Thunderstorms Increase Mercury Wet Deposition

Atmospheric wet deposition of mercury in urban Jinan, eastern China: Speciation, scavenging process and potential sources

Understanding the speciation and related influence factors of Hg in wet deposition is important to predict the fate and transport of mercury in the atmosphere. In this study, event-based samples of rainwater were collected for one year in Jinan, a northern city in eastern China. The volume-weighted mean concentration of total mercury (THg) in rainwater was 34.8 ng L^-1, comparable to levels in some inland cities in China and were significantly higher than those in North America, Korea and Japan. Most of the Hg in rainwater was associated with particulates, accounted for 15.2-92.9% of THg with a mean of 66.9%, which might be attributed to the scavenging effects of high particulate-bound mercury concentrations in ambient air in urban Jinan. Dissolved mercury (DHg) accounted for 33.1% of THg, in which Hg(OH)₂, HgClOH, HgCl₂ and Hg(NH₃)₂²⁺ are the dominant species based on the chemical equilibrium modeling simulations. THg concentrations in rainwater decreased as the rainfall amount increased owing to the dilution effect and 5 mm rainfall might be a threshold for the full wash-out capability of atmospheric Hg. For a continuous rain event, the proportion of DHg in THg could increase from 7.1% to 84.8% with the rainfall processing, especially for the species of HgClOH and HgCl₂ under the influence of rainwater pH. Positive matrix factorization (PMF) analysis suggested that the major sources of Hg in rainwater were combustion emissions, marine sources, industrial emissions, as well as complexation process, which contributed to 51.4%, 24.7%, 12.2%, and 11.7% of the THg, respectively. For the specific species, the main sources varied with different Hg species, in which combustion emissions contributed one third to one half of each species sum to particulate mercury (PHg), HgClOH, HgCl₂, HgBrOH and HgBrCl followed by marine sources and industrial emissions. Cluster analysis of backward trajectories revealed that polluted air masses, transported from southeast Shandong, Anhui and Jiangsu Provinces, as well as Beijing-Tianjin-Hebei region, contributed to high Hg concentration in rainwater in Jinan.

Evaluating atmospheric mercury (Hg) uptake by vegetation in a chemistry-transport model

Mercury (Hg), a neurotoxic heavy metal, is transferred to marine and terrestrial ecosystems through atmospheric transport. Recent studies have highlighted the role of vegetation uptake as a sink for atmospheric elemental mercury (Hg⁰) and a source of Hg to soils. However, the global magnitude of the Hg⁰ vegetation uptake flux is highly uncertain, with estimates ranging 1000-4000 Mg per year. To constrain this sink, we compare simulations in the chemical transport model GEOS-Chem with a compiled database of litterfall, throughfall, and flux tower measurements from 93 forested sites. The prior version of GEOS-Chem predicts median Hg⁰ dry deposition velocities similar to litterfall measurements from Northern hemisphere temperate and boreal forests (∼0.03 cm s^-1), yet it underestimates measurements from a flux tower study (0.04 cm s^-1vs. 0.07 cm s^-1) and Amazon litterfall (0.05 cm s^-1vs. 0.17 cm s^-1). After revising the Hg⁰ reactivity within the dry deposition parametrization to match flux tower and Amazon measurements, GEOS-Chem displays improved agreement with the seasonality of atmospheric Hg⁰ observations in the Northern midlatitudes. Additionally, the modelled bias in Hg⁰ concentrations in South America decreases from +0.21 ng m^-3 to +0.05 ng m^-3. We calculate a global flux of Hg⁰ dry deposition to land of 2276 Mg per year, approximately double previous model estimates. The Amazon rainforest contributes 29% of the total Hg⁰ land sink, yet continued deforestation and climate change threatens the rainforest's stability and thus its role as an important Hg sink. In an illustrative worst-case scenario where the Amazon is completely converted to savannah, GEOS-Chem predicts that an additional 283 Mg Hg per year would deposit to the ocean, where it can bioaccumulate in the marine food chain. Biosphere-atmosphere interactions thus play a crucial role in global Hg cycling and should be considered in assessments of future Hg pollution.

