Globally, riverine inputs deliver more terrestrial mercury (Hg) to the coastal ocean than direct atmospheric deposition. Once in coastal environments, Hg can sorb to particles, become buried in sediments, and undergo methylation, facilitating its entry into food webs. However, at regional scales, Hg sources and transport pathways remain poorly constrained due to limited observations and uncertainties in source apportionment. Here, we combine Hg concentrations with Hg and organic carbon stable isotopic signatures in surface sediments to trace Hg sources along an estuarine-to-marine gradient in the southern Bay of Biscay (Atlantic Ocean). In-situ transformation processes had a negligible impact on Hg isotopic composition, supporting the use of Hg isotopes as a tracer for Hg source apportionment. Hg isotopic composition (δ 202 Hg) was significantly correlated to carbon (δ 13 C), revealing distinct terrestrial and marine endmembers, with an enrichment in lighter isotopes in estuarine sediments (δ 202 Hg -0.83 ± 0.15‰, δ 13 C -27.3 ± 0.50‰) compared to shelf and canyon marine sediments (δ 202 Hg -0.54 ± 0.16‰, δ 13 C -25.2 ± 0.80‰). A binary-mixing model constrained by the Hg-C isotope relationship suggests a progressive transition from a predominantly terrestrial Hg pool in estuarine samples (79%) towards decreasing terrestrial contributions in offshore marine samples (15%). Despite their lower terrestrial fraction, offshore sediments contained the largest terrestrial Hg stock, reflecting efficient terrestrial Hg export across the estuarine-to-marine boundary. Our results demonstrate that combined Hg and C stable isotope observations provide a powerful framework for Hg source apportionment, offering the potential to directly constrain the land-to-ocean transfer of Hg into coastal sediments.

Recent studies and OECD (Organization for Economic Cooperation and Development) reports provide roadmaps to reduce dispersal of mismanaged plastic waste to aquatic environments. Here, we use a coupled land-ocean-atmosphere model to simulate global plastic and microplastic dispersal for different OECD policy scenarios toward 2060. We establish a global plastic budget for the year 2015, with revised estimates of the total marine plastic pool of 263 teragrams (Tg, million tons), and land to sea plastic transport of 14 Tg per year, implying four to nine times larger leakage than OECD estimates. Model simulation of two ambitious policy scenarios show a peak in land to sea transport of total plastics of 23 Tg per year around 2045 and a decrease thereafter. Environmental concentrations of small microplastics remain high after 2060 due to continuous fragmentation of legacy mismanaged waste on land and indicate the need for remediation of legacy terrestrial plastic waste in policy instruments.

The coastal ocean receives more mercury from riverine input (land-derived Hg) than from direct atmospheric deposition. Land-derived Hg is primarily particle-bound and buried in coastal sediment, where it may undergo methylation. However, its contribution to coastal sediment Hg loads remains poorly constrained. Here, we investigate Hg sources along an estuarine-to-marine gradient in the Bay of Biscay (Atlantic Ocean) combining Hg and carbon (C) stable isotopic composition in sediments. We find a significant (p<0.05) difference in δ202Hg and δ13C between estuarine (δ202Hg -0.83 ± 0.15‰, δ13C -27.3 ± 0.50‰) and marine (δ202Hg -0.54 ± 0.16‰, δ13C -25.2 ± 0.80‰) sediments, and a significant correlation (p<0.05) between δ13C and δ202Hg. While in-situ processes had a negligible impact on Hg isotopic composition, the observed trends could be explained by the mixing of land-derived Hg (e.g. vegetational uptake, anthropogenic) and direct atmospheric deposition to the marine environment. Applying a binary-mixing model with regional endmembers to δ13C and δ202 signatures reveals a near 1:1 relationship between land-derived Hg and terrestrial C along our estuarine-to-marine gradient. This relationship, if confirmed for other regions, suggests that joint observations of C stable isotopes and Hg concentrations could directly constrain the transfer of land-derived Hg into coastal sediments.

