Atmospheric source of mercury to the ocean constrained by isotopic model

Domain: earth_science
Published: 2025-07-01
Editorial outcome: minor_revision

Abstract
Mercury is a potent neurotoxin that poses significant health risks to humans, primarily through seafood consumption. Atmospheric deposition is the largest source of oceanic mercury, in either oxidized (HgII) or elemental (Hg0) form. Understanding the relative contributions of atmospheric HgII and Hg0 to the ocean is essential for accurately assessing global mercury budgets. Earlier even mercury isotope (Δ200Hg) analyses suggested equivalent HgII/Hg0 contributions but neglected spatial variations in atmospheric Δ200Hg signatures. Here, we developed a 3D atmospheric model incorporating mercury chemistry and isotopic fractionation to address this limitation. Our simulations reveal distinct atmospheric Δ200Hg patterns and quantify their deposition to the ocean. Constrained by observed Δ200Hg data in the ocean, we propose an updated deposition ratio of atmospheric HgII to Hg0 to the ocean, which may exceed 2:1, higher than the previously reported 1:1. Our findings are crucial for assessing atmospheric mercury dispersal and predicting the recovery of marine ecosystems.

Introduction
Mercury (Hg) is a pervasive global pollutant with severe health impacts1. It is released into the environment from both human activities, such as fossil fuel burning and mining, and natural processes like oceanic emissions and wildfires2,3. Once in the atmosphere, Hg exists mainly as gaseous elemental mercury (Hg0), which makes up over 80% of atmospheric Hg and can remain airborne for months. The oxidized forms of mercury (HgII), including both gaseous [HgII(g)] and particle-bound species [HgII(p)], are less abundant but are rapidly removed by deposition4–6. Atmospheric deposition of HgII and Hg0 represents the major source of Hg to marine environments, influencing Hg biomagnification and cycling in the ocean7,8. However, the precise contributions of these Hg forms to oceanic Hg levels remain uncertain.

Direct observations of Hg0 and HgII deposition fluxes over the ocean are limited. Recent atmospheric model studies report varying ratios of HgII deposition compared to Hg0 uptake overs ocean, accompanied by distinct Hg deposition fluxes. The models, including different assumptions in Hg emissions and atmospheric redox chemistry, indicated that HgII deposition over the modern ocean is 2:1 to 3:1 compared to Hg0 uptake, with gross fluxes ranging from 3900 to 4600 Mg yr−1 and 1300 to 2000 Mg yr−1, respectively4,5. These models are often evaluated against atmospheric Hg concentration data near the ground and the troposphere, as well as wet deposition fluxes observed over the land, leaving such a ratio largely unconstrained.

Isotopic signatures provide a more reliable means of constraining these fluxes. Mercury has seven isotopes that exhibit both mass-dependent (MDF, δ202Hg) and mass-independent fractionation (MIF) signatures, including odd-MIF (Δ199Hg and Δ201Hg) and even-MIF (Δ200Hg and Δ204Hg)9,10. Even-MIF is known to be generated in the upper troposphere through photochemical reactions, making it a conservative tracer in surface environments that can be used to trace atmospheric inputs of Hg11–13. A recent study by Jiskra et al.14 compared atmospheric and marine Δ200Hg signatures, suggesting an atmospheric HgII:Hg0 contribution ratio of 1:1. This ratio is significantly lower than those reported in previous modeling studies. Based on this, a recent coupled atmosphere-land-ocean model updated the global Hg budget, suggesting a ratio of 1.5:1, but with much higher fluxes across the air-sea interface, estimating HgII deposition and Hg0 uptake at 4860 Mg yr−1 and 3270 Mg yr−1, respectively15.

However, most of the atmospheric isotope data used by Jiskra et al.14 are derived from measurements on land or in coastal regions. Relying on land-based observations to trace oceanic Hg sources may introduce significant uncertainties. Firstly, previous observations and modeling have revealed substantial global spatial variation in atmospheric Hg isotopes16–19. Secondly, current isotopic observations primarily focus on Hg0 and wet deposition, with a notable lack of isotopic characterization for dry deposition of Hg over the ocean. Accurate isotopic characterization of atmospheric end-members is crucial for effectively constraining sea-air Hg exchange using isotopes. But the absence of direct observations of Hg isotope signatures for different forms of Hg in the marine atmosphere limit the effectiveness of these constraints.

We propose that recent advancements in three-dimensional atmospheric Hg isotope modeling offer a promising opportunity to address this issue18,19. In this study, we aim to update the relative contributions of HgII and Hg0 to oceanic Hg by using more accurate Δ200Hg signatures in atmospheric end-members. We integrate Hg isotopes and isotopic fractionation into the GEOS-Chem model to develop an atmospheric Hg isotope model. Based on the even-MIF mechanisms outlined in our recent study (see Methods for details), we present an optimized modeling scenario for atmospheric even-MIF. This model elucidates the Δ200Hg signatures in global atmospheric Hg deposition, constrained by observed Δ200Hg in global atmospheric samples. By comparing these with observed oceanic Δ200Hg values, we propose an updated ratio of atmospheric HgII to Hg0 deposition to the ocean of over 2:1, higher than the previously reported 1:1 ratio. Our results indicate that spatial differentiation of atmospheric Δ200Hg signatures significantly impacts the findings, with important implications for understanding the global Hg cycle.

Results
Modelevaluation
We developed a three-dimensional isotopic model for atmospheric Hg, incorporating state-of-the-art Hg chemistry, to elucidate the potential processes driving even-MIF. Our model results show comparable spatial variability with globally observed Hg concentrations and wet deposition fluxes (Supplementary Discussion, Text 1, Supplementary Fig. 1). Additionally, our modeling results reveal pronounced spatial variations in atmospheric Δ200Hg, particularly for HgII species, a phenomenon not previously documented (Fig. 1). Since the model outputs are annual averages, we utilize background observations, from various environments—including the poles, remote mountains, coastal regions, and marine boundary layer (MBL) areas—for modeling validation. The model results reproduce the observed Δ200Hg signatures in global atmospheric samples, including Hg0 (represented by Δ200Hg0), HgII(g) [represented by Δ200HgII(g)], HgII(p) [represented by Δ200HgII(p)], and precipitation [represented by Δ200HgII(pre)]. As illustrated in Fig. 1a, the observed Δ200Hg0 shows a mean value −0.07±0.02‰ (n=48), which is closely matched by modeled Δ200Hg0 of −0.07±0.01‰ (n=48). The modeled mean Δ200Hg0 also aligns with observations from the Arctic (about −0.07‰)20–23, remote mountain regions (about −0.07‰)11,24–26, and MBL (about −0.06‰)27–30. Similarly, the observed Δ200HgII(p) has a mean value of 0.06±0.05‰ (n=13), and our model yields a consistent value of 0.06±0.06‰ (n=13) (Fig. 1b). Specifically, the model successfully reproduces the mean Δ200HgII(p) values observed in remote mountain regions (about 0.10‰)11,28,31,32 and coast MBL regions (about 0.06‰)28,33–36, including the Huaniao Island (about 0.08‰) at Northwest Pacific Ocean32.

The modeled Δ200HgII(g) distribution closely mirrors that of Δ200HgII(p) (Fig. 1c), which can be attributed to the degassing process that produces HgII(g) from HgII(p)4. Although observations of Δ200HgII(g) are limited, two studies have measured Δ200HgII(g) in Grand Bay, USA (0.18±0.07‰)34 and Pic du Midi, France (0.15±0.06‰)11, with our model yielding comparable mean values of 0.16‰ and 0.17‰, respectively. The modeled Δ200HgII(pre) also exhibits a consistent mean value with observations, at 0.15±0.04‰ (n=15, Fig. 1d), and shows a significant positive correlation with observed data, with a correlation coefficient of 0.62, reproducing the latitudinal variation of observations (Fig. 1e). Notably, the modeled Δ200HgII(pre) values (0.10‰ and 0.07‰) aligns well with observations from the Island of Hawaii (0.14±0.05‰) and the R/V Kilo Moana (open ocean station ALOHA, 0.08‰)37, which are representative of Δ200HgII(pre) observed over the open Pacific Ocean. In summary, our model successfully simulates the gradient of Δ200Hg values, with the highest values in precipitation samples, followed by HgII(p), and the lowest in Hg0. The close agreement between our modeled and observed values, which approximate a 1:1 relationship (Fig. 1f), underscores the reliable performance of our Hg isotope model.

The distinct variations in Δ200Hg values among HgII species—including gaseous (HgII(g)), particulate (HgII(p)), and precipitation-derived (HgII(pre))—arise from chemical fractionation processes. Comparison with the global atmospheric Hg redox rate (Supplementary Fig. 2a, d) reveals that regions dominated by net Hg oxidation, such as parts of the central Indian and Pacific Oceans, exhibit relatively lower Δ200Hg values in HgII(g), HgII(p), and wet deposition (Fig. 1b, d). These patterns reflect the influence of specific fractionation mechanisms: Br-initiated oxidation dominates in the marine boundary layer (Supplementary Fig. 2a), while even-MIF processes occur during OH-initiated oxidation and HgII(p) photoreduction (see Methods). Elevated Δ200HgII values in African regions further highlight the spatial variability of chemical fractionation. High PM2.5 concentrations driven by dust emissions (Supplementary Fig. 2e) promote HgII(p) formation, while concurrent high photolysis frequencies (Supplementary Fig. 2f) induce strong net HgII(p) reduction (Supplementary Fig. 2d). Additionally, significant OH-initiated chemical activity (Supplementary Fig. 2b) also contributes to this regional Δ200Hg signal. We decompose the fractionation process and find that the OH-initiated chemistry and HgII(p) photoreduction pathways induce distinct Δ200HgII distribution patterns, as these reactions prevail in distinct atmospheric layers (Supplementary Fig. 3). Both processes contribute to the elevated Δ200HgII values over the African land mass (Supplementary Fig. 3). The modeled Δ200HgII signatures at the surface are also influenced by the upper atmosphere. The free troposphere, marked by intense redox rates4,5 (Supplementary Fig. 2g and h), generates high Δ200Hg values in HgII species (Supplementary Fig. 2f). Subsequent convective transport (Supplementary Fig. 4d, e) delivers these species to the surface, thereby affecting surface Δ200HgII signatures. Moreover, compared with regions such as East and South Asia, the African region has relatively lower anthropogenic Hg emissions that is characterized with a close to zero Δ200Hg signal, further enhancing the importance of chemical fractionation processes in this area.