Dissociation of Mercuric Oxides Drives Anomalous Isotope Fractionation during Net Photo-oxidation of Mercury Vapor in Air

The atmosphere is the primary medium for long-distance transport and transformation of elemental mercury (Hg), a potent neurotoxin. The recent discovery of mass-independent fractionation (MIF) of even-mass Hg isotopes (even-MIF, measured as Δ²⁰⁰Hg and Δ²⁰⁴Hg) in the atmosphere is surprising and can potentially serve as a powerful tracer in understanding Hg biogeochemistry. Far-ultraviolet (UVC) light-induced gas-phase reactions have been suspected as a likely cause for even-MIF, yet the mechanism remains unknown. Here, we present the first experimental evidence of large-scale even-MIF caused by UVC-induced (wavelength: 254 nm) Hg oxidation in synthetic air at the pressure (46-88 kPa) and temperature (233-298 K) resembling those of the lower atmosphere. We observe negatively correlated Δ²⁰⁰Hg and Δ²⁰⁴Hg signatures with values as low as -50‰ and as high as 550‰, respectively, in the remaining atomic Hg pool. The magnitude of even-MIF signatures decreases with decreasing pressure with the Δ²⁰⁰Hg/Δ²⁰⁴Hg ratio being similar to that observed in global precipitation. This even-MIF can be explained by photodissociation of mercuric oxides that are photochemically formed in the UVC-irradiated Hg-O₂ system. We propose that similar processes occurring in the atmosphere, where mercuric oxide species serve as intermediates, are responsible for the observed even-MIF in the environment.

Decadal trends of mercury cycling and bioaccumulation within Everglades National Park

Mercury (Hg) contamination has been a persistent concern in the Florida Everglades for over three decades due to elevated atmospheric deposition and the system's propensity for methylation and rapid bioaccumulation. Given declines in atmospheric Hg concentrations in the conterminous United States and efforts to mitigate nutrient release to the greater Everglades ecosystem, it was vital to assess how Hg dynamics responded on temporal and spatial scales. This study used a multimedia approach (water and biota) to examine Hg and methylmercury (MeHg) dynamics across a 76-site network within the southernmost portion of the region, Everglades National Park (ENP), from 2008 to 2018. Hg concentrations across matrices showed that air, water, and biota from the system were inextricably linked. Temporal patterns across matrices were driven primarily by hydrologic and climatic changes in the park and no evidence of a decline in atmospheric Hg deposition from 2008 to 2018 was observed, unlike other regions of the United States. In the Shark River Slough (SRS), excess dissolved organic carbon and sulfate were also consistently delivered from upgradient canals and showed no evidence of decline over the study period. Within the SRS a strong positive correlation was observed between MeHg concentrations in surface water and resident fish. Within distinct geographic regions of ENP (SRS, Marsh, Coastal), the geochemical controls on MeHg dynamics differed and highlighted regions susceptible to higher MeHg bioaccumulation, particularly in the SRS and Coastal regions. This study demonstrates the strong influence that dissolved organic carbon and sulfate loads have on spatial and temporal distributions of MeHg across ENP. Importantly, improved water quality and flow rates are two key restoration targets of the nearly 30-year Everglades restoration program, which if achieved, this study suggests would lead to reduced MeHg production and exposure.

Event-Based Atmospheric Precipitation Uncovers Significant Even and Odd Hg Isotope Anomalies Associated with the Circumpolar Vortex

