives this climate-related variability in atmospheric mercury concentrations and fluxes, we apply empirical orthogonal function analysis to identify influential meteorological variables, and geographic regions where atmospheric measurements are most sensitive to anthropogenic mercury changes. We discuss how these insights can inform the mercury science-policy community on monitoring site location and metric selection.

Mercury (Hg) emissions from natural surfaces contribute a large portion of total Hg emissions in a global scale; however, there is a huge uncertainty on natural emissions inventory. According to UNEP, natural emissions and re-emissions can be categorized into five groups including biomass burning, soil surface, vegetation, geogenic, and oceans. Among these categories, Hg emissions from soil surfaces contribute about 30% of total Hg emissions. In this study, air-soil surface exchange flux of Hg was measured from various soil types including deciduous and coniferous forest floors, grassland, crop field, and lawn throughout a year using dynamic flux chamber (DFC) made of FEP (Fluorinated Ethylene Propylene) Teflon film. Tekran 2537B was used to detect the Hg concentrations inside and outside of DFC. Influencing factors for Hg exchange flux were investigated, and empirical equations estimating Hg flux were derived based on the measurements. In forest floor, different characteristics of Hg emission were observed for leaf-on (summer, autumn) and leaf-off period (spring). Volumetric water content in soil was the most important parameter during leaf-on period because solar radiation nearly reached on the surface due to the high leaf area index (LAI). On the other hand, the most important factors were UV and soil temperature for leaf-off period. In open fields, Hg emission was mainly controlled by soil temperature and UV irradiation for all period. Hg emission flux was also measured from hazardous waste landfill area, and it peaked about 3 h before solar radiation peaked, possibly because of reducible Hg being abundant at the soil surface. Four different models were empirically derived for Hg fluxes from deciduous and coniferous forest soils during leaf-on and leaf-off periods and from open field. In future, one governing equation which can depict Hg exchange flux from all kinds of soil surfaces will be developed, and hopefully presented at the conference.

Long term atmospheric mercury measurements at Underhill, VT (VT99) and Huntington Forest, NY (NY20) from 1992 to 2014 and 2005 to 2014, respectively, were used to determine concentration trends using the Mann-Kendalls tau test with Sens slope estimator. Data from these sites, generally downwind of large Hg sources in the Midwestern U.S., are the longest record of ambient Hg concentrations available in the U.S. At VT99 concentrations of gaseous element mercury (GEM), gaseous oxidized mercury (GOM) and particle bound mercury (PBM) declined with rates of -1.8%, -3.2% and -6.7% per year, respectively. At NY20 GEM and GOM concentrations declined with rates of -1.6% and -7.8% per year. However, PBM concentrations increased at a rate of 2.0% per year likely related to wood combustion. A trajectory ensemble analysis using potential source contribution function indicates the source locations associated with high mercury concentration changed from the Toronto-Buffalo and Pennsylvania areas to east-coast urban centers. The declining GEM concentrations in the Northeastern United States are positively correlated with decreasing SO2 emissions in the upwind area. Overall, the results indicate that decreased mercury concentrations measured during the past decade are consistent with decreased Hg emissions from regional point sources and that increasing global emissions have not overwhelmed those decreases.

Long-term monitoring data of ambient mercury (Hg) on a global scale to assess its emission, transport, atmospheric chemistry, and deposition processes is vital to understanding the impact of Hg pollution on the environment. The Global Mercury Observation System (GMOS) started in November 2010 with the overall goal to develop a coordinated global observing system to monitor Hg on a global scale, including a large network of ground-based monitoring stations, ad hoc oceanographic cruises and measurement flights in the troposphere and lower stratosphere. To date, more than 40 ground-based monitoring sites constitute the global network covering many regions where little to no observational data were available before GMOS. This work presents atmospheric Hg concentrations recorded worldwide, analyzing Hg measurement results in terms of temporal trends, seasonality and comparability within the network. Major findings highlighted a clear gradient of atmospheric Hg concentrations between the Northern and Southern hemispheres, confirming that the gradient observed is mostly driven by local and regional sources, which can be anthropogenic, natural or a combination of both. In order to understand the atmospheric cycling and seasonal depositional characteristics of Hg, wet deposition samples were also collected at 17 selected GMOS monitoring sites providing new insight into baseline concentrations of THg concentrations in precipitation particularly in regions, such as the Southern Hemisphere and tropical areas where wet deposition were never investigated before, opening the way for additional measurements across the GMOS network and new findings in future modeling studies highlighting the need of 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 Hg concentrations in air and precipitation with the overarching benefit which clearly consists in the advancement of knowledge about Hg processes on global scale.