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1. Previously unaccounted atmospheric mercury deposition in a midlatitude deciduous forest

   https://doi.org/10.1073/pnas.2105477118  

   Obrist, Daniel; Roy, Eric M.; Harrison, Jamie L.; Kwong, Charlotte F.; Munger, J. William; Moosmüller, Hans; Romero, Christ D.; Sun, Shiwei; Zhou, Jun; Commane, Róisín (July 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Mercury is toxic to wildlife and humans, and forests are thought to be a globally important sink for gaseous elemental mercury (GEM) deposition from the atmosphere. Yet there are currently no annual GEM deposition measurements over rural forests. Here we present measurements of ecosystem–atmosphere GEM exchange using tower-based micrometeorological methods in a midlatitude hardwood forest. We measured an annual GEM deposition of 25.1 µg ⋅ m −2 (95% CI: 23.2 to 26.7 1 µg ⋅ m −2 ), which is five times larger than wet deposition of mercury from the atmosphere. Our observed annual GEM deposition accounts for 76% of total atmospheric mercury deposition and also is three times greater than litterfall mercury deposition, which has previously been used as a proxy measure for GEM deposition in forests. Plant GEM uptake is the dominant driver for ecosystem GEM deposition based on seasonal and diel dynamics that show the forest GEM sink to be largest during active vegetation growing periods and middays, analogous to photosynthetic carbon dioxide assimilation. Soils and litter on the forest floor are additional GEM sinks throughout the year. Our study suggests that mercury loading to this forest was underestimated by a factor of about two and that global forests may constitute a much larger global GEM sink than currently proposed. The larger than anticipated forest GEM sink may explain the high mercury loads observed in soils across rural forests, which impair water quality and aquatic biota via watershed Hg export. 

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2. Atmosphere-terrestrial exchange of gaseous elemental mercury: parameterization improvement through direct comparison with measured ecosystem fluxes

   https://doi.org/10.1039/c9em00341j  

   Khan, T. R.; Obrist, D.; Agnan, Y.; Selin, N. E.; Perlinger, J. A. (October 2019, Environmental Science: Processes & Impacts)

   To simulate global mercury (Hg) dynamics in chemical transport models (CTMs), surface-atmosphere exchange of gaseous elemental mercury, Hg 0 , is often parameterized based on resistance-based dry deposition schemes coupled with a re-emission function, mainly from soils. Despite extensive use of this approach, direct evaluations of this implementation against field observations of net Hg 0 exchange are lacking. In this study, we evaluate an existing net exchange parameterization (referred to here as the base model) by comparing modeled fluxes of Hg 0 to fluxes measured in the field using micrometeorological techniques. Comparisons were performed in two terrestrial ecosystems: a grassland site in Switzerland and an Arctic tundra site in Alaska, U.S., each including summer and winter seasons. The base model included the dry deposition and soil re-emission parameterizations from Zhang et al. (2003) and the global CTM GEOS-Chem, respectively. Comparisons of modeled and measured Hg 0 fluxes showed large discrepancies, particularly in the summer months when the base model overestimated daytime net deposition by approximately 9 and 2 ng m −2 h −1 at the grassland and tundra sites, respectively. In addition, the base model was unable to capture a measured nighttime net Hg 0 deposition and wintertime deposition. We conducted a series of sensitivity analyses and recommend that Hg simulations using CTMs: (i) reduce stomatal uptake of Hg 0 over grassland and tundra in models by a factor 5–7; (ii) increase nighttime net Hg 0 deposition, e.g. , by increasing ground and cuticular uptake by reducing the respective resistance terms by factors of 3–4 and 2–4, respectively; and (iii) implement a new soil re-emission parameterization to produce larger daytime emissions and lower nighttime emissions. We also compared leaf Hg 0 uptake over the growing season estimated by the dry deposition model against foliar Hg measurements, which revealed good agreement with the measured leaf Hg concentrations after adjusting the base model as suggested above. We conclude that the use of resistance-based models combined with the new soil re-emission flux parameterization is able to reproduce observed diel and seasonal patterns of Hg 0 exchange in these ecosystems. This approach can be used to improve model parameterizations for other ecosystems if flux measurements become available. 

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3. Elevated oxidized mercury in the free troposphere: analytical advances and application at a remote continental mountaintop site

   https://doi.org/10.5194/acp-24-9615-2024  

   Derry, Eleanor J; Elgiar, Tyler R; Wilmot, Taylor Y; Hoch, Nicholas W; Hirshorn, Noah S; Weiss-Penzias, Peter; Lee, Christopher F; Lin, John C; Hallar, A Gannet; Volkamer, Rainer; et al (January 2024, Atmospheric Chemistry and Physics)