The determination of the mass-independent fractionation of even Hg isotopes (even-MIF, Δ²⁰⁰Hg) in atmospheric samples adds another intriguing feature to the Hg isotope system. Despite our lack of sufficient experimental verification and the momentary absence of a valid mechanism to explain its occurrence, even-MIF could be instrumental in understanding the cycle and deposition of atmospheric Hg. In contrast to slightly positive Δ²⁰⁰Hg values (<0.30‰) frequently observed in most atmospheric samples, large Δ²⁰⁰Hg values (up to 1.24‰) determined in precipitation from Peterborough (Ontario, Canada) stand out and could provide valuable information for the origin of the even-MIF mystery. We now report a systematic analysis of high-resolution rainfall and snowfall samples collected in winter during cold weather at Peterborough, Canada. Dissolved and particulate Hg both displayed large variations of odd-MIF (from -0.93‰ to 2.02‰ for Δ¹⁹⁹Hg), which may result from long-range transportation, as the negative odd-MIF in particulate Hg is likely a result of long-distance transport of arctic atmospheric Hg(II). Dissolved Hg revealed significant even-MIF values (from 0.25‰ to 1.19‰ for Δ²⁰⁰Hg) and a negative relationship between Δ²⁰⁰Hg and Δ²⁰⁴Hg, which provide further evidence for the previously proposed conceptual model of Δ²⁰⁰Hg. Disconnected odd-MIF and even-MIF trends were detected in sequentially collected precipitation samples, which further suggests different sources or mechanisms for Δ¹⁹⁹Hg and Δ²⁰⁰Hg. Particularly, the high Δ²⁰⁰Hg values highlight the transport of stratospheric Hg through a polar vortex to the sampling region, stimulating further systematic investigation. The new Δ²⁰⁰Hg data for particulate Hg add to existing information on atmospheric Hg(II) worldwide, suggesting a global distribution of Hg characterized by even-MIF in the atmosphere, and further constrain the model of atmospheric Hg deposition.

A Simplified Approach to Modeling the Dispersion of Mercury from Precipitation to Surface Waters—The Bay of Kaštela Case Study

Wet deposition is the main source of mercury (Hg) from the atmosphere to the Earth’s surface. However, the processes that govern the dispersion of deposited Hg in seawater are currently not well understood. To address this issue, total mercury (THg) concentrations in surface seawaters and precipitation were determined on a monthly basis in the Bay of Kaštela (Central Adriatic Sea). Following the assumption that deposited THg is diluted in the seawater bulk due to mixing processes, an exponential decay-like model was developed and the wet deposition of THg was normalized based on periods between precipitation events and seawater sampling. Normalized wet deposition of THg showed significant correlation with the THg gradient in surface seawater after removal of an outlier. To explain the observed outlier, further data normalization included wind data to account for enhanced seawater mixing due to strong winds. Wind-normalized THg deposition of all datapoints showed significant correlation with the THg gradient in surface seawater. The correlation showed that the THg gradient in surface seawater of 0.378 pg L−1 m−1 corresponds to THg wet deposition of 1 ng m−2 after including the influence of wind speed on seawater mixing.

Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury

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

Interannual Variability of Air-Sea Exchange of Mercury in the Global Ocean: The "Seesaw Effect" in the Equatorial Pacific and Contributions to the Atmosphere

Air-sea exchange of gaseous elemental mercury (Hg(0)) is influenced by different meteorological factors and the availability of Hg in seawater. Here, we use the MITgcm ocean model to explore the interannual variability of this flux and the influence of oceanographic and atmospheric dynamics. We apply the GEOS-Chem model to further simulate the potential impact of the evasion variability on the atmospheric Hg levels. We find a latitudinal pattern in Hg(0) evasion with a relatively small variability in mid-latitudes (3.1-6.7%) and a large one in the high latitudes and Equator (>10%). Different factors dominate the patterns in the equatorial (wind speed), mid- (oceanic flow and temperature), and high-latitudinal (sea-ice, temperature, and dynamic processes) oceans. A seesaw pattern of Hg(0) evasion anomaly (±5-20%) in the equatorial Pacific is found from November to next January between El Niño and La Niña years, owing to the anomalies in wind speed, temperature, and vertical mixing. Higher atmospheric Hg level (2%-5%) are simulated for Hg(0) evasion fluxes with three-month lag, associated with the suppression of upwelling in the beginning of the El Niño event. Despite of the uncertainties, this study elucidates the spatial patterns of the interannual variability of the ocean Hg(0) evasion flux and its potential impact on atmospheric Hg levels.