Plastics occupy a central role in the global economy, yet cause significant damage to ecosystems and human health. Recent studies and OECD reports have provided comprehensive roadmaps to reduce the environmental impacts of plastics, based on coordinated global action to reduce plastic consumption and improve waste management and recycling. Leakage of mismanaged plastic waste to aquatic environments, including the ocean, is a key policy variable, yet not well constrained in plastics life cycle analysis. Here we use a coupled land-ocean-atmosphere box model to simulate global plastic and microplastic dispersal for different policy scenarios. We update the global plastic and microplastic budget for the year 2015. Based on a revised estimate of the total marine plastic pool of 263 Tg, we constrain land to sea transport of plastics at 14 Tg y-1 for the year 2015, implying 4 to 7 times larger leakage than OECD estimates. Model simulation of two ‘global action’ policy scenarios, attaining near-zero mismanaged waste and >50% recycling by 2060, show a peak in land to sea transport of total plastics of 23 Tg y-1 around 2045 and a decrease thereafter. Land to sea transfer of microplastic, however, remains high during the 21st century due to its continued supply from the fragmentation of legacy mismanaged waste on land. Consequently, exposure to small microplastic, <300 μm, in air, terrestrial runoff, marine waters and sediment is estimated to increase 3 to 6-fold by 2060, compared to 2019, and can only be curbed by including remediation of terrestrial mismanaged plastic waste in policy scenarios.

The main drivers of mercury (Hg) compound distribution in seawater are poorly understood, calling for novel spatial and seasonal observations of potential transformations. However, scientific progress is hindered by a lack of intercomparability among incubation studies and the infrequent inclusion of dissolved gaseous mercury species (DGM = elemental Hg (Hg(0)) + dimethyl Hg (DMHg)) despite their importance in the biogeochemical cycle of Hg. We perform a comprehensive quality assessment on our proposed incubation protocol at near ambient concentrations (similar to 10 x ambient background) including the formation of DGM on three distinct coastal seawaters and discuss intercomparability with previous experimental approaches. We establish an excellent mass balance for tracer isotopes both excluding DGM (199Hg = 99.5% and 201Hg = 100.4%, median) and including DGM (199Hg = 100.3% and 201Hg = 101.7%, median). We find a good median relative standard deviation for experimental triplicates (199Hg(II) similar to 1.7%, MM201Hg similar to 1.5%, 199DGM similar to 10%, 201Hg(II) similar to 13%, and 201DGM similar to 22%), enabling the accurate determination of methylation, demethylation, and reduction rate constants at femtomolar concentration levels. We observed Hg(0) formation from MMHg, potentially indicating reductive demethylation. This study highlights the practicability and importance of incorporating DGM species (here, Hg(0), eventually DMHg) in future incubation studies.

Terrestrial volcanism is known to emit mercury (Hg) into the atmosphere. However, despite many years of investigation, its net impact on the atmospheric Hg budget remains insufficiently constrained, in part because the transformations of Hg in volcanic plumes as they age and mix with background air are poorly understood. Here we report the observation of complete gaseous elemental mercury (GEM) depletion events in dilute and moderately aged (~3-7 hours) volcanic plumes from Piton de la Fournaise on Réunion Island. While it has been suggested that co-emitted bromine could, once photochemically activated, deplete GEM in a volcanic plume, we measured low bromine concentrations in both the gas- and particle phase and observed complete GEM depletion even before sunrise, ruling out a leading role of bromine chemistry here. Instead, we hypothesize that the GEM depletions were mainly caused by gas-particle interactions with sulfate-rich volcanic particles (mostly of submicron size), abundantly present in the dilute plume. We consider heterogeneous GEM oxidation and GEM uptake by particles as plausible manifestations of such a process and derive empirical rate constants. By extrapolation, we estimate that volcanic aerosols may scavenge 210 Mg y-1 (67 – 480 Mg y-1) of Hg from the atmosphere globally, acting effectively as atmospheric mercury sink. While this estimate is subject to large uncertainties, it highlights that Hg transformations in aging volcanic plumes must be better understood to determine the net impact of volcanism on the atmospheric Hg budget and Hg deposition pathways.