Isotope signature of atmospheric deposition
The model elucidates the Δ200Hg signatures of atmospheric sources deposited into the ocean, including Hg0 uptake, HgII dry deposition, and HgII wet deposition. The Δ200Hg signatures in deposited Hg are influenced by the corresponding signatures in atmospheric Hg. As shown in Fig. 2a, Hg0 uptake by the ocean generally shows negative Δ200Hg0 signatures, with a median value of −0.06‰ [interquartile range (IQR), −0.07‰ to −0.05‰]. In contrast, HgII dry deposition, encompassing both HgII(g) and HgII(p), displays distinct spatial variations in Δ200Hg signatures (Fig. 2b). Globally, HgII dry deposition to the ocean has a median Δ200Hg value of 0.00‰ (IQR, −0.02‰ to 0.03‰). And near-shore regions show higher Δ200Hg values compared to the open ocean, with median values of 0.04‰ (IQR, 0.01‰ to 0.09‰) and 0.01‰ (IQR, −0.01‰ to 0.03‰), respectively. The modeled Δ200Hg values in wet deposition to the ocean are generally positive, with a median of 0.08‰ (IQR, 0.06‰ to 0.12‰) for the global ocean and 0.11‰ (IQR, 0.09‰ to 0.13‰) for near-shore regions (Fig. 2c). The spatial patterns in both dry and wet deposition display similar trends, largely driven by the Δ200HgII distributions (Fig. 1 and Supplementary Fig. 4). Although HgII dry deposition occurs primarily in the boundary layer, the Δ200Hg signatures of HgII(g) and HgII(p) therein are significantly influenced by upper-atmospheric processes—similar to the Δ200Hg observed in precipitation due to the washout of HgII species from higher altitudes. Moreover, elevated Δ200Hg values in both dry and wet deposition near coastal regions are also influenced by terrestrial transport, as on-land Hg chemistry exhibits stronger reduction rates (Supplementary Fig. 2), leading to higher Δ200Hg in HgII species.

Our results reveal distinct differences between the Δ200Hg signatures of atmospheric dry deposition HgII and wet deposition HgII over the ocean—a detail not fully emphasized in previous studies14. Since dry deposition of HgII occurs primarily in the boundary layer, while wet deposition of HgII originates mainly from higher atmospheric layers, the Δ200Hg values for wet deposition are considerably higher than those for dry deposition. In Jiskra’s study14, the Δ200Hg characteristics of dry- and wet-deposited HgII were not distinguished; instead, they were combined into a HgII end-member with Δ200Hg values ranging from 0.13‰ to 0.15‰. Compared to our modeling results, this approach likely overestimates the Δ200Hg of the dry deposition HgII, leading to an underestimation of HgII deposition’s contribution to oceanic Hg. Our model provides more detailed Δ200Hg signatures of atmospheric Hg sources, offering a more accurate resolution of the contribution of atmospheric Hg to oceanic Hg.

Atmospheric sources to ocean
We use Δ200Hg data to evaluate the atmospheric contribution of Hg to the ocean, as Δ200Hg is uniquely produced in the atmosphere and remains conserved in ocean. We exclude other potential marine Hg sources, such as hydrothermal emissions and river inputs, since hydrothermal Hg flux is minimal38 and riverine Hg primarily affects coastal regions39. Figure 3a compares the modeled latitudinal variations of Δ200Hg in atmospheric Hg depositions with observations in ocean samples (see method). The deposited Δ200Hg values exhibit three different gradients, with wet deposition showing the highest values, followed by HgII dry deposition, and then Hg0 uptake. The observed interquartile ranges (P25–P75) of Δ200Hg values in ocean samples fall between these three end-members of Hg depositions, suggesting that the relative proportions of these three depositions to oceanic Hg can be differentiated based on Δ200Hg isotope mass balance.

The Δ200Hg0 values exhibit minimal variation with latitude, with only a slight downward trend observed in lower latitudes (Fig. 3a). In contrast, Δ200Hg in both dry and wet deposition of HgII shows distinct latitudinal variations, with an increasing trend in low-latitude regions. This corresponds with an upward trend in oceanic Δ200Hg values in the same areas, suggesting a greater contribution of atmospheric HgII to oceanic Hg compared to atmospheric Hg0. Additionally, both Hg0 and dry-deposited HgII display lower Δ200Hg values compared to observed values in certain regions, such as between 20°S and 40°S and 50°N and 70°N, indicating a more significant role for wet deposition in these areas. The overall Δ200Hg values in both wet and dry deposition of HgII align more closely with observed oceanic Δ200Hg, indicating that atmospheric HgII might be a more substantial contributor to the global ocean Hg pool than atmospheric Hg0.

The Hg deposition proportions and isotopic signatures of this model can be further used to quantify the contribution of atmospheric Hg to marine Hg. As illustrated in Fig. 3b, the majority of Δ200Hg values in the modeled total atmospheric deposition (purple area) are comparable to the interquartile range of oceanic observations. This suggests that the relative proportions of the three types of Hg deposition in our model are largely reasonable for explaining the oceanic Δ200Hg. However, the modeled Δ200Hg values are also slightly lower than the observations between 20°S to 40°S, 10°N to 20°N, and 50°N to 70°N, implying a potential underestimation of the relative contribution from HgII wet deposition or, alternatively, an overestimation of Hg0 uptake and HgII dry deposition in these regions. Indeed, previous studies have shown that the GEOS-Chem model underestimates HgII precipitation in equatorial tropical areas, partly due to its limitations in accurately predicting cloud mass in these regions7,40. Additionally, when combined our modeled Δ200Hg with Jiskra’s Hg deposition flux data14, the total Δ200Hg also shows lower values in the regions between 20°S to 40°S and 50°N to 70°N (Supplementary Fig. 5), further supporting this conclusion.

Figure 3c exhibits the proportions of the three types of depositions to the ocean calculated by the model. The proportion of Hg0 tends to decrease in low-latitude tropical regions, while the proportion of HgII wet deposition tends to increase. This trend may explain why ocean samples from these regions exhibit higher Δ200Hg values (Fig. 3a), as wet deposition in these areas is associated with high positive Δ200Hg values (Fig. 3b). Globally, atmospheric Hg0 contributes 34% to oceanic Hg, while atmospheric HgII accounts for the remaining 66%, with 38% from dry deposition and 28% from wet deposition. This indicates a 2:1 ratio of atmospheric HgII to Hg0 deposition over the ocean. Given the potential underestimation of wet deposition’s contribution in the model as indicated above, we suggest this ratio likely exceeds 2:1, significantly higher than the 1:1 ratio reported in the previous study14. This also implies that the contribution of atmospheric HgII wet deposition to marine Hg may be greater than 28%, suggesting that the combined contribution of HgII dry deposition and Hg0 uptake could be less than 72%.

Uncertainty
The uncertainties in this study may stem from the simulation of even-MIF of atmospheric Hg. While our model can replicate Δ200Hg values comparable to atmospheric observations, the fractionation mechanisms and enrichment factors for even-MIF in the model have not been definitively established. Therefore, we advocate for further experimental and theoretical investigations on this matter. Additionally, the Hg isotope signatures of source emissions exhibit standard deviations ranging from 0.02‰ to 0.05‰, as indicated in Table 1. To evaluate the impact of the upper and lower bounds (maximum or minimum isotopic composition) of various emission sources on the modeling results, we performed a sensitivity analysis. The findings indicate that uncertainties in source emissions can lead to shifts of ±0.02‰ to ±0.03‰ in the simulated Δ200Hg0, Δ200HgII(g), Δ200HgII(p), and Δ200HgII(pre), which minimally impact the analysis of these Δ200Hg values. Another uncertainty arises from the assumption that all oceanic Hg originates from the atmosphere, which may not be applicable to coastal regions where rivers are the predominant source39. Uncertainties may also result from the limited observational coverage of the ocean. Current observations do not encompass the entire ocean, particularly in open ocean regions like the central Indian Ocean and the Pacific Ocean. This uneven spatial distribution of observations may influence the analysis of the results.

Another source of uncertainty arises from the delineation of the Hg cycle of our isotope model. However, the impact on our conclusion is limited as the simulation of deposition fluxes and their isotopes in our model operates relatively independently. To validate this, we conducted sensitivity tests using different enrichment factors (E200Hg) during the chemical fractionation process. As illustrated in Supplementary Fig. 6, two scenarios with various E200Hg yielded consistent ratios of the three forms of Hg occupancy, although the simulated Δ200Hg values varied significantly. Moreover, the isotopic observations in atmospheric and ocean samples are independent of the simulations, providing robust constraints on the modeled Δ200Hg signatures in both atmospheric Hg and its deposition. This confirms the feasibility of using modeled Δ200Hg values to trace oceanic Hg sources.

The modeling uncertainties may arise from the omission of the lightning process that induces even-MIF, as proposed by recent experimental studies41. As a result, this could influence the simulation of Δ200Hg signatures in wet deposition, given that precipitation events are often associated with lightning. However, according Sun et al.41, it is still unrealistic to attribute global significance to Hg(3P)-induced oxidation (Supplementary Discussion, Text S2). Another potential source of bias is the model resolution. For example, the 4°×5° GEOS-Chem model may fail to accurately capture high-altitude precipitation events in North America42, potentially leading to an underestimation of Hg elution with positive Δ200HgII values from higher altitudes in this region. Additionally, although the model reproduces the existing observations well, these are primarily concentrated in East Asia, North America, Europe, and parts of the Arctic. Consequently, we recommend that future studies focus on more remote regions, such as South America, Africa, and the open ocean.

Implications
Current observations lack the direct measurement of fluxes in both HgII and Hg0 deposition over the ocean. The Δ200Hg data and modeling indicate that previous findings, which suggested an equal contribution (1:1) of atmospheric HgII and Hg0 to the ocean14, may be biased. Our analysis proposes a higher relative contribution of HgII, with a ratio of might over 2:1, and highlights the significant spatial heterogeneity of Δ200Hg in atmospheric Hg deposition as a key factor in resolving isotopic signatures at the Earth’s surface, including both land and ocean. The greater relative contribution of atmospheric HgII to marine Hg may impact the marine ecosystems, as various forms of Hg in the ocean affect methylmercury production43,44. This has direct implications for human health due to the consumption of seafood containing methylmercury. Conversely, since Hg0 constitutes the majority of atmospheric Hg, its smaller relative contribution to oceanic Hg affects its atmospheric lifetime and ultimately influences the benefits of reducing anthropogenic Hg emissions.

