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1. A synthesis of mercury research in the Southern Hemisphere, part 1: Natural processes

   https://doi.org/10.1007/s13280-023-01832-5  

   Schneider, Larissa; Fisher, Jenny A.; Diéguez, María C.; Fostier, Anne-Hélène; Guimaraes, Jean R. D.; Leaner, Joy J.; Mason, Robert (March 2023, Ambio)

   Abstract Recent studies demonstrate a short 3–6-month atmospheric lifetime for mercury (Hg). This implies Hg emissions are predominantly deposited within the same hemisphere in which they are emitted, thus placing increasing importance on considering Hg sources, sinks and impacts from a hemispheric perspective. In the absence of comprehensive Hg data from the Southern Hemisphere (SH), estimates and inventories for the SH have been drawn from data collected in the NH, with the assumption that the NH data are broadly applicable. In this paper, we centre the uniqueness of the SH in the context of natural biogeochemical Hg cycling, with focus on the midlatitudes and tropics. Due to its uniqueness, Antarctica warrants an exclusive review of its contribution to the biogeochemical cycling of Hg and is therefore excluded from this review. We identify and describe five key natural differences between the hemispheres that affect the biogeochemical cycling of Hg: biome heterogeneity, vegetation type, ocean area, methylation hotspot zones and occurence of volcanic activities. We review the current state of knowledge of SH Hg cycling within the context of each difference, as well as the key gaps that impede our understanding of natural Hg cycling in the SH. The differences demonstrate the limitations in using NH data to infer Hg processes and emissions in the SH. 

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2. Global seasonal distribution of CH ₂ Br ₂ and CHBr ₃ in the upper troposphere and lower stratosphere

   https://doi.org/10.5194/acp-22-15049-2022  

   Jesswein, Markus; Fernandez, Rafael P.; Berná, Lucas; Saiz-Lopez, Alfonso; Grooß, Jens-Uwe; Hossaini, Ryan; Apel, Eric C.; Hornbrook, Rebecca S.; Atlas, Elliot L.; Blake, Donald R.; et al (January 2022, Atmospheric Chemistry and Physics)

   Abstract. Bromine released from the decomposition of short-lived brominated source gases contributes as a sink of ozone in the lower stratosphere.The two major contributors are CH2Br2 and CHBr3.In this study, we investigate the global seasonal distribution of these two substances, based on four High Altitude and Long Range Research Aircraft (HALO) missions, the HIAPER Pole-to-Pole Observations (HIPPO) mission, and the Atmospheric Tomography (ATom) mission.Observations of CH2Br2 in the free and upper troposphere indicate a pronounced seasonality in both hemispheres, with slightly larger mixing ratios in the Northern Hemisphere (NH).Compared to CH2Br2, CHBr3 in these regions shows larger variability and less clear seasonality, presenting larger mixing ratios in winter and autumn in NH midlatitudes to high latitudes.The lowermost stratosphere of SH and NH shows a very similar distribution of CH2Br2 in hemispheric spring with differences well below 0.1 ppt, while the differences in hemispheric autumn are much larger with substantially smaller values in the SH than in the NH.This suggests that transport processes may be different in both hemispheric autumn seasons, which implies that the influx of tropospheric air (“flushing”) into the NH lowermost stratosphere is more efficient than in the SH.The observations of CHBr3 support the suggestion, with a steeper vertical gradient in the upper troposphere and lower stratosphere in SH autumn than in NH autumn.However, the SH database is insufficient to quantify this difference.We further compare the observations to model estimates of TOMCAT (Toulouse Off-line Model of Chemistry And Transport) and CAM-Chem (Community Atmosphere Model with Chemistry, version 4), both using the same emission inventory of Ordóñez et al. (2012).The pronounced tropospheric seasonality of CH2Br2 in the SH is not reproduced by the models,presumably due to erroneous seasonal emissions or atmospheric photochemical decomposition efficiencies.In contrast, model simulations of CHBr3 show a pronounced seasonality in both hemispheres, which is not confirmed by observations.The distributions of both species in the lowermost stratosphere of the Northern and Southern hemispheres are overall well captured by the models with the exception of southern hemispheric autumn,where both models present a bias that maximizes in the lowest 40 K above the tropopause, with considerably lower mixing ratios in the observations.Thus, both models reproduce equivalent flushing in both hemispheres, which is not confirmed by the limited available observations.Our study emphasizes the need for more extensive observations in the SH to fully understand the impact of CH2Br2 and CHBr3 on lowermost-stratospheric ozone loss and to help constrain emissions. 

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3. Impacts of Volcanic Emissions on the Global Biogeochemical Mercury Cycle: Insights From Satellite Observations and Chemical Transport Modeling

   https://doi.org/10.1029/2023GL104667  

   Geyman, Benjamin M.; Thackray, Colin P.; Jacob, Daniel J.; Sunderland, Elsie M. (November 2023, Geophysical Research Letters)

   Abstract Volcanism is the largest natural source of mercury (Hg) to the biosphere. However, past Hg emission estimates have varied by three orders of magnitude. Here, we present an updated central estimate and interquartile range (232 Mg a⁻¹; IQR: 170–336 Mg a⁻¹) for modern volcanic Hg emissions based on advances in satellite remote sensing of sulfur dioxide (SO₂) and an improved method for considering uncertainty in Hg:SO₂emissions ratios. Atmospheric modeling shows the influence of volcanic Hg on surface atmospheric concentrations in the extratropical Northern Hemisphere is 1.8 times higher than in the Southern Hemisphere. Spatiotemporal variability in volcanic Hg emissions may obscure atmospheric trends forced by anthropogenic emissions at some locations. This should be considered when selecting monitoring sites to inform global regulatory actions. Volcanic emission estimates from this work suggest the pre‐anthropogenic global atmospheric Hg reservoir was 580 Mg, 7‐fold lower than in 2015 (4,000 Mg). 

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4. Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry

   https://doi.org/10.1038/s41467-024-49789-7  

   Landis, Joshua D; Obrist, Daniel; Zhou, Jun; Renshaw, Carl E; McDowell, William H; Nytch, Christopher J; Palucis, Marisa C; Del_Vecchio, Joanmarie; Montano_Lopez, Fernando; Taylor, Vivien F (December 2024, Nature Communications)

   Abstract Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments. 

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5. Recent Tropical Expansion: Natural Variability or Forced Response?

   https://doi.org/10.1175/JCLI-D-18-0444.1  

   Grise, Kevin M.; Davis, Sean M.; Simpson, Isla R.; Waugh, Darryn W.; Fu, Qiang; Allen, Robert J.; Rosenlof, Karen H.; Ummenhofer, Caroline C.; Karnauskas, Kristopher B.; Maycock, Amanda C.; et al (March 2019, Journal of Climate)

   null (Ed.)

   Abstract Previous studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability. 

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