Potential sources, scavenging processes, and source regions of mercury in the wet deposition of South Korea

In this study, the potential sources, scavenging processes, and emission regions for Hg in wet deposition were investigated in rural (Jeju), suburban (Gwangju), and urban sites (Incheon and Seoul) of South Korea. The annual volume-weighted mean concentrations of Hg in wet deposition were four to five times higher in Incheon (16.6 ng L^-1) and Seoul (22.5 ng L^-1) than in Jeju (4.0 ng L^-1) and Gwangju (4.1 ng L^-1). The variations in the Hg concentrations in wet deposition of Jeju and Gwangju were related to Cl^-, Na⁺, Mg²⁺, and K⁺ originating from marine and crustal sources, and those in Incheon and Seoul were related to SO₄^2-, NO₃^-, and NH₄⁺ emitted from anthropogenic sources. The below-cloud scavenging was considered a major inclusion process of Hg in Jeju and Gwangju, while the within-cloud scavenging was suggested in Incheon and Seoul, based on the results of correlation analysis with Hg and major ions in wet deposition, and meteorological data. The cluster analysis of backward trajectories demonstrated that the Hg concentration in wet deposition was highest in the cluster transported from Hebei and Shandong of China in Gwangju, but in Seoul, the Hg concentrations of each cluster were comparable. This suggests that regional transport is the major source of Hg in the wet deposition of Gwangju while local transport provides substantial amount of Hg in the wet deposition of Seoul. This was further supported by the results of concentration-weighted trajectories: the most probable source region was east China for Gwangju, and the mid-west of South Korea and east China for Seoul. It is noted that the peak methylmercury concentrations were found every spring with simultaneous increases in atmospheric Al, Ca, Mg, and Fe concentrations, indicating a concurrence with Asian dust. The formation process of methylmercury in Asian dust should be confirmed in future studies.

Mercury wet depositions study at suburban, agriculture and traffic sampling sites

This study aimed to measure and discuss the relationship of ambient air precipitations with respect to mercury wet depositions at suburban, agriculture and traffic three characteristic sampling sites during the year of 2019. In addition, the mercury volume weighted mean concentrations (VWM) at three characteristic sampling sites were also calculated. Finally, the ambient mercury wet depositions data obtained in this study to various world sampling sites were also compared and discussed in this study. The results indicated that the average mercury wet depositions for suburban, agriculture and traffic areas were 0.62, 0.55 and 2.32 ng/m² min, respectively. And the average mercury VWM values were 0.9, 0.72 and 1.85 ng/m² min for suburban, agriculture and traffic sites, respectively. In addition, the highest VWM and wet depositions for mercury both occurred in March at traffic and suburban areas. And the mercury wet depositions displayed a declined trend when the month was moved from March to July at both traffic and suburban sampling sites. In addition, the relationship between wet depositions and precipitations was low to moderate correlated in traffic area, while the relationship between wet depositions and precipitations was insignificant at both suburban and agriculture areas. Moreover, the average highest mercury wet deposition occurred in Nepal when compared to the other world sites. In addition, the average value of mercury wet depositions in Nepal was about 17.23 times to that of data obtained in this study during the period of 2007-2019. Finally, the average highest VWM (ng/L) occurred in the China. In addition, the average value mercury VWM in China was about 14.82 times to that of data obtained in this study during the period of 2007-2019.

An updated review of atmospheric mercury

The atmosphere is a key component of the biogeochemical cycle of mercury, acting as a reservoir, transport mechanism, and facilitator of chemical reactions. The chemical and physical behavior of atmospheric mercury determines how, when, and where emitted mercury pollution impacts ecosystems. In this review, we provide current information about what is known and what remains uncertain regarding mercury in the atmosphere. We discuss new ambient, laboratory, and theoretical information about the chemistry of mercury in various atmospheric media. We review what is known about mercury in and on solid- and liquid-phase aerosols. We present recent findings related to wet and dry deposition and spatial and temporal trends in atmospheric mercury concentrations. We also review atmospheric measurement methods that are in wide use and those that are currently under development.