The updated ratio provides an important constraint for the global Hg budget, offering insights into the sea-air Hg fluxes. In global modeling studies, sea-air exchange fluxes can be adjusted within the model by modifying relevant parameters to achieve a self-consistent Hg balance between sea and air4,5,15. For example, a coupled atmosphere-land-ocean model15 suggests atmospheric Hg0 and HgII deposition fluxes of 3270 Mg yr−1 and 4860 Mg yr−1, respectively, constrained by the previously believed 1:1 HgII to Hg0 ratio. This indicates an ocean Hg0 emission of 7220 Mg yr−1 resulted from adapting faster air-sea exchange velocities. Our study suggests that the oceanic Δ200Hg data may not fully support this budget.

Our results effectively differentiate the spatial variation of Δ200Hg in atmospheric Hg and distinguish between the Δ200Hg signatures of dry and wet deposition over the ocean. This demonstrates the advantages of three-dimensional atmospheric Hg isotope modeling, which effectively captures global variations in atmospheric Hg isotopes, compensates for the inhomogeneity of observational data, and provides new perspectives on using Hg isotopes to analyze the global Hg cycle. To better constrain the global Hg cycle, we propose that both atmospheric and oceanic Hg balance and isotopic composition should be integrated. Future research should focus on developing coupled atmosphere-land-ocean isotope models, which could serve as a powerful tool for constraining the global Hg cycle.

Methods
Model description
The GEOS-Chem (version 12.9.0) platform is used to develop the Hg isotope model, featuring a resolution of 4° latitude by 5° longitude and 47 vertical layers18,19. The model employs the native MERRA-2 (4°×5°) meteorological reanalysis data produced by NASA’s Global Modeling and Assimilation Office (GMAO). Anthropogenic Hg emissions are sourced from Streets et al.45, while natural emissions are cited from Shah et al.4. Atmospheric HgII, including HgII(g) and HgII(p), experiences both dry and wet deposition over the ocean. Dry deposition of Hg following the resistance-in-series scheme of Wesely (1989)46. Over the ocean, dry deposition velocity for the atmospheric HgII(g) is biologically unreactive and has a highly high Henry’s law constant47. Dry deposition of HgII(p) is calculated according to the aerosol deposition scheme48,49. Sea-salt aerosol uptake of HgII also acts as a dry deposition sink of Hg in the marine boundary layer6. And wet scavenging of HgII follows the scheme proposed by Amos et al.50 and Liu et al.51. The exchange of Hg0 between the air-sea interface is controlled by the following equations48:

Flux_up = K_w × C_Hg0

Flux_down = K_w × C_Hga / H

Where Flux represents the flux of Hg0 from the ocean to the atmosphere, and K_w denotes the mass transfer coefficient, which was developed based on field experiments with volatile and nonvolatile tracers52. C_Hg0 is the concentration of dissolved Hg0 in seawater. We utilize the state-of-the-art chemistry mechanism of atmospheric Hg, which includes mainly four redox pathways4: (i) the oxidation of Hg0 to HgII by Br; (ii) the oxidation of Hg0 to HgII by OH; (iii) the oxidation of Hg0 to HgII by Cl; (iv) photoreduction of HgII(p) to Hg0. This mechanism also integrates the photoreduction of gaseous HgII53, a process that occurs primarily within the Br-initiated pathway and significantly influences global Hg deposition. The oxidant field includes debromination from sea-salt aerosol in the marine atmosphere54,55, which enhances the Br radicle concentration in the marine boundary compared to previous Br fields56. The model simulation covers the period from 2016 to 2018, with the first two years used for model initialization and the third year dedicated to result analysis.

Isotope notion and fractionation
We follow the notation established by Blum and Bergquist10 to express the isotopic ratio difference simulated species and the NIST-3133 standard, measured in permil (‰):

δxxxHg(m) = [(xxx/198Hg / xxx/198Hg_NIST3133) − 1] × 1000

MIF is calcuated as below:

Δ199Hg = δ199Hg − δ202Hg × 0.252

Δ200Hg = δ200Hg − δ202Hg × 0.502

Δ201Hg = δ201Hg − δ202Hg × 0.752

Δ204Hg = δ204Hg − δ202Hg × 1.493

In our calculations, the xxx/198Hg ratios are treated as constants, derived from the atomic abundances of Hg isotopes in the NIST-3133 standard10,57: 196Hg=0.155%, 198Hg=10.04%, 199Hg=16.94%, 200Hg=23.14%, 201Hg=13.17%, 202Hg=29.73%, and 204Hg=6.83%. For the Hg isotope inventories, we synthesize the Hg isotopic composition of each source and calculate its isotopic ratio (xxx/198Hg) using Eqs. (3)–(7) (Table 1). These ratios allow us to divide total Hg emissions into seven isotopes, which can be input into the model as individual tracers. After running the model, it outputs the concentrations and fluxes of these isotopes, enabling us to calculate the isotope signatures using Eqs. (3)–(7).

Our model incorporates isotopic fractionation during chemical processes, treating these fractionations as kinetic processes. The chemical reaction coefficients are slightly adjusted for various isotopes by applying multiple fractionation factors (α), thus achieving isotope fractionation during these chemical reactions18,19. In the model, 198Hg is treated as the reference isotope, maintaining the chemical reaction coefficients (K_198) consistent with the standard GEOS-Chem. The calculations of K_xxx for other six isotopes follow the MDF kinetic laws and utilizes reported enrichment factors for specifically processes. The K_xxx is for calculating K_xxx is as follows:

ln(α_xxx/198Hg_MDF) = ε_xxxHg / 1000

ln(α_xxx/198Hg_MIF) = E_xxxHg / 1000

ln(α_xxx/198Hg_MDF) = β_xxx × ln(α_202/198Hg)

α_xxx/198Hg_tot = α_xxx/198Hg_MDF × α_xxx/198Hg_MIF

K_xxx = K_198 × α_xxx/198Hg_tot

Where the ε_xxx and E_xxx represent the reported enrichment factors for the modeled isotopes signatures. These factors are calculated as the difference between the MDF and MIF signatures as product over reactant Hg pool. The calculation of α_xxx/198Hg_MDF follows the kinetic laws governing Hg transfer, where β takes values as −0.507, 0.252, 0.502, 0.752, and 1.493 for 196Hg, 199Hg, 200Hg, 201Hg, 202Hg, and 204Hg, respectively10. The α_xxx/198Hg_tot represents the processes-based kinetic fractionation factors and is defined as the ratio of xxx/198Hg for the product to that in the reactant. The α_xxx/198Hg_tot comprises two parts: α_xxx/198Hg_MDF and α_xxx/198Hg_MIF. In this study, we focus on the mass-independent fractionations of even mass isotopes (represented by Δ200Hg). While previous studies11–13,41,58 have documented elevated Δ200Hg values in precipitation and aerosols, the mechanisms driving even-MIF remain poorly constrained. Chemical processes of Hg0 photo-oxidation in the tropopause12,13 and HgII photolysis on aerosols mediated by magnetic halogen nuclei11 have been proposed as potential sources. Experimental evidence further implicates mercuric oxide photodissociation in UVC-exposed Hg–O systems41. Recent detections of positive Δ200Hg in high-altitude gaseous HgII(g), particulate Hg (HgII(p)), and marine/urban aerosols59–63 underscore the spatial complexity of these processes. Here, we evaluate atmospheric Hg redox pathways using the state-of-the-art atmospheric Hg chemical frameworks4, identifying HgII(p) photoreduction and OH-initiated reactions are plausible dominant even-MIF drivers. Our model successfully reproduces global Δ200Hg distributions (Supplementary Discussion Text S2), resolving key uncertainties in the environmental contexts and chemical pathways of even-MIF generation.

Mercury isotope data
We synthesized representative atmospheric observations of Δ200Hg. These observations are far away from high anthropogenic emission areas, especially for East Asia, where the atmospheric Δ200Hg signatures are significant influenced by anthropogenic emissions. The selected observations are conducted at poles, remote mountain regions, coast areas, marine boundary layer. Most of the land-based observations are from Asia and North America and are presented as average values, including Hg0, HgII(p), and precipitation [HgII(pre)] samples. All the atmospheric isotope data are presented in Supplementary Table 1. For oceanic Hg isotopic signatures, we utilize the synthesized data from Jiskra et al.14, which includes total Hg in seawater, particle-bound Hg, sediment, and biota samples. These data are collected from four oceans: the Mediterranean Sea, Atlantic Ocean, Pacific Ocean, and Southern Ocean. The median Δ200Hg for oceanic samples are as follows: 0.03‰ (IQR, 0.01‰ to 0.05‰; n=87) for the Mediterranean Sea, 0.02‰ (IQR, −0.01‰ to 0.05‰; n=122) for the Atlantic Ocean, 0.06‰ (IQR, 0.03‰ to 0.08‰; n=295) for the Pacific Ocean, 0.00‰ (IQR, −0.03‰ to 0.08‰; n=110) for the Southern Ocean. All Δ200Hg signatures for various sample categories in the oceans are detailed in Jiskra et al.14.

Data availability
All data generated in this study are available in the main text, the supplementary information, and the research group website: https://www.ebmg.online/mercury. The data for model evaluation are synthesized from references and presented in Supplementary Information/Source data file. Source data are provided with this paper.