Background: The relative distribution and importance of monomethylmercury (MMHg) and dimethylmercury (DMHg) in seawater is still under debate. A lack of comparability between measurements at sub-picomolar levels hampered the further understanding of the biogeochemical Hg cycle. To overcome this, we assessed the relative standard measurement uncertainties (U-ex,U-r) for direct measurements of MMHg and DMHg by species-specific isotope dilution ICP-MS and cryo-focusing GC-ICP-MS at femtomolar concentrations. Furthermore, U-ex,U-r was determined for the indirect determination of DMHg (DMHgcalc = MeHg - MMHg) and MeHg (MeHgcalc = MMHg + DMHg) to compare the two methodologies.Results: Expanded U-ex,U-r (confidence interval of 95%) for cryo-focusing GC-ICP-MS was 14.4 (<50 fM) and 14.2% (>50 fM) and for SS-ID GC-ICP-MS 5.6 (<50 fM) and 3.7% (>50 fM). For concentrations above 50 fM, U-ex,U-r for DMHgcalc was always lower than for direct measurements (14.2%). For MeHgcalc, on the other hand, U-ex,U-r was always higher for concentrations above 115 fM (range: 3.7-13.9%) than for direct measurements (3.7%). We evaluated the comparability of directly measured and calculated DMHg and MeHg concentrations based on Hg speciation measurements for two vertical profiles in the Mediterranean Sea. We show that directly measured and indirectly determined DMHg and MeHg concentrations yield comparable results.Significance: Our results validate the application of the indirect method for the determination of DMHg if a direct measurement method with a low U-ex,U-r such as isotope dilution is used for MMHg and MeHg measurements. The validation of the indirect measurement approach opens new possibilities to generate more precise and accurate DMHg data in the global ocean.

Atmospheric mercury (Hg) observations in the lower free troposphere (LFT) can give important insights into Hg redox chemistry and can help constrain Hg background concentrations on a regional level. Relatively continuous sampling of LFT air, inaccessible to most ground-based stations, can be achieved at high-altitude observatories. However, such high-altitude observatories are rare, especially in the Southern Hemisphere (SH), and atmospheric Hg in the SH LFT is unconstrained. To fill this gap, we continuously measured gaseous elemental mercury (GEM; hourly) and reactive mercury (RM; integrated over ∼ 6–14 d) for 9 months at Maïdo mountain observatory (2160 m a.s.l.) on remote Réunion Island (21.1∘ S, 55.5∘ E) in the tropical Indian Ocean. GEM exhibits a marked diurnal variation characterized by a midday peak (mean: 0.95 ng m−3; SD: 0.08 ng m−3) and a nighttime low (mean: 0.78 ng m−3; SD: 0.11 ng m−3). We find that this diurnal variation is likely driven by the interplay of important GEM photo-reemission from the islands' vegetated surfaces (i.e. vegetation + soil) during daylight hours (8–22 ng m−2 h−1), boundary layer influences during the day, and predominant LFT influences at night. We estimate GEM in the LFT based on nighttime observations in particularly dry air masses and find a notable seasonal variation, with LFT GEM being lowest from December to March (mean 0.66 ng m−3; SD: 0.07 ng m−3) and highest from September to November (mean: 0.79 ng m−3; SD: 0.09 ng m−3). Such a clear GEM seasonality contrasts with the weak seasonal variation reported for the SH marine boundary layer but is in line with modeling results, highlighting the added value of continuous Hg observations in the LFT. Maïdo RM is 10.6 pg m−3 (SD: 5.9 pg m−3) on average, but RM in the cloud-free LFT might be about twice as high, as weekly–biweekly sampled RM observations are likely diluted by low-RM contributions from the boundary layer and clouds.

Even though anthropogenic mercury (Hg) emissions to the atmosphere are ∼2.5 times higher in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH), atmospheric Hg concentrations in the NH are only ∼1.5 times higher than in the SH. Global Hg models attribute this apparent discrepancy to large SH oceanic Hg emissions or to interhemispheric exchange of Hg through the atmosphere. However, no observational data set exists to serve as a benchmark to validate whether these coarse‐resolution models adequately represent the complex dynamics of interhemispheric Hg exchange. During the 2015–2016 El Niño, we observed at mount Chacaltaya in the tropical Andes a ∼50% increase in ambient Hg compared to the year before, coinciding with a shift in synoptic transport pathways. Using this event as a case study, we investigate the impact of interhemispheric exchange on atmospheric Hg in tropical South America. We use HYSPLIT to link Hg observations to long‐range transport and find that the observed Hg increase relates strongly to air masses from the tropical Pacific upper troposphere (UT), a region directly impacted by interhemispheric exchange. Inclusion of the modeled seasonality of interhemispheric air mass exchange strengthens this relationship significantly. We estimate that interhemispheric exchange drives Hg seasonality in the SH tropical Pacific UT, with strongly enhanced Hg between July and October. We validate this seasonality with previously published aircraft Hg observations. Our results suggest that the transport of NH‐influenced air masses to tropical South America via the Pacific UT occurs regularly but became more detectable at Chacaltaya in 2015–2016 because of a westward shift in air mass origin.