   Abstract. Mercury (Hg) is a global atmospheric pollutant. In its oxidized form (HgII), it can readily deposit to ecosystems, where it may bioaccumulate and cause severe health effects. High HgII concentrations are reported in the free troposphere, but spatiotemporal data coverage is limited. Underestimation of HgII by commercially available measurement systems hinders quantification of Hg cycling and fate. During spring–summer 2021 and 2022, we measured elemental (Hg0) and oxidized Hg using a calibrated dual-channel system alongside trace gases, aerosol properties, and meteorology at the high-elevation Storm Peak Laboratory (SPL) above Steamboat Springs, Colorado. Oxidized Hg concentrations displayed diel and episodic behavior similar to previous work at SPL but were approximately 3 times higher in magnitude due to improved measurement accuracy. We identified 18 multi-day events of elevated HgII (mean enhancement of 36 pg m−3) that occurred in dry air (mean ± SD of relative humidity = 32 ± 16 %). Lagrangian particle dispersion model (HYSPLIT–STILT, Hybrid Single-Particle Lagrangian Integrated Trajectory–Stochastic Time-Inverted Lagrangian Transport) 10 d back trajectories showed that the majority of transport prior to events occurred in the low to middle free troposphere. Oxidized Hg was anticorrelated with Hg0 during events, with an average (± SD) slope of −0.39 ± 0.14. We posit that event HgII resulted from upwind oxidation followed by deposition or cloud uptake during transport. Meanwhile, sulfur dioxide measurements verified that three upwind coal-fired power plants did not influence ambient Hg at SPL. Principal component analysis showed HgII consistently inversely related to Hg0 and generally not associated with combustion tracers, confirming oxidation in the clean, dry free troposphere as its primary origin. 

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4. Improved mechanistic model of the atmospheric redox chemistry of mercury

   Shah, V.; Jacob, D. J.; Thackray, C. P.; Wang, X.; Sunderland, E. M.; . Dibble, T. S.; Saiz-Lopez, A; Černušák, I.; Kellö, V.; Castro, P. J.; et al (July 2021, Environmental science and technology)

   null (Ed.)

   We present a new chemical mechanism for Hg(0)/ Hg(I) / Hg(II) atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg(0) by Br atoms and OH radicals, with subsequent oxidation of Hg(I) by ozone and radicals, re-speciation of gaseous Hg(II) in aerosols and cloud droplets, and speciated Hg(II) photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg(0) concentrations and Hg(II) wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg(0) to Hg(II). Ozone is the principal Hg(I) oxidant, enabling the efficient oxidation of Hg(0) to Hg(II) by OH. BrHgOH and Hg(OH)2 are the initial Hg(II) products of Hg0 oxidation, re-speciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of Hg(II) to Hg(0) takes place largely through photolysis of aqueous Hg(II)-organic complexes. 71% of model Hg(II) deposition is to the oceans. Major mechanism uncertainties for atmospheric Hg chemistry modeling include the concentrations of Br atoms, the stability and reactions of Hg(I), and the speciation of Hg(II) in aerosols and clouds with implications for photoreduction. 

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5. A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

   https://doi.org/10.5194/acp-17-6353-2017  

   Horowitz, Hannah M.; Jacob, Daniel J.; Zhang, Yanxu; Dibble, Theodore S.; Slemr, Franz; Amos, Helen M.; Schmidt, Johan A.; Corbitt, Elizabeth S.; Marais, Eloïse A.; Sunderland, Elsie M. (January 2017, Atmospheric Chemistry and Physics)

   Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg⁰. Oxidation to water-soluble Hg^II plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg⁰∕Hg^II redox chemistry in the GEOS-Chem global model and examine the implications for the global atmospheric Hg budget and deposition patterns. Our simulation includes a new coupling of GEOS-Chem to an ocean general circulation model (MITgcm), enabling a global 3-D representation of atmosphere–ocean Hg⁰∕Hg^II cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg⁰ oxidant and that second-stage HgBr oxidation is mainly by the NO₂ and HO₂ radicals. The resulting chemical lifetime of tropospheric Hg⁰ against oxidation is 2.7 months, shorter than in previous models. Fast Hg^II atmospheric reduction must occur in order to match the  ∼ 6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM ≡ Hg⁰+Hg^II(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase Hg^II–organic complexes in aerosols and clouds, resulting in a TGM lifetime of 5.2 months against deposition and matching both mean observed TGM and its variability. Model sensitivity analysis shows that the interhemispheric gradient of TGM, previously used to infer a longer Hg lifetime against deposition, is misleading because Southern Hemisphere Hg mainly originates from oceanic emissions rather than transport from the Northern Hemisphere. The model reproduces the observed seasonal TGM variation at northern midlatitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but it does not reproduce the lack of seasonality observed at southern hemispheric marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM–ozone relationship indicative of fast Hg⁰ oxidation, but we show that this relationship provides only a weak test of Hg chemistry because it is also influenced by mixing. The model reproduces observed Hg wet deposition fluxes over North America, Europe, and China with little bias (0–30%). It reproduces qualitatively the observed maximum in US deposition around the Gulf of Mexico, reflecting a combination of deep convection and availability of NO₂ and HO₂ radicals for second-stage HgBr oxidation. However, the magnitude of this maximum is underestimated. The relatively low observed Hg wet deposition over rural China is attributed to fast Hg^II reduction in the presence of high organic aerosol concentrations. We find that 80% of Hg^II deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for Hg^II reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO₂ and NO₂ for second-stage HgBr oxidation. 

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