Code availability
In adherence to principles of transparency and reproducibility, we have made our code accessible to facilitate the replication and validation of our findings. All Hg isotope model code is available at the research group website (https://www.ebmg.online/mercury)64. The original GEOS-Chem code can be obtained at: https://github.com/geoschem/geos-chem (https://doi.org/10.5281/zendo.15242410)65.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "The study by Song et al entitled “Atmospheric source of mercury to the ocean constrained by isotopic model” uses D200Hg (even-MIF anomalies) to decipher the relative proportion of Hg(0) and Hg(II) deposition over the oceans. The study applies D200Hg into a global scale atmosphere-chemistry-transport model to evaluate Hg(0):Hg(II) deposition and compares the modelled results with the measured ones. In general, I praise the authors for undertaking such difficult task and for simulating D200Hg deposition on a global basis. The model also simulates various Hg species including Hg(0) and Hg(II) in gas, particulate, and wet phases relative to the study by Jiskra, which took a simpler approach of modeling Hg(0) and Hg(II) deposition.\nDespite the effort, I must say that the model is built upon a rather questionable assumption that even-MIF anomalies occur primarily via photoreduction of Hg(II)p rather than Hg photo-oxidation as suggested by many prior studies. It is true that further theoretical and experimental work is needed to better understand the mechanism(s) resulting in even-MIF anomalies. At the same time, the current notion that photo-oxidation causes even-MIF should not be discredited, which has been built upon few experimental works and lots of measured/compilated dataset. Perhaps, this is what has generated numerous review papers on even-MIF anomalies (Blum and Johnson 2017 Reviews in Minerology & Geochemistry, Cai and Chen 2015 Science Bulletin etc…). I think this assumption, while tested by the model, is what’s driving negative D200Hg values in Hg(II)g, Hg(II)p, and wet deposition in the central Indian and Pacific Oceans (line 124-126). The problem is that the simulated values go against the actual measured data, which reported positive D200Hg in dry and wet Hg(II) species from the Pacific Ocean (including biota) (Motta et al 2019 Global Biogeochemical Cycles, Motta et al 2022 PNAS). Also, more positive D200Hg in nearshore environment is confounding, because riverine influence would likely result in widespread and negative D200Hg in the coastal oceans.\nI understand that the beauty of modeling is to identify/verify certain mechanisms (in this case photoreduction vs photo-oxidation). And I still praise the authors for the exploration and hard work of building a sophisticated model. If I may respectfully suggest a way to go about this is to first show the readers why/how photo-oxidation, especially in the tropopause (to my knowledge the atmospheric column matters due to UV exposure), does not produce measured results and if so WHERE does measured-modelled mismatch occur most noticeably? I think this is a good starting point to figure out whether there may be other Hg sources or mechanisms, which may cancel off the observed even-MIF. The second part is to actually show that photoreduction does better than photo-oxidation (if this is true…).\nOverall, it is difficult to recommend this paper for revision or publication as a present form. But I do agree that Hg(0):Hg(II) deposition (2:1) modelled by a prior study is overestimated and strongly encourage the authors to move this work forward with a tailed assumption.", "reviewer_3": "Quantifying the contribution of atmospheric Hg to ocean is a significant issue to determine the Hg budget globally. Here the authors innovatively integrate Hg isotope into a three dimensional atmospheric Hg isotope model and elucidate the Δ200Hg signatures of atmospheric Hg. This pioneering combination of traditional numerical model and emerging isotope method aid to reassess Hg deposition to ocean and thus reduce the uncertainty of current understanding on Hg cycling. This work is interesting and with significance. We recommend publication after addressing the following comments.\n1. Model evaluation. Brief description of method integration should be introduced at the beginning of results as it is a significant innovation of this study.\n2. Line 125-128 It is generally assumed that Δ200Hg is from the upper atmospheric Hg oxidation. What is the theory base for the assumption that even-MIF is driven particulate HgII photoreduction?\n3. Figure 2 (a), how to explain the white strip around antarctica? Although the exact Δ200Hg values between HgII dry deposition and wet deposition vary, their spatial distributions are quite similar. Higher values have been observed at the coastal areas. How to explain suc phenomena?\n4. Line 298-302 Existing global emissions with spatial distribution do not contain the detail isotope information. How do you consider the isotope fractions in emissions?\n5. Line 311-315 Direct gaseous HgII reduction has been proved to be important and to significantly impact Hg deposition distribution. Why do not you consider it in this study? What is the impact of such process is missing?\nSaiz-Lopez, A.; Sitkiewicz, S. P.; Roca-Sanjuan, D.; Oliva-Enrich, J. M.; Davalos, J. Z.; Notario, R.; Jiskra, M.; Xu, Y.; Wang, F.; Thackray, C. P.; Sunderland, E. M.; Jacob, D. J.; Travnikov, O.; Cuevas, C. A.; Acuna, A. U.; Rivero, D.; Plane, J. M. C.; Kinnison, D. E.; Sonke, J. E., Photoreduction of gaseous oxidized mercury changes global atmospheric mercury speciation, transport and deposition. Nat Commun 2018, 9."}, "author_response": "Author response to comments on MS# NCOMMS-24-61583, “Atmospheric Source of Mercury to the Ocean Constrained by Isotopic Model”\nWe would like to thank the two reviewers and editor for their feedback on the manuscript. We very much appreciate their time and have considered these comments carefully. We present our response below, as well as changes to the main text (reviewer comments are black font), author response are blue, and quotes from the revised manuscript are purple). Line numbers (L###) refer to revised manuscript (clear version).\nReviewers' comments:\nReviewer #1 (Remarks to the Author):\nThe study by Song et al entitled “Atmospheric source of mercury to the ocean constrained by isotopic model” uses D200Hg (even-MIF anomalies) to decipher the relative proportion of Hg(0) and Hg(II) deposition over the oceans. The study applies D200Hg into a global scale atmosphere-chemistry-transport model to evaluate Hg(0):Hg(II) deposition and compares the modelled results with the measured ones. In general, I praise the authors for undertaking such difficult task and for simulating D200Hg deposition on a global basis. The model also simulates various Hg species including Hg(0) and Hg(II) in gas, particulate, and wet phases relative to the study by Jiskra, which took a simpler approach of modeling Hg(0) and Hg(II) deposition.\nWe thank you for your recognition and your valuable feedback. In the revised version we have redesigned our simulations and added new analyses to address your major concerns.\n1.①Despite the effort, I must say that the model is built upon a rather questionable assumption that even-MIF anomalies occur primarily via photoreduction of Hg(II)p rather than Hg photo-oxidation as suggested by many prior studies. It is true that further theoretical and experimental work is needed to better understand the mechanism(s) resulting in even-MIF anomalies. At the same time, the current notion that photo-oxidation causes even-MIF should not be discredited, which has been built upon few experimental works and lots of measured/compilated dataset. Perhaps, this is what has generated numerous review papers on even-MIF anomalies (Blum and Johnson 2017 Reviews in Minerology & Geochemistry, Cai and Chen 2015 Science Bulletin etc…).② I think this assumption, while tested by the model, is what’s driving negative D200Hg values in Hg(II)g, Hg(II)p, and wet deposition in the central Indian and Pacific Oceans (line 124-126). The problem is that the simulated values go against the actual measured data, which reported positive D200Hg in dry and wet Hg(II) species from the Pacific Ocean (including biota) (Motta et al 2019 Global Biogeochemical Cycles, Motta et al 2022 PNAS). ③Also, more positive D200Hg in nearshore environment is confounding, because riverine influence would likely result in widespread and negative D200Hg in the coastal oceans.\n① We appreciate the reviewer’s comments regarding the processes underlying even-MIF anomalies. Earlier studies have suggested that even-MIF may result from the photo-oxidation of Hg0 in the tropopause (Cai and Chen, 2016; Chen et al., 2012). More recent investigations, however, propose that the photolysis of atmospheric Hgᴵᴵ is responsible for the observed even-MIF signals. For instance, Fu et al. (2021a) demonstrated that even-MIF could arise from Hgᴵᴵ photolysis on aerosols containing magnetic halogen nuclei, and experimental work has indicated that the photodissociation of mercuric oxides under UVC irradiation may also trigger atmospheric even-MIF (Sun et al., 2022). Despite these advances, these hypotheses have not yet been validated on a global scale. To further clarify our findings, we have supplemented and redesigned our fractionation simulation experiments. In response to the reviewer’s subsequent suggestion, we first demonstrate that the photooxidation of Hg0 in the tropopause cannot reproduce the measured results, particularly the latitudinal variation of Δ²⁰⁰Hg in precipitation. Subsequently, based on the state-of-the-art atmospheric mercury chemistry mechanism (Shah et al., 2021), we evaluated various redox pathways and found that the photoreduction of particulate Hgᴵᴵ and OH-initiated reactions may predominantly drive even-MIF in the atmosphere. The specific revisions are detailed in our response to the comment #2.\n② We carefully examined the data referenced by the reviewer and found that our model successfully reproduces the observations. First, regarding the dry HgII species from the Pacific Ocean—cited in Motta et al. (2022) and originally reported by Fu et al. (2019) from measurements on Huaniao Island in the northwest Pacific—our model yields an annual mean Δ²⁰⁰Hg of 0.06‰, which closely agrees with the observed value of 0.08 ± 0.04‰ (Fu et al., 2019). Second, for the wet HgII species, Motta et al. (2019) reported Δ²⁰⁰Hg signatures in wet deposition at two sites: the Island of Hawaii and aboard the R/V Kilo Moana (open ocean station ALOHA), with mean values of 0.14 ± 0.05‰ and 0.08‰, respectively. Our simulations yield annual mean Δ²⁰⁰Hg values in precipitation of 0.10‰ and 0.07‰ at these sites, respectively, which are in good agreement with the observations. Overall, our simulation results are consistent with the observational data.\nIn our original manuscript, the statement that \"regions undergoing net Hg oxidation tend to exhibit lower or even negative Δ²⁰⁰Hg values in HgII(g), HgII(p), and wet deposition, particularly in the central Indian and Pacific Oceans\" was inaccurate. In the revised version, we have incorporated the two observations mentioned by the reviewer and revised the presentation accordingly. The following are the revised sentences:\nLine 107-110: Specifically, the model successfully reproduces the mean Δ200HgII(p) values observed in remote mountain regions (about 0.10‰) (Fu et al., 2021a; Fu et al., 2021b; Fu et al., 2018; Fu et al., 2019) and coast MBL regions (about 0.06‰) (Fu et al., 2018; Li et al., 2020; Rolison et al., 2013; Sun et al., 2021; Yu et al., 2020), including the Huaniao Island (about 0.08‰) at Northwest Pacific Ocean (Fu et al., 2019).