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Anthropogenic mercury emissions are transported through the atmosphere as gaseous elemental mercury (Hg(0)) before they are deposited to Earth’s surface. Strong seasonality in atmospheric Hg(0) concentrations in the Northern Hemisphere has been explained by two factors: anthropogenic Hg(0) emissions are thought to peak in winter due to higher energy consumption, and atmospheric oxidation rates of Hg(0) are faster in summer. Oxidation-driven Hg(0) seasonality should be equally pronounced in the Southern Hemisphere, which is inconsistent with observations of constant year-round Hg(0) levels. Here, we assess the role of Hg(0) uptake by vegetation as an alternative mechanism for driving Hg(0) seasonality. We find that at terrestrial sites in the Northern Hemisphere, Hg(0) co-varies with CO2, which is known to exhibit a minimum in summer when CO2 is assimilated by vegetation. The amplitude of seasonal oscillations in the atmospheric Hg(0) concentration increases with latitude and is larger at inland terrestrial sites than coastal sites. Using satellite data, we find that the photosynthetic activity of vegetation correlates with Hg(0) levels at individual sites and across continents. We suggest that terrestrial vegetation acts as a global Hg(0) pump, which can contribute to seasonal variations of atmospheric Hg(0), and that decreasing Hg(0) levels in the Northern Hemisphere over the past 20 years can be partly attributed to increased terrestrial net primary production.

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg⁰ to the atmosphere where it is oxidized to reactive HgII compounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized HgI and HgII species postulate their photodissociation back to Hg⁰ as a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg⁰, HgI, and HgII species in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of HgI and HgII leads to insufficient Hg oxidation globally. The combined efficient photoreduction of HgI and HgII to Hg⁰ competes with thermal oxidation of Hg⁰, resulting in a large model overestimation of 99% of measured Hg⁰ and underestimation of 51% of oxidized Hg and ∼66% of HgII wet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3–6-mo range based on observed atmospheric Hg variability. These results show that the HgI and HgII photoreduction processes largely offset the efficiency of bromine-initiated Hg⁰ oxidation and reveal missing Hg oxidation processes in the troposphere.

The Minamata Convention on Mercury (Hg) entered into force in 2017, committing its 116 parties (as of January 2019) to curb anthropogenic emissions. Monitoring of atmospheric concentrations and trends is an important part of the effectiveness evaluation of the convention. A few years ago (in 2017) we reported an increasing trend in atmospheric Hg concentrations at the Cape Point Global Atmosphere Watch (GAW) station in South Africa (34.3535∘ S, 18.4897∘ E) for the 2007–2015 period. With 2 more years of measurements at Cape Point and the 2012–2017 data from Amsterdam Island (37.7983∘ S, 77.5378∘ E) in the remote southern Indian Ocean, a more complex picture emerges: at Cape Point the upward trend for the 2007–2017 period is still significant, but no trend or a slightly downward trend was detected for the period 2012–2017 at both Cape Point and Amsterdam Island. The upward trend at Cape Point is driven mainly by the Hg concentration minimum in 2009 and maxima in 2014 and 2012. Using ancillary data on 222Rn, CO, O3, CO2, and CH4 from Cape Point and Amsterdam Island, the possible reasons for the trend and its change are investigated. In a companion paper this analysis is extended for the Cape Point station by calculations of source and sink regions using backward-trajectory analysis.

Due to their long half-life, polychlorinated biphenyls (PCBs) tend to contaminate not only coastal areas, but they travel over long distances, eventually reaching remote areas such the Arctic. The physical and biogeochemical features of every coastal area govern the main distribution patterns of freshly introduced PCBs into the marine system. Some of these processes are determined by chemical properties of the individual PCB congener. Thus, atmospheric influx along with ad/absorption on non-living organic material, photolytical and biological degradation processes vary from one PCB congener to another. For a detailed fate analysis of individual congeners, we developed a new chemical model, based on the GOTM-ECOSMO-FABM model framework. Here, we exemplarily present results for PCB153 based on 1D simulations of four regions in the North-Baltic Sea. The study area is characterized by different hydrodynamic and biogeochemical conditions. We investigate the impact of resuspension, mixing and the biological pump, sea ice and tides on the final phasal distribution of PCB153. Different combinations of these factors lead to the development of different areas of PCB153 accumulation, with the formation of hotspot areas, and influence the total uptake and concentration of PCB153 in the water column. As a result, two major dynamics determine the fate of PCB153 in the coastal ocean: (i) Primary production leads to PCB153 being adsorbed by organic material. Partitioning to organic material and sedimentation of organic particles removes dissolved PCB153 from the surface ocean and increases atmospheric influx. (ii) Tidal-induced resuspension and mixing control the benthic–pelagic exchange of PCB153 and its distribution in the water column. Depending on the resuspension regime and stratification, sediments can become a permanent (Gotland Deep, the Baltic Sea) or seasonal sink for PCB153. In regions with seasonal stratification and high near bottom turbulence (Northern North Sea), resuspension events can lead to pronounced peaks in PCB153 concentrations and can therefore have a major impact on bioaccumulation. Under the conditions of permanent mixing and high bottom turbulence (Southern North Sea, Bothnian Bay), pollutants are hardly accumulating in sediments.