Since 1950 humans have introduced 8300 teragrams (Tg, 1012 g, millions of metric tons) of plastic polymers into the Earth’s surface environment. Accounting for the dispersal and fate of produced plastics and fragmented microplastics in the environment has been challenging. Recent studies have fueled debate on the global river budget for plastic transport to oceans, the sinking and beaching of marine plastics and the emission and deposition of atmospheric microplastics. Here we define a global plastics cycle and budget, and develop a box model of plastics cycling, including the fragmentation and transport of large and small microplastics (LMP, SMP) within coupled terrestrial, oceanic and atmospheric reservoirs. We force the model with historical plastics production and waste data, and explore how macroplastics, LMP and SMP propagate through the reservoirs from 1950 to 2015 and beyond. We find that considerable amounts of plastics reside most likely in the deep ocean (82 Tg), in shelf sediments (116 Tg), on beaches (1.8 Tg) and, as a result of marine emissions, in the remote terrestrial surface pool (28 Tg). Business as usual or maximum feasible reduction and discard scenarios show similar, 4-fold increases in atmospheric and aquatic ecosystem SMP exposure by 2050, because future plastics mobilization is controlled by releases from the large terrestrial discarded plastics reservoir (3500 Tg). Zero-release from 2025 onwards illustrates recovery of P and LMP reservoirs on centennial time scales, while SMP continue to cycle in air, soil, and surface ocean for millennia. Limiting dramatic future dispersal of plastics requires, in addition to reducing use and waste, remediation of the large terrestrial legacy plastics pool.

Clouds are made of droplets that arise from the activation of suitable aerosol particles (termed cloud condensation nuclei, CCN). In the activation process, water vapor saturation ratio exceeds a critial ratio enabling CCN runaway-growth to cloud droplet sizes. The number concentration of cloud droplets (CDNC) is highly dependent on the aerosol population properties (size distribution and composition), relative humidity, and the vertical wind component. While the activation of CCN consisting of non-volatile particulate matter is fairly well understood, the same process involving semi-volatile organic vapors (SVOCs) has received less attention despite their significant presence in ambient air. A recent cloud parcel modeling study shows substanial CDNC enhancement due to SVOC condensation (Topping et al., 2013). Surprisingly, the topic has not been widely investigated nor the results replicated with other cloud parcel models (CPM). Thus, in the current study we seek to quantify the CDNC enhancement by SVOC condensation using a recently developed CPM framework (Lowe et al., 2020, in prep.). Moreover, the CPM initialization is performed, for the first time, with state-of-the art measurement data including measured SVOC data for multiple airmass types. Here, the CPM, which uses spectral microphysics for the simulation of CCN activation and hydrometeor growth, also includes a SVOC condensation equation analogous to those of water vapor. Equilibrium initialization of the SVOC volatility basis set (VBS) partitioning coefficients is performed iteratively, and constrained by the organic to inorganic ratio in the particle phase determined by ambient measurements performed at the Chacaltaya Global Atmospheric Watch (GAW) Station located at 5240 m a.s.l. in the Bolivian Andes, in spring 2018. The uniquely comprehensive data set recorded, which tracks all of the relevant aerosol population characteristics in near real-time, reveals a high degree of variability in aerosol composition, size distribution and loading depending on the air mass origin. Lagrangian backward simulations during the measurement period at Chacaltaya GAW reveal at least 18 significantly different airmass origins (Aliaga et al., 2020, in prep.). Such variability served multiple model initialization scenarios for individual case studies. We will show a suite of CDNC enhancements by SVOC condensation under different initialization scenarios actualized in data recorded at Chacaltaya GAW Station, including airmasses originating from the Amazon (biomass burning and biogenic VOCs), Andean plateau (volcanic activity), and La Paz/El Alto metropolitan areas (anthropogenic emissions). References: Topping, D., Connolly, P. and McFiggans, G., 2013. Cloud droplet number enhanced by co-condensation of organic vapours. Nature Geoscience, 6(6), p.443.