\nLine 115-121: The modeled Δ200HgII(pre) also exhibits a consistent mean value with observations, at 0.15 ± 0.04‰ (n = 15, Figure 1d), and shows a significant positive correlation with observed data, with a correlation coefficient of 0.62. Notably, the modeled Δ200HgII(pre) values (0.10‰ and 0.07‰) aligns well with observations from the Island of Hawaii (0.14 ± 0.05‰) and the R/V Kilo Moana (open ocean station ALOHA, 0.08‰)(Motta et al., 2019), which are representative of Δ200HgII(pre) observed over the open Pacific Ocean.\nLine 133-144: The distinct variations in Δ200Hg values among HgII species—including gaseous (HgII(g)), particulate (HgII(p)), and precipitation-derived (HgII(pre))—arise from chemical fractionation processes. Comparison with the global atmospheric Hg redox rate (Figure S2) reveals that regions dominated by net Hg oxidation, such as parts of the central Indian and Pacific Oceans, exhibit lower Δ200Hg values in HgII(g), HgII(p), and wet deposition (Figure 1b-d). This pattern reflects underlying fractionation processes, with Br-initiated oxidation predominating in the marine boundary layer (Figure S2) and even-MIF occurring during OH-initiated oxidation and HgII(p) photoreduction (see Methods). Elevated Δ200HgII values in regions like Africa are linked to upper atmospheric processes. The free troposphere, marked by intense redox cycling (Horowitz et al., 2017; Shah et al., 2021), generates high Δ200Hg values in HgII species (Figure S3). These species are transported to the surface via convection, significantly influencing wet deposition.\n③ We agree that the Δ²⁰⁰Hg signatures in nearshore environments should closely resemble those from riverine inputs, as previous studies have shown that mercury in the coastal ocean is primarily derived from riverine sources (Liu et al., 2021). However, riverine inputs exert minimal influence on the overall oceanic mercury budget. Our study focuses exclusively on atmospheric mercury and its deposition. Global mercury budgets indicates that riverine mercury contributes approximately 1,000 Mg y⁻¹ to the ocean (Liu et al., 2021; Zhang et al., 2023), compared with atmospheric inputs of roughly 8,000 Mg y⁻¹, with only about 6% of riverine mercury reaching the open ocean (Zhang et al., 2019). Moreover, the observed Δ²⁰⁰Hg signatures in marine samples, as reported by Jiskra et al. (2021), are derived from the open ocean. These factors indicate that nearshore atmospheric mercury deposition does not significantly affect the comparison between our modeled data and ocean observations.\nOur model exclusively simulates the Δ200Hg associated with atmospheric mercury deposition and its impact on marine mercury inputs, without accounting for riverine contributions. Consequently, it does not capture Δ200Hg signatures in nearshore environments. We will address this limitation in future work by developing fully coupled isotopic models.\n2. I understand that the beauty of modeling is to identify/verify certain mechanisms (in this case photoreduction vs photo-oxidation). And I still praise the authors for the exploration and hard work of building a sophisticated model. If I may respectfully suggest a way to go about this is to first show the readers why/how photo-oxidation, especially in the tropopause (to my knowledge the atmospheric column matters due to UV exposure), does not produce measured results and if so WHERE does measured-modelled mismatch occur most noticeably? I think this is a good starting point to figure out whether there may be other Hg sources or mechanisms, which may cancel off the observed even-MIF. The second part is to actually show that photoreduction does better than photo-oxidation (if this is true…).\nWe thank the reviewer for the suggestion. Accordingly, we have conducted additional scenario simulations to illustrate our results and have incorporated the following description in the Methods section and Supporting Information.\nMethod:\nLine 381-394: In this study, we focus on the mass-independent fractionations of even mass isotopes (represented by Δ200Hg). While previous studies (Bergquist, 2018; Cai and Chen, 2016; Chen et al., 2012; Fu et al., 2021a; Sun et al., 2022) have documented elevated Δ200Hg values in precipitation and aerosols, the mechanisms driving even-MIF remain poorly constrained. Chemical processes of Hg0 photo-oxidation in the tropopause (Cai and Chen, 2016; Chen et al., 2012) and HgII photolysis on aerosols mediated by magnetic halogen nuclei (Fu et al., 2021a) have been proposed as potential sources. Experimental evidence further implicates mercuric oxide photodissociation in UVC-exposed Hg–O₂ systems (Sun et al., 2022). Recent detections of positive Δ200Hg in high-altitude gaseous HgII(g), particulate Hg (HgII(p)), and marine/urban aerosols (AuYang et al., 2022; Huang et al., 2022; Liu et al., 2022; Qiu et al., 2021a; Qiu et al., 2021b) underscore the spatial complexity of these processes. Here, we evaluate atmospheric Hg redox pathways using the state-of-the-art atmospheric Hg chemical frameworks (Shah et al., 2021), identifying HgII(p) photoreduction and OH-initiated reactions as dominant even-MIF drivers. Our model successfully reproduces global Δ200Hg distributions (Supplementary Text S2), resolving key uncertainties in the environmental contexts and chemical pathways of even-MIF generation.\nSupporting Information Text:\nLine 38-103: We validated even-MIF using a state-of-the-art atmospheric Hg chemistry mechanism (Shah et al., 2021). Our analysis demonstrated that OH-initiated oxidation and HgII(p) photoreduction may dominate atmospheric even-MIF. We evaluated simulations against two observational constraints: (i) the mean Δ200Hg values for Hg0, HgII(p), HgII(g), and HgII(pre) and (ii) their spatial variability (Figure 1, Table S2). Given the uncertainty surrounding the fractionation factor (E), we varied these factors to assess whether the processes could reproduce global observations. Initially, we conducted four simulations based on the hypothesis that even-MIF is solely driven by Hg0 oxidation in the tropopause (Cai and Chen, 2016; Chen et al., 2012). As shown in Figure S6, at a low fractionation factor (E = 2.6‰, ten times the observed net oxidation factor (Fu et al., 2021a)), the simulation results failed to capture the gradients in Δ200Hg values across different Hg forms. As the fractionation factor increased, Δ200Hg values for different Hg forms began to diverge, yet none could be reproduced simultaneously. Notably, the observed mean values and variability of Δ200HgII for wet deposition were not replicated, with a spatial correlation coefficient (R2) of only ~0.05. This is particularly significant because wet deposition, originating from the upper atmosphere, provides the most representative data for even-MIF in that region. These findings suggest that even at higher fractionation factors (e.g., E = 260‰), although elevated Δ200HgII signatures in the upper atmosphere may influence the lower atmosphere (Figures S8 and S9), the simulations could not simultaneously reproduce both mean Δ200Hg values and spatial variability across different atmospheric Hg forms. Therefore, our modeling results indicate that the hypothesis of even-MIF occurring exclusively at the tropopause may not fully explain the global Δ200Hg observations.\nWe next evaluated the potential impact of HgII(p) photoreduction, as proposed by Fu et al. (2021a). By applying various fractionation factors, we found that when E = -0.78‰, the model reproduced observed mean Δ200Hg values for different Hg forms and achieved a spatial correlation coefficient of 0.46 between modeled and observed Δ200Hg in precipitation (Figure S10a). However, given the complexity of atmospheric Hg chemistry and the uncertainties surrounding the even-MIF mechanism, we further explored potential contributions from additional redox pathways. The latest atmospheric Hg chemistry model incorporates gaseous HgII reduction and includes four key redox pathways: (i) Br-initiated reactions, (ii) OH-initiated reactions, (iii) Cl-initiated reactions, and (iv) HgII(p) photoreduction. In the following section, we focus on evaluating the Br-, Cl-, and OH-initiated pathways.\nThe Br-, Cl-, and OH-initiated pathways each involve multiple oxidation and reduction processes. To assess their contributions to even-MIF, we selected OH-initiated pathway and examined differences in simulation results after applying fractionation to individual redox reactions within each pathway. Our analysis revealed strong positive correlations between simulated Δ200Hg values when even-MIF occurs through different OH-initiated reactions (Figure S11). This suggests that once even-MIF is introduced in any oxidation or reduction step within the OH-initiated pathway, the Δ200Hg signal propagates through subsequent redox transformations, ultimately shaping the final isotopic signature. As a result, the spatial distribution of simulated Δ200Hg values remains consistent across all OH-initiated reactions. Consequently, a single representative reaction can be selected to model the even-MIF effect for the entire OH-initiated pathway in further analyses. The same approach applies to Br- and Cl-initiated pathways. In addition, we further find that the observed latitudinal variation in Δ200HgII(pre) signatures provides an effective constraint for evaluating different chemical pathways. As shown in Figure S12, when even-MIF is attributed to Br- or Cl-initiated pathways, the modeled Δ200HgII(pre) values fail to reproduce the observed latitudinal trends, yielding low correlation coefficients (R2 = 0.04 and 0.02, respectively). In contrast, when even-MIF occurs via the OH-initiated pathway, the modeled Δ200HgII(pre) values show a positive correlation with observations (R2 = 0.35). These results suggest that the OH-initiated pathway also has the potential to explain global variations in observed Δ200Hg signatures.\nOur analysis identifies three possible processes capable of reproducing global Δ200Hg observations in simulations: (1) HgII(p) photoreduction, (2) OH-initiated reactions, and (3) a combination of both. To evaluate these scenarios within the constraints of global observations, we tested each process using different fractionation factors. As shown in Figure S10, the optimal simulated results were obtained at E = -0.78‰ for HgII(p) photoreduction, E = -1.82‰ for OH-initiated reactions, and E = -0.52‰ for the combined process. Notably, when even-MIF occurs in both HgII(p) photoreduction and OH-initiated reactions (Figure S10e–f), the simulations align more closely with observations. In particular, the modeled Δ200HgII(pre) signatures show the highest correlation with observations (R2 = 0.62), compared R2 = 0.35 and 0.37 for the first two scenarios (Figure S10e–f). These findings suggest that, based on the latest atmospheric Hg chemical mechanisms, OH-initiated oxidation and HgII(p) photoreduction likely play a dominant role in driving atmospheric even-MIF.