This paper provides a detailed, thorough analysis of air pollution by benzo[a]pyrene (BaP) in the Czech Republic. The Czech residential sector is responsible for more than 98.8% of BaP, based on the national emission inventory. According to the data from 48 sites of the National Air Quality Monitoring Network, the range of annual average concentration of BaP ranges from 0.4 ng·m−3 at a rural regional station to 7.7 ng·m−3 at an industrial station. Additionally, short-term campaign measurements in small settlements have recorded high values of daily benzo[a]pyrene concentrations (0.1–13.6 ng·m−3) in winter months linked to local heating of household heating. The transboundary contribution to the annual average concentrations of BaP was estimated by the CAMx model to range from 46% to 70% over most of the country. However, the contribution of Czech sources can exceed 80% in residential heating hot spots. It is likely that the transboundary contribution to BaP concentrations was overestimated by a factor of 1.5 due to limitations of the modeling approach used. During the period of 2012–2018, 35–58% of the urban population in the Czech Republic were exposed to BaP concentrations above target. A significant decreasing trend, estimated by the Mann-Kendall test, was found for annual and winter BaP concentrations between 2008 and 2018.

Persistent organic pollutants (POPs) are of considerable concern due to their well-recognized toxicity and their potential to bioaccumulate and engage in long-range transport. These compounds are semi-volatile and, therefore, create a partition between vapour and condensed phases in the atmosphere, while both phases can undergo chemical reactions. This work describes the extension of the Community Multiscale Air Quality (CMAQ) modelling system to POPs with a focus on establishing an adaptable framework that accounts for gaseous chemistry, heterogeneous reactions, and gas-particle partitioning (GPP). The effect of GPP is assessed by implementing a set of independent parameterizations within the CMAQ aerosol module, including the Junge–Pankow (JP) adsorption model, the Harner–Bidleman (HB) organic matter (OM) absorption model, and the dual Dachs–Eisenreich (DE) black carbon (BC) adsorption and OM absorption model. Use of these descriptors in a modified version of CMAQ for benzo[a]pyrene (BaP) results in different fate and transport patterns as demonstrated by regional-scale simulations performed for a European domain during 2006. The dual DE model predicted 24.1 % higher average domain concentrations compared to the HB model, which was in turn predicting 119.2 % higher levels compared to the baseline JP model. Evaluation with measurements from the European Monitoring and Evaluation Programme (EMEP) reveals the capability of the more extensive DE model to better capture the ambient levels and seasonal behaviour of BaP. It is found that the heterogeneous reaction of BaP with O3 may decrease its atmospheric lifetime by 25.2 % (domain and annual average) and near-ground concentrations by 18.8 %. Marginally better model performance was found for one of the six EMEP stations (Košetice) when heterogeneous BaP reactivity was included. Further analysis shows that, for the rest of the EMEP locations, the model continues to underestimate BaP levels, an observation that can be attributed to low emission estimates for such remote areas. These findings suggest that, when modelling the fate and transport of organic pollutants on large spatio-temporal scales, the selection and parameterization of GPP can be as important as degradation (reactivity).

Benzo[ a ]pyrene (BaP) has been proven to be toxic and carcinogenic. Since 2010, the European Union officially established target values for BaP concentrations in ambient air. In this study BaP concentrations over Europe have been modelled using a modified version of the chemistry transport model Community Multiscale Air Quality (CMAQ) which includes the relevant reactions of BaP. CMAQ has been run using different emission datasets for the years 1980, 2000, and 2020 as input data. In this study, the changes in BaP concentrations between 1980 and 2020 are evaluated and regions which exceed the European annual target value of 1 ng/m 3 are identified, i.e. the Po Valley, the Paris metropolitan area, the Rhine-Ruhr area, Vienna, Madrid, and Moscow. Additionally, the impact of emission reductions on atmospheric concentrations of BaP is investigated. Between 1980 and 2000, half of the BaP emission reductions are due to lower emissions from industrial sources. These emission reductions, however, only contribute to one third of the total ground-level BaP concentration reduction. Further findings are that between 2000 and 2020, a large part (40%) of the BaP concentration reduction is not due to changes in BaP emissions but caused by changes in emissions of criteria pollutants which have an impact on the formation of ozone.