A Coupled Global Atmosphere-Ocean Model for Air-Sea Exchange of Mercury: Insights into Wet Deposition and Atmospheric Redox Chemistry

Air-sea exchange of mercury (Hg) is the largest flux between Earth system reservoirs. Global models simulate air-sea exchange based either on an atmospheric or ocean model simulation and treat the other media as a boundary condition. Here we develop a new modeling capability (NJUCPL) that couples GEOS-Chem (atmospheric model) and MITgcm (ocean model) at the native hourly model time step. The coupled model is evaluated against high-frequency simultaneous measurements of elemental mercury (Hg⁰) in both the atmosphere and surface ocean obtained during five published cruises in the Atlantic, Pacific, and Southern Oceans. Results indicate that the calculated global Hg net evasion flux is 12% higher for the online model than the offline model. We find that the coupled online model captures the spatial pattern of the observations; specifically, it improves the representation of peak seawater Hg⁰ (Hg⁰_aq) concentration associated with enhanced precipitation in the intertropical convergence zone in both the Atlantic and the Pacific Oceans. We investigate the causes of the observed Hg⁰_aq peaks with two sensitivity simulations and show that the high Hg⁰_aq concentrations are associated with elevated convective cloud mass flux and bromine concentrations in the tropical upper troposphere. Observations of elevated Hg⁰_aq concentrations in the western tropical Pacific Ocean merit further study involving BrO vertical distribution and cloud resolving models.

Understanding factors influencing the detection of mercury policies in modelled Laurentian Great Lakes wet deposition

We used chemical transport modelling to better understand the extent to which policy-related anthropogenic mercury emissions changes (a policy signal) can be statistically detected in wet deposition measurements in the Great Lakes region on the subdecadal scale, given sources of noise. In our modelling experiment, we consider hypothetical regional (North American) and global (rest of the world) policy changes, consistent with existing policy efforts (Δglobal = -18%; Δregional = -30%) that divide an eight-year period. The magnitude of statistically significant (p < 0.1) pre- and post-policy period wet deposition differences, holding all else constant except for the policy change, ranges from -0.3 to -2.0% for the regional policy and -0.8 to -2.7% for the global policy. We then introduce sources of noise-trends and variability in factors that are exogenous to the policy action-and evaluate the extent to which the policy signals can still be detected. For instance, technology-related variability in emissions magnitude and speciation can shift the magnitude of differences between periods, in some cases dampening the policy effect. We have found that the interannual variability in meteorology has the largest effect of the sources of noise considered, driving deposition differences between periods to ±20%, exceeding the magnitude of the policy signal. However, our simulations suggest that gaseous elemental mercury concentration may be more robust to this meteorological variability in this region, and a stronger indicator of local/regional emissions changes. These results highlight the potential challenges of detecting statistically significant policy-related changes in Great Lakes wet deposition within the subdecadal scale.

The superstatistical nature and interoccurrence time of atmospheric mercury concentration fluctuations

The probability density function (PDF) of the time intervals between subsequent extreme events in atmospheric Hg⁰ concentration data series from different latitudes has been investigated. The Hg⁰ dynamic possesses a long-term memory autocorrelation function. Above a fixed threshold Q in the data, the PDFs of the interoccurrence time of the Hg⁰ data are well described by a Tsallis q-Exponential function. This PDF behavior has been explained in the framework of superstatistics, where the competition between multiple mesoscopic processes affects the macroscopic dynamics. An extensive parameter μ, encompassing all possible fluctuations related to mesoscopic phenomena, has been identified. It follows a χ ²-distribution, indicative of the superstatistical nature of the overall process. Shuffling the data series destroys the long-term memory, the distributions become independent of Q, and the PDFs collapse on to the same exponential distribution. The possible central role of atmospheric turbulence on extreme events in the Hg⁰ data is highlighted.

Mercury Wet Scavenging and Deposition Differences by Precipitation Type

We analyze the effect of precipitation type on mercury wet deposition using a new database of individual rain events spanning the contiguous United States. Measurements from the Mercury Deposition Network (MDN) containing single rainfall events were identified and classified into six precipitation types. Mercury concentrations in surface precipitation follow a power law of precipitation depth that is modulated by precipitation system morphology. After controlling for precipitation depth, the highest mercury deposition occurs in supercell thunderstorms, with decreasing deposition in disorganized thunderstorms, quasi-linear convective systems (QLCS), extratropical cyclones, light rain, and land-falling tropical cyclones. Convective morphologies (supercells, disorganized, and QLCS) enhance wet deposition by a factor of at least 1.6 relative to nonconvective morphologies. Mercury wet deposition also varies by geographic region and season. After controlling for other factors, we find that mercury wet deposition is greater over high-elevation sites, seasonally during summer, and in convective precipitation.