\nThe following figures are added in Supporting Information for illustration:\nFigure S6. Comparison of simulated and observed mercury isotope signatures in atmospheric compartments. Modeled Δ200Hg values for elemental mercury (Hg0, yellow), particulate-bound and gaseous oxidized mercury (HgII(p) and HgII(g), blue), and precipitation-derived HgII (HgII(pre), green), compared with global observational data. Isotopic fractionation is exclusively attributed to Hg0 oxidation within the tropopause layer. The baseline simulation incorporates a mass-independent fraction factor (E) of 0.26‰ derived from Fu et al. (2021a). The enrichment factors (E) are defined as the ratio of product to reactant, reflecting the enrichment of 200Hg isotopes in HgII and the depletion in Hg0 during redox process. To reconcile model-observation discrepancies, we systematically enhanced E values by orders of magnitude (10× increments) across sensitivity experiments. Coefficient of determination (R) values quantify agreement between simulated and measured Δ200Hg for: Hg0 (yellow), HgII(p) and HgII(g) (blue), and wet-deposited Hgᴵᴵ (green). Error bars represent the reported stand deviation of measured Δ200Hg for: Hg0 (yellow), HgII(g) (blue), and wet-deposited Hgᴵᴵ (green) (1δ). Error bars for HgII(p) represent spatial variability in observational and modeled datasets. The enrichment factors (E) are defined as the ratio of product to reactant, reflecting the enrichment of 200Hg isotopes in HgII and the depletion in Hg0 during redox process.\nFigure S7. Comparison of simulated and observed latitudinal Δ200Hg signatures in precipitation.\nFigure S8. Global distribution pattern of modeled Δ200Hg signatures by various fractionation factors. Even-MIF occurs only in the Hg0 oxidation process at tropopause layer. (a-d) Modeled Δ200Hg0 signatures; (e-h) Modeled Δ200HgII (sum of HgII(g) and HgII(p)) signatures. (i-k) Modeled Δ200HgII(pre) signatures.\nFigure S9. Vertical distribution patterns of modeled Δ200HgII in global atmosphere by various fractionation factors. Even-MIF only occurs in Hg0 oxidation at tropopause layer.\nFigure S10. Comparison of optimal simulation and observation of Δ200Hg signatures in atmospheric compartments and their latitudinal variations. (a-b) Even-MIF generated in HgII(p) photoreduction process, with fractionation factor of -0.78‰. (c-d) Even-MIF generated in both OH-initiated chemical processes. The even-MIF is applied in HgIIOHO + CO → HgIOH process, representing the OH-initiated pathway, with fractionation factor of -1.82‰. (e-f) Even-MIF generated in both HgII(p) photoreduction and OH-initiated chemical processes, with fractionation factor of -0.52‰. It should be noted that OH-initiated pathway represents the redox reactions that induced by OH radicals. As that when even-MIF arises from any oxidation or reduction reaction in OH-initiated reactions, the modeled Δ200Hg signal can propagate through subsequent redox steps, affecting the final product (Figure S11).\nFigure S11. Comparison of Δ200Hg signatures modeled by OH-initiated redox reactions when using the same fractionation factors (|E| = 1.0‰). Further analysis revealed strong positive correlations between simulated Δ200Hg values when even-MIF occurs through different OH-initiated chemical reactions (Figure S9). This indicates that when even-MIF arises from any oxidation or reduction reaction in OH-initiated processes, the Δ200Hg signal can propagate through subsequent redox steps, affecting the final product. As a result, the spatial distribution of simulated Δ200Hg values exhibits a high degree of similarity across all OH-initiated reactions. Consequently, we can select a single chemical reaction simulation to represent the entire OH-initiated pathway in further analyses. Same as the Br-initiated pathway.\nFigure S12. Comparison of simulated and observed latitudinal Δ200Hg signatures in precipitation. (a) Even-MIF generates in Br-initiated chemical pathway, with the selected process of HgIIBrO reduction by CO (E = -1.0‰). (c) Even-MIF generates in Cl-initiated chemical pathway, with the selected process of HgIIClO reduction by CO (E = -1.0‰) (c) Even-MIF generates in OH-initiated chemical pathway, with the selected process of HgIIOHO reduction by CO (E = -1.0‰)\n3.Overall, it is difficult to recommend this paper for revision or publication as a present form. But I do agree that Hg(0):Hg(II) deposition (2:1) modelled by a prior study is overestimated and strongly encourage the authors to move this work forward with a tailed assumption.\nWe thank the reviewer for the supportive feedback. In the revised manuscript, we have bolstered our conclusions by incorporating numerous scenario-based simulation experiments, which significantly enhance the robustness of our findings.\nReviewer #2 (Remarks to the Author):\nQuantifying the contribution of atmospheric Hg to ocean is a significant issue to determine the Hg budget globally. Here the authors innovatively integrate Hg isotope into a three dimensional atmospheric Hg isotope model and elucidate the Δ200Hg signatures of atmospheric Hg. This pioneering combination of traditional numerical model and emerging isotope method aid to reassess Hg deposition to ocean and thus reduce the uncertainty of current understanding on Hg cycling. This work is interesting and with significance. We recommend publication after addressing the following comments.\nThanks to the reviewer for the recognition and suggestion.\n1. Model evaluation. Brief description of method integration should be introduced at the beginning of results as it is a significant innovation of this study.\nWe added the following description at the beginning:\nLine 90-91: We developed a three-dimensional isotopic model for atmospheric Hg, incorporating state-of-the-art Hg chemistry, to elucidate the potential processes driving even-MIF.\n2. Line 125-128 It is generally assumed that Δ200Hg is from the upper atmospheric Hg oxidation. What is the theory base for the assumption that even-MIF is driven particulate HgII photoreduction?\nA similar concern was raised by Reviewer #1 (comment #2). Earlier studies have suggested that even‐MIF may result from the photo-oxidation of Hg0 in the tropopause (Cai and Chen, 2016; Chen et al., 2012). More recent investigations, however, propose that the photolysis of atmospheric HgII is responsible for the observed even-MIF signals. For example, Fu et al. (2021a) demonstrated that even-MIF could arise from HgII photolysis on aerosols containing magnetic halogen nuclei, and experimental work has indicated that the photodissociation of mercuric oxides under UVC irradiation may also trigger atmospheric even-MIF (Sun et al., 2022). Despite these advances, these hypotheses have yet to be validated on a global scale.\nTo further clarify our findings, we have supplemented and redesigned our fractionation simulation experiments. We first demonstrate that the photo-oxidation of Hg0 in the tropopause cannot reproduce the measured results, particularly the latitudinal variation of Δ200Hg in precipitation. Subsequently, based on the state-of-the-art atmospheric Hg chemistry mechanism (Shah et al., 2021), we evaluated various redox pathways and found that the photoreduction of particulate HgII and OH-initiated reactions may be the primary drivers of even-MIF in the atmosphere. The specific revisions are detailed in both the Methods and Supporting Information sections, as outlined in comment #2 by Reviewer #1.\n3. ①Figure 2 (a), how to explain the white strip around antarctica? ②Although the exact Δ200Hg values between HgII dry deposition and wet deposition vary, their spatial distributions are quite similar. Higher values have been observed at the coastal areas. How to explain such phenomena?\nFigure 2(a) presents the annual mean Δ²⁰⁰Hg signatures for the oceanic uptake of atmospheric Hg0. In polar regions, sea-air Hg exchange is restricted by sea-ice cover, resulting in null model outputs that appear as white stripes. We have incorporated this description in the figure caption, adjusted the color bar range, and re-presented Figure 2 as follows:\nFigure 2. Annual mean Δ200Hg signatures of atmospheric Hg deposited in the ocean. Hg0 uptake by the ocean is calculated using equations (1) and (2) as described in section 2.1, representing the uptake of atmospheric Hg0. HgII deposition includes both dry and wet deposition of HgII(g) and HgII(p). (a) Δ200Hg in Hg0 uptake by the ocean. The white strip data nulls caused by sea ice cover. (b) Δ200Hg in the dry deposition of HgII, including both direct deposition and uptake by sea salt. (c) Δ200Hg in the wet deposition of HgII.\n②Distinct spatial variations in the Δ²⁰⁰Hg values of dry and wet deposition are influenced by both chemical fractionation processes and transport. Although dry deposition of HgII occurs primarily in the boundary layer, the Δ²⁰⁰Hg signatures of both gaseous and particulate HgII are substantially affected by inputs from the upper atmosphere. The free troposphere, characterized by intense redox cycling, produces elevated Δ²⁰⁰Hg values in HgII species (Figure S3), which are then transported to the surface via convection. Consequently, higher Δ²⁰⁰Hg values in coastal dry deposition indicate a stronger influence of atmospheric transport. Since wet deposition mainly originates from the upper atmosphere through the washout of Hg HgII species, the spatial distributions of Δ²⁰⁰Hg in both dry and wet deposition exhibit similar trends. In addition, terrestrial transport also contributes to elevated coastal Δ²⁰⁰Hg values, as on-land atmospheric Hg chemistry exhibits higher reduction rates (Figure S2), resulting in elevated Δ²⁰⁰Hg for HgII species.\nWe have added the following discussion in this section:\nLine 158-165: The spatial patterns in both dry and wet deposition display similar trends, largely driven by the Δ200HgII distributions (Figure 1 and Figure S3). Although HgII dry deposition occurs primarily in the boundary layer, the Δ200Hg signatures of HgII(g) and HgII(p) therein are significantly influenced by upper-atmospheric processes—similar to the Δ200Hg observed in precipitation due to the washout of HgII species from higher altitudes. Moreover, elevated Δ200Hg values in both dry and wet deposition near coastal regions are also likely influenced by terrestrial transport, as on-land Hg chemistry exhibits stronger reduction rates (Figure S2), leading to higher Δ200Hg in HgII species.\n4. Line 298-302 Existing global emissions with spatial distribution do not contain the detail isotope information. How do you consider the isotope fractions in emissions?\nWe developed an emission inventory containing isotopic information from various sources. The isotopic signatures were compiled from published field observations of anthropogenic, soil, oceanic, biomass, and geogenic emission sources. The following table, presented in the Methods section, provides further details.\nTable 1 Mercury flux and Δ200Hg signatures of source emissions\nSources Anthropogenica Soilb Oceanc Biomassd Geogenice\nFlux (Mg y-1) 2200 850 4800 300 300\nΔ200Hg(‰) 0.00 −0.01 0.04 −0.04 0.00\nmean\n1sd 0.03 0.03 0.05 0.02 0.02\nThe fluxes of Hg emission sources are from Shah et al. (2021). aThe isotope signatures of anthropogenic emission are cited from (Sun et al., 2019; 2016). bΔ200Hg emission from soil is adopted from Sun et al. (2019) and Wang et al. (2019). cWe estimate the Δ200Hg emission from ocean is consistent with that in oceanic samples (Jiskra et al., 2021). dΔ200Hg in biomass emission are compiled from references (Fu et al., 2018; Jiskra et al., 2015; Kurz et al., 2020; Obrist et al., 2017; Wang et al., 2020; Yu et al., 2016). eΔ200Hg in geogenic emission are adopted from volcanic emission reported by Sun et al. (2016).