The ability of monomethylmercury (MMHg+) to bioaccumulate in seafood is of concern due to its neurotoxic properties. The challenge in understanding MMHg+ bioaccumulation lies in the fact that its levels in higher trophic levels result from both bioconcentration and biomagnification. Furthermore, Hg can occur in several chemical species, including Hg2+ and MMHg+, which both bioaccumulate. Although the dominant pathway for MMHg+ bioaccumulation into seafood is the bioconcentration of MMHg+ in primary producers and the subsequent biomagnification to higher trophic levels, other pathways can contribute to MMHg+ bioaccumulation. In this study, we use a fully coupled 1D water column Hg bioaccumulation model to quantify how total bioaccumulation of Hg2+ and uptake of MMHg+ from the water (bioconcentration) in consumers affects the bioaccumulation of MMHg+ in high-trophic-level fish. The study is performed in three setups representing hydrodynamic conditions representative of the North and Baltic Seas. We find that Hg2+ bioaccumulation does not influence the bioaccumulation of MMHg+ but the bioconcentration of MMHg+ plays an important role. Although direct bioconcentration accounts for < 15 % of MMHg+ bioaccumulation in cod, the cumulative effect of bioconcentration on all trophic levels increases the MMHg+ content of cod by 28 %–49 %. These results demonstrate that bioconcentration in consumers is essential to accurately model the bioaccumulation of MMHg+ at higher trophic levels.

Methylmercury (MeHg) is a bioaccumulative neurotoxin that poses a risk to human health through seafood consumption. Sponges have unique mercury (Hg) profiles. Measurements show an unusually high inorganic Hg (iHg) content in Low Microbial Abundance (LMA) sponges and an even higher iHg content in High Microbial Abundance (HMA) sponges, while MeHg concentrations remain low, particularly in HMA sponges. Combined with the recently improved understanding of the important ecological role of sponges as a food source for other biota, this suggests that their low MeHg content may influence MeHg transfer within benthic food webs. In this study, we used a 1D water-column model to investigate the bioaccumulation of MeHg in sponges. It has been hypothesized that the low MeHg content in HMA sponges may result from active MeHg demethylation. Our model results indicate that the consumption of dissolved organic matter (DOM) can already explain the low observed MeHg content in LMA sponges, and higher DOM consumption in HMA sponges can account for the even lower MeHg levels in HMA species. Alternatively, if MeHg demethylation occurs, a low rate of 1 % d.sup.-1 could explain the differences between LMA and HMA sponges. Although DOM consumption increases iHg bioaccumulation in both sponge types, it does not explain the extremely high iHg concentrations observed. Finally, our model suggests that HMA sponges could potentially reduce MeHg concentrations in benthic fish by up to 53 % when they dominate at the base of the food web. These findings highlight the potentially important role of sponges in Hg cycling and indicate that sponge-dominated systems could help reduce MeHg accumulation in benthic food webs.

Mercury (Hg) is a toxic pollutant that poses significant risks to marine ecosystems and human health as a result of bioaccumulation. Despite its known hazards, the processes that govern Hg bioaccumulation within the marine food web are poorly understood. This study examines the role of the marine ecosystem in Hg cycling in highly productive coastal seas. We integrate Hg biotic uptake, release and transformation into the ECOSMO E2E marine ecosystem model, coupled with the MERCY v2.0 marine Hg cycling model. Incorporating bioaccumulation into the model leads to a 44 % increase in total methylmercury (tMeHg) concentrations in coastal pelagic waters, from 0.059 to 0.092 pM, compared to a model without bioaccumulation. Bioaccumulation and binding of Hg to organic matter contribute to elevated Hg levels in surface waters. Furthermore, cyanobacteria-driven reduction of Hg.sup.2+ to Hg.sup.0 decreases average marine Hg concentrations by up to 10 % above the mixed layer depth in the Gotland Deep and 20 % in shallow Baltic Sea regions, and increases Hg.sup.0 evaporation in the Baltic Sea, reducing Hg inflow into the North Sea. We quantify a 1 % increase in tMeHg per 4.5 mg C m.sup.-3 biota biomass. Finally, we show that bioaccumulation decreases the burial of Hg by 13 kg yr.sup.-1 increasing Hg export to the Atlantic Ocean and the English Channel. These findings highlight the importance of ecosystem feedback on marine Hg cycling and demonstrate the need to integrate biological processes into Hg cycling models.