\n5. Line 311-315 Direct gaseous HgII reduction has been proved to be important and to significantly impact Hg deposition distribution. Why do not you consider it in this study? What is the impact of such process is missing?\n(Saiz-Lopez, A.; Sitkiewicz, S. P.; Roca-Sanjuan, D.; Oliva-Enrich, J. M.; Davalos, J. Z.; Notario, R.; Jiskra, M.; Xu, Y.; Wang, F.; Thackray, C. P.; Sunderland, E. M.; Jacob, D. J.; Travnikov, O.; Cuevas, C. A.; Acuna, A. U.; Rivero, D.; Plane, J. M. C.; Kinnison, D. E.; Sonke, J. E., Photoreduction of gaseous oxidized mercury changes global atmospheric mercury speciation, transport and deposition. Nat Commun 2018, 9.)\nPhotoreduction of gaseous HgII has been incorporated into the chemical mechanism employed in this study. This process primarily occurs via the Br-initiated redox pathway, reducing approximately 4000 Mg y-1 of atmospheric HgII to HgI and Hg0 species. We have added the following discussion in the Methods section for clarification:\nWe utilize the state-of-the-art chemistry mechanism of atmospheric Hg, which includes mainly four redox pathways (Shah et al., 2021): (i) the oxidation of Hg0 to HgII by Br; (ii) the oxidation of Hg0 to HgII by OH; (iii) the oxidation of Hg0 to HgII by Cl; (iv) photoreduction of HgII(p) to Hg0. This mechanism also integrates the photoreduction of gaseous HgII (Saiz-Lopez et al., 2018), a process that occurs primarily within the Br-initiated pathway and significantly influences global Hg deposition."}, {"round": 2, "reviewer_comments": {"reviewer_2": "After some period of rehashing through Fu et al 2021 and Sun et al 2022, I am still not fully convinced that photoreduction is responsible for the positive even-MIF anomalies. To my understanding, Fu et al 2021 relies upon 199/200 slopes established by prior studies to come up a theoretical concept on the even-MIF anomalies. The experimental study conducted by Sun et al 2022, on the other hand, is little more convincing.\nHaving said that, it is certainly true that the mechanism driving positive even-MIF is still inconclusive and requires years of experimental, theoretical, observational, and modeling work. This perhaps applies even to the conventional notion that even-MIF occurs via tropospheric photo-oxidation as I previously suggested. As a scientist, I think it is important to stay open to all possibility and continuously generate multiple and REASONABLE scientific evidences on the potential mechanisms.\nIn this regard, I am willing to stay open and consider this paper for publication in Nature Communications. For the next set of revision, I kindly ask the authors to disclose more thorough comparison of measured vs modelled D200Hg instead of showing the average values (Fig 1e). More specifically, I ask the authors to plot a spatial comparison between measured vs modelled D200Hg, similar to the one shown in Fig S1. Fig S1 shows a map of measured Hg concentrations in symbols and modelled data in the background. As a reviewer, I am concerned how the measurement D200Hg is biased towards China, US, Europe, and some Arctic and this could cause scrutiny in the future. I encourage the authors to show frankly that the scientific basis on photoreduction and OH-oxidation driving positive D200Hg come from limited measurement data.\nThis leads to my next comment. The highly positive D200Hg observed almost solely in Africa is highly concerning, especially given that we have no measurement data from Africa. I think all readers would agree that Africa in Fig 1 stands out too much. The authors mentioned in the revised line 140-141 that “Elevated D200Hg values in regions like Africa are linked to upper atmospheric processes. The free troposphere, marked by intense redox cycling, generates high D200Hg values in Hg(II) species. These species are transported to the surface via convection, significantly influencing wet deposition.” The first question that comes up in my mind is: why is this not the case in other regions of the world? A recent study by Saiz-Lopez et al (2025 in Science Advances) show that the magnitude of tropospheric Hg oxidation is generally proportional to the regions of high anthropogenic Hg emissions. This assumption is also not very convincing based on the little net Hg oxidation reported by Shah et al near the equator. Does this have to do with the abundance of OH at the equator then?\nOverall, I encourage the authors to treat this spatial pattern (D200Hg) more cautiously and provide reasonable scientific evidence that, when we actually begin to generate measurement data from Africa, we would indeed see anomalously high D200Hg.", "reviewer_3": "The comments have been well addressed in the revised manuscript. I recommend to publish it directly."}, "author_response": "Author response to comments on MS# NCOMMS-24-61583A-Z, “Atmospheric Source of Mercury to the Ocean Constrained by Isotopic Model”\nWe would like to thank the two reviewers for their feedback on the manuscript. We greatly appreciate their time and have considered all the comments carefully. We present our response below, as well as changes to the main text (reviewer comments are black font), author response are blue, and quotes from the revised manuscript are purple). Line numbers (L###) refer to revised manuscript.\nREVIEWER COMMENTS\nReviewer #2 (Remarks to the Author):\n1. After some period of rehashing through Fu et al 2021 and Sun et al 2022, I am still not fully convinced that photoreduction is responsible for the positive even-MIF anomalies. To my understanding, Fu et al 2021 relies upon 199/200 slopes established by prior studies to come up a theoretical concept on the even-MIF anomalies. The experimental study conducted by Sun et al 2022, on the other hand, is little more convincing.\nWe thank the reviewer for this insightful comment. Our results do not contradict with Sun et al. (2022)1, who suggested that photodissociation of HgO in UVC-irradiated systems could induce even-MIF. However, we believe this process may not generate sufficient even-MIF to explain global observations. First, the troposphere lacks UVC radiation due to ozone absorption in the stratosphere, and HgO photolysis mechanisms in the troposphere are minimal1-3. Second, while the stratosphere contains UVC radiation, the low atmospheric pressure in this region results in both the production and photolysis rates of HgO being very low1,2. Third, although Sun et al. (2022)1 proposed that this mechanism could occur under lightning conditions in the troposphere, it is unlikely to cause a global even-MIF for various forms of Hg. For example, it does not explain the even-MIF observed for HgII(p) in urban and marine boundary layers. We agree with the reviewer that the UVC-irradiated Hg-O system might induce even-MIF, as suggested by Sun et al. (2022)1, but its contribution to global even-MIF remains highly uncertain. Our results show that only the tropopause and stratospheric even-MIF sources cannot reproduce global observations (see below).\nRecent work by Saiz-Lopez et al (2025)2 has expanded upon the concept proposed by Sun et al (2022), hypothesizing a potential stratospheric contribution to global even-MIF. Considering this, we conducted additional scenario simulations incorporating even-MIF in the stratosphere to explore the possibility of global transport of Δ200Hg signals originating there. The modeled results did not align with the observations, suggesting that if even-MIF occurs exclusively in the stratosphere, this mechanism alone cannot account for the observed Δ200Hg signatures. We have included a detailed discussion of this in the Text S2 section of the Supplementary Information.\nText S2: We then conducted simulations based on the hypothesis that even-MIF dominates in the stratosphere. A recent study suggests that the photolysis of mercuric oxides, formed from ultraviolet-radiation triplet Hg atoms [Hg(3P)], may induce even-MIF1, but this process should not generate sufficient even-MIF to explain global observations. As the troposphere lacks UVC radiation due to ozone absorption in the stratosphere, and HgO photolysis mechanisms in the troposphere are minimal1-3. While Sun et al.1 proposed that this mechanism might occur under lightning conditions in the troposphere, it is unlikely to produce global even-MIF for various forms of Hg. Recent research on stratospheric Hg chemistry has expanded on this concept, hypothesizing a potential stratospheric contribution to global even-MIF2. To explore this, we simulated the global transport of Δ200Hg signals generated in the stratosphere. High fractionation factors (E = 100‰ and 200‰) were applied to the Hg oxidation process, which results in the enrichment of 200Hg isotopes in atmospheric HgII species and the depletion of 200Hg isotopes in Hg0. As shown in Figure S11, the modeled mean Δ200Hg values for HgII (mean: 0.11 ± 0.25‰) were comparable to observations (mean: 0.09 ± 0.06‰) at E = 200‰. However, the modeled Δ200Hg values for Hg0 and HgII(pre) deviated significantly from the observations. Additionally, the modeled Δ200Hg distribution patterns did not match the observations, with R2 values of 0.09 for HgII and 0.06 for HgII(pre).\nAdditionally, we have included a discussion in the uncertainty section regarding the omission of the lightning process in the model, which may influence the simulation results.\nL 293-298: The modeling uncertainties may arise from the omission of the lightning process that induces even-MIF, as proposed by recent experimental studies1. As a result, this could influence the simulation of Δ200Hg signatures in wet deposition, given that precipitation events are often associated with lightning. However, according to Sun et al.1, it is still unrealistic to attribute global significance to Hg(3P)-induced oxidation (Supplementary Discussion, Text S2).\nConstrained by the available observational data, our simulations are consistent with the observations. However, the modeling results cannot rule out the possibility proposed by Fu et al. (2021)4. In response to Comment #3, we emphasize the limitation of observational data in the revised text.\nFigure S11. Global distribution pattern of modeled Δ200Hg signatures when even-MIF occurs in stratosphere. Δ200Hg signatures for Hg0, HgII(g), HgII(p) and HgII(pre) species when fractionation factors of (a-d) E = 100‰ and (e-h) E = 200‰, respectively.\n2. Having said that, it is certainly true that the mechanism driving positive even-MIF is still inconclusive and requires years of experimental, theoretical, observational, and modeling work. This perhaps applies even to the conventional notion that even-MIF occurs via tropospheric photo-oxidation as I previously suggested. As a scientist, I think it is important to stay open to all possibility and continuously generate multiple and REASONABLE scientific evidences on the potential mechanisms.\nWe appreciate the comment. Constrained by observational data, our results suggest that both HgII(p) photoreduction and OH-initiated reactions are plausible drivers for even-MIF. Importantly, the OH-initiated pathway involves a series of redox reactions. This means that our findings cannot rule out the possibility that even-MIF generated during Hg oxidation in the lower troposphere. Since the even-MIF signal can be transferred along the following chemical reactions, causing similarity in simulation results under reactions in the same pathway. We have included a discussion in the Text S2 section of the SI.\nText S2: These findings suggest that, based on the latest atmospheric Hg chemical mechanisms, OH-initiated oxidation and HgII(p) photoreduction likely play a dominant role in driving atmospheric even-MIF. Note that the OH-initiated pathway involves a series of redox reactions, meaning our findings cannot exclude the possibility that any oxidation or reduction processes within this pathway may contribute to even-MIF. Once the even-MIF signal is generated, it can be transferred through subsequent chemical reactions, leading to similar simulation results within the same pathway. Identifying the specific dominant reaction for even-MIF will require further investigation.\nIn response to this comment, we have also revised the text to temper assertive claims and clarify the exploratory nature of our conclusions. Key modifications include:\nL27-29\nOriginal: \"we propose an updated ratio of atmospheric HgII to Hg0 deposition to the ocean of over 2:1, rather than the previously reported 1:1.\"\nRevised: \"we propose an updated deposition ratio of atmospheric HgII to Hg0 to the ocean, which may exceed 2:1, higher than the previously reported 1:1.\"\nL84\nOriginal: \"replacing the previously reported 1:1 ratio. \"\nRevised: \"higher than the previously reported 1:1 ratio.\"\nL98\nOriginal: \"The model accurately reproduces…\"\nRevised: \"The model results reproduce…\"\nL184\nOriginal: \"a detail not emphasized in previous studies. \"\nRevised: \"a detail not fully emphasized in previous studies. \"\nL222-223\nOriginal: \"indicating that atmospheric HgII is a more substantial contributor…\"\nRevised: \"indicating that atmospheric HgII might be a more substantial contributor…\"\nL328-329\nOriginal: \"Our study suggests that this budget is not supported by the oceanic Δ200Hg data.\"\nRevised: \"Our study suggests that the oceanic Δ200Hg data may not fully support this budget.\"\nL330\nOriginal: \"Our results contrast with previous research, as our model effectively differentiates…\"\nRevised: \"Our results effectively differentiate…\"\nL425-426\nOriginal: \"identifying HgII(p) photoreduction and OH-initiated reactions as dominant even-MIF drivers.\"\nRevised: \"identifying HgII(p) photoreduction and OH-initiated reactions are plausible dominant even-MIF drivers.\"\n3. In this regard, I am willing to stay open and consider this paper for publication in Nature Communications. For the next set of revision, I kindly ask the authors to disclose more thorough comparison of measured vs modelled D200Hg instead of showing the average values (Fig 1e). More specifically, I ask the authors to plot a spatial comparison between measured vs modelled D200Hg, similar to the one shown in Fig S1. Fig S1 shows a map of measured Hg concentrations in symbols and modelled data in the background. As a reviewer, I am concerned how the measurement D200Hg is biased towards China, US, Europe, and some Arctic and this could cause scrutiny in the future. I encourage the authors to show frankly that the scientific basis on photoreduction and OH-oxidation driving positive D200Hg come from limited measurement data.\nWe greatly appreciate the reviewer’s insightful comment. In the original manuscript, Figure 1a-d presented a comparison of the observation point data. However, the points are too small to visual and we are sorry for this inconvenience. In the revised version, we have updated Figure 1 by enlarging the observation points to facilitate comparison with the modeled values. Additionally, we have included a latitudinal comparison plot of modeled and observed Δ200Hg values in wet deposition, which illustrates the spatial distribution of both modeled and observed data. This is particularly relevant as the observed Δ200Hg signal is most pronounced in the wet deposition. Furthermore, we have retained Figure 1f (formerly Figure 1e in the original manuscript), as it effectively demonstrates the capabilities of the simulation.\nFig. 1 | Distribution patterns of modeled Δ200Hg signatures in surface atmosphere. a Comparison of modeled and observed Δ200Hg0 values. b Comparison of modeled and observed Δ200HgII(p) values. c Comparison of modeled and observed Δ200HgII(g) values. d Comparison of modeled and observed Δ200HgII(pre) values. Cycles represent corresponding observational data. e Comparison of simulated and observed latitudinal Δ200Hg signatures in precipitation. f Comparison of observed and modeled annual mean Δ200Hg values in atmospheric samples. Error bars represent the reported stand deviation of measured Δ200Hg for: Hg0 (yellow), HgII(g) (blue), and wet-deposited HgII(green) (1δ). Error bars for HgII(p) represent spatial variability in observational and modeled datasets (1δ).\nWe also added the following sentence for discussing the latitudinal variation of Δ200HgII(pre):\nL114-117: The modeled Δ200HgII(pre) also exhibits a consistent mean value with observations, at 0.15 ± 0.04‰ (n = 15, Figure 1d), and shows a significant positive correlation with observed data, with a correlation coefficient of 0.62, reproducing the latitudinal variation of observations (Fig. 1e).\nWe agree with the reviewers that the current observations are limited, and have therefore included a discussion of this in the uncertainty section, as outlined below:\nL301-303: Additionally, although the model reproduces the existing observations well, these are primarily concentrated in East Asia, North America, Europe, and parts of the Arctic. Consequently, we recommend that future studies focus on more remote regions, such as South America, Africa, and the open ocean.\n4. This leads to my next comment. The highly positive D200Hg observed almost solely in Africa is highly concerning, especially given that we have no measurement data from Africa. I think all readers would agree that Africa in Fig 1 stands out too much. The authors mentioned in the revised line 140-141 that “Elevated D200Hg values in regions like Africa are linked to upper atmospheric processes. The free troposphere, marked by intense redox cycling, generates high D200Hg values in Hg(II) species. These species are transported to the surface via convection, significantly influencing wet deposition.” The first question that comes up in my mind is: why is this not the case in other regions of the world? A recent study by Saiz-Lopez et al (2025 in Science Advances) show that the magnitude of tropospheric Hg oxidation is generally proportional to the regions of high anthropogenic Hg emissions. This assumption is also not very convincing based on the little net Hg oxidation reported by Shah et al near the equator. Does this have to do with the abundance of OH at the equator then?\nOverall, I encourage the authors to treat this spatial pattern (D200Hg) more cautiously and provide reasonable scientific evidence that, when we actually begin to generate measurement data from Africa, we would indeed see anomalously high D200Hg.\nOur model results suggest that both HgII(p) photoreduction and OH-initiated reactions may drive even-MIF, which means that the modeled pattern of Δ200HgII signatures are linked to the spatial distribution of chemical reactions. We separately analyzed the effects of the two chemical fractionation pathways and found that Δ200Hg produced by both pathways exhibits various characteristic distributions in both the surface and upper atmosphere (Figure S3). The both pathways contribute to an increase in Δ200Hg values in the African region. Furthermore, the upper atmosphere tends to have relatively high Δ200Hg values, which can influence the lower atmosphere through convective transport. The absence of similar patterns in other regions is attributed to weaker chemical reactions and isotope fractionation in those areas and the upper atmosphere.\nThe changes made in the main text are as follows:\nL142-159: Elevated Δ200HgII values in African regions further highlight the spatial variability of chemical fractionation. High PM concentrations driven by dust emissions 2.5 (Supplementary Fig. 2e) promote HgII(p) formation, while concurrent high photolysis frequencies (Supplementary Fig. 2f) induce strong net HgII(p) reduction (Supplementary Fig. 2d). Additionally, significant OH-initiated chemical activity (Supplementary Fig. 2b) also contributes to this regional Δ200Hg signal. We decompose the fractionation process and find that the OH-initiated chemistry and HgII(p) photoreduction pathways induce distinct Δ200HgII distribution patterns, as these reactions prevail in distinct atmospheric layers (Supplementary Fig. 3). Both processes contribute to the elevated Δ200HgII values over the African landmass (Supplementary Fig. 3). The modeled Δ200HgII signatures at the surface are also influenced by the upper atmosphere. The free troposphere, marked by intense redox rates3,5 (Supplementary Figs. 2g and 2h), generates high Δ200Hg values in HgII species (Supplementary Fig. 2f). Subsequent convective transport (Supplementary Figs. 4d and 4e) delivers these species to the surface, thereby affecting surface Δ200HgII signatures. Moreover, compared with regions such as East and South Asia, the African region has relatively lower anthropogenic Hg emissions that is characterized with a close to zero Δ200Hg signal, further enhancing the importance of chemical fractionation processes in this area.\nThe net atmospheric Hg oxidation near the equator is relatively low due to the high rates of HgII(p) photoreduction in these regions. Simultaneously, stronger OH-initiated chemistry increases the fractionation induced by these processes, elevating the simulated Δ200HgII values in equatorial regions, such as Africa and South America. This pattern is correlated with the distribution of OH abundance. In the revised manuscript, we present the rates of OH oxidation and HgII(p) reduction in the free troposphere (Figure S2e-f), providing additional support for our conclusions.\nFigure S2. Annual mean chemistry rates of (a) Hg0 oxidation by Br, (b) Hg0 oxidation by OH, (c) reduction of HgII(p), and (d) net oxidation of Hg0 in the surface atmosphere. (e) Annual mean of PM concentration. (f) Annual mean of photolysis frequencies of NO . In the model, the 2.5 2 photolysis frequencies of HgII(p) are sclaed from the local photolysis frequency of NO 3. Annual 2 zonal mean of (g) Hg0 oxidation by OH and (h) HgII photoreduction rates.\nFigure S3. Distribution pattern of modeled Δ200HgII at different layer by generated by OH-initialed chemistry and HgII(p) photoreduction processes (E =0.52‰). (a-e) modeled Δ200HgII(g) and (f-j) Δ200HgII(p) through OH-initialed chemistry pathway. (k-o) modeled Δ200HgII(g) and (p-t) Δ200HgII(p) through HgII(p) photoreduction pathway. The 1 layer represents the surface atmosphere. The other layers represent various pressure and altitude levels: 5 layer, 944hPa and 0.589 km; 10 layer, 868hPa and 0.856 km; 15 layer, 771hPa and 2.25 km; 20 layer, 625 hPa and 3.89 km.\nFigure S4. Annual zonal mean of (a) Δ200Hg0, (b) Δ200HgII(p), (c) Δ200HgII(g) over the ocean. The spatial distribution of (d) Δ200HgII(g) and (e) Δ200HgII(p) in the free troposphere, with the showing Δ200HgII(g) values of < 0‰ or > 0.15‰, and Δ200HgII(p) values of < 0‰ or > 0.20‰.\nReviewer #3 (Remarks to the Author):\nThe comments have been well addressed in the revised manuscript. I recommend to publish it directly.\nWe thank the reviewer for recognizing the improvements in our revised manuscript."}]

Decision letter