Stable Mercury Isotopes Revealing Photochemical Processes in the Marine Boundary Layer

Abstract

Atmospheric mercury (Hg) can be operationally defined into three types: gaseous elemental Hg (GEM), gaseous oxidized Hg (GOM), and particle-bound Hg (PBM). GEM is the most abundant Hg species in the global atmosphere (approximately 90%) and is well mixed due to its prolonged lifetime (0.5-1 year) and stable chemical properties. As GOM and PBM are more soluble in water than GEM and have a shorter lifetime (days to weeks), they are the predominant Hg species deposited to ecosystems through wet and dry deposition (Selin, 2009). The mutual transformations between these three Hg species have an important influence on the transportation and deposition of atmospheric Hg on a global scale (Schroeder & Munthe, 1998). The marine boundary layer (MBL) represents the atmospheric area affected by the oceanic surface, and its height significantly varies between middle-high latitudes (dozens to hundreds of meters) and low latitudes (up to 2000 m) (Hedgecock et al., 2005). The circulation of atmospheric Hg in the MBL differs from that in the planetary boundary layer (PBL) because of the significant differences in meteorological conditions and chemical compositions between the MBL and the PBL (e.g., humidity, sea salt aerosols, and oxidation mechanisms) (

Sources and Transformation Mechanisms of Atmospheric Particulate Bound Mercury Revealed by Mercury Stable Isotopes

“…City-specific mean δ 202 Hg, Δ 199 Hg, and Δ 200 Hg values of PBM during the study period ranged from −2.00 to −0.78, −0.04 to 0.47, and 0.02 to 0.06‰ ( n = 10), respectively, while the overall mean from all the 10 cities were −1.16, 0.22, and 0.05‰, respectively (Figure and Table S3). PBM isotopic compositions observed in the present study align well with the pool of global PBM data, which are characterized by significantly negative δ 202 Hg (mean = −1.64 to −0.69‰), slightly negative to significantly positive Δ 199 Hg (mean = −0.21 to 0.83‰), and positive Δ 200 Hg (mean: 0.01 to 0.14‰) signatures (Table S3), − ,, with the exception of the observations in polar regions (mean δ 202 Hg = 0.03 to 1.05‰, mean Δ 199 Hg = −0.38 to −0.28‰, mean Δ 200 Hg = −0.01 to 0.00‰, n = 2) (Table S3). , By comparison, PBM at urban sites worldwide showed relatively lower Δ 199 Hg and Δ 200 Hg values as compared to observations at Chinese rural sites, in marine air, and at a coastal site in the United States (Figure ).…”

“…Mean (±1sd) of δ 202 Hg vs Δ 199 Hg (A) and δ 202 Hg vs Δ 200 Hg (B) for PBM collected in the 10 megacities in this study. Observations of δ 202 Hg, Δ 199 Hg, and Δ 200 Hg for PBM in rural areas of China, oceans, − urban areas of South Asia, , and USA, as well as Hg(0) (GEM) in urban and rural areas of China ,, were also compiled from previous studies. Error bars are 1sd of the mean values.…”

“…Recent studies have explored the MIF variability of even-Hg isotopes (e.g., Δ 200 Hg), which is thought to be produced during photochemical Hg redox reactions at high altitudes and distinguishable among atmospheric Hg species [e.g., Δ 200 Hg of Hg(0) vs GOM and wet deposition]. ,,, Δ 200 Hg has been used to track the pathways of atmospheric Hg deposition to Earth surface reservoirs. ,,, City-specific mean PBM Δ 200 Hg obtained in the present study ranged from 0.02 to 0.06‰, while higher values (0.01 to 0.14‰) have been reported from rural areas in China ( n = 3), a coastal site in the United States, and over oceans (Figure ). − PBM in the atmosphere is sourced from primary anthropogenic PBM emissions, gas-particle partitioning of GOM emitted from anthropogenic sources, and gas-particle partitioning of GOM produced during atmospheric Hg(0) oxidation. PBM and GOM directly emitted from anthropogenic sources are expected to have Δ 200 Hg values around 0.00 ± 0.03‰. , The positive shift in the observed Δ 200 Hg PBM (higher values than ∼0.00‰ mentioned above) could be mainly caused by gas-particle partitioning of GOM produced by Hg(0) oxidation, which is observed to carry significantly positive Δ 200 Hg values. , Here we use Δ 200 Hg to estimate the fraction of PBM sourced from atmospheric Hg(0) oxidation ( F Hg(0)‑oxidation ) based on eqs and . where Δ 200 Hg sample is the observed Δ 200 Hg values of PBM samples, Δ 200 Hg PBM&GOM‑anthropogenic is the Δ 200 Hg values of anthropogenic emitted PBM and GOM, Δ 200 Hg Hg(0)‑oxidation is the Δ 200 Hg signature of GOM produced by Hg(0) oxidation, and F anthropogenic and F Hg(0)‑oxidation are the fractions of PBM derived from anthropogenic emissions (PBM and gas-particle partitioning of GOM emitted from anthropogenic sources) and gaseous Hg(0) oxidation followed by gas-particle partitioning of GOM, respectively.…”

“…MDF of Hg isotopes can be caused by many physicochemical processes, including reduction, methylation/demethylation, sorption, and evaporation . Large odd-MIF of Hg isotopes are mainly associated with photochemical redox reactions due to the magnetic isotope effect (MIE). − Small magnitudes of odd-MIF (e.g., <0.6‰) have also been observed in abiotic dark Hg­(II) reduction, Hg­(II) complexation with thiol, and abiotic dark Hg(0) oxidation (mainly due to equilibrium isotope exchange) experiments, which are accompanied by a discernible Δ 199 Hg/Δ 201 Hg regression slope of ∼1.6 compared to the MIE (slope of ∼1.0) due to the nuclear volume effect (NVE). − Even-MIF anomalies are mainly observed in atmospheric samples and are assumed to be exclusively produced during atmospheric redox reactions at high altitudes. − Recently, measurements of the Hg isotopic composition in atmospheric aerosols have been carried out at urban, − remote, , oceanic, − and polar sites , worldwide, from which it was proposed that industrial emissions, subsequent in-aerosol photoreduction, and conversion of Hg(0) to PBM are crucial factors regulating the variations in PBM isotopic composition. ,,, Oxidation processes were also speculated to be of potential importance in an oceanic study, but the detailed mechanisms remain largely unknown . Hence, the links between heterogeneous Hg reactions in aerosols and PBM isotopic signature have yet to be established.…”

Stable Isotopes Reveal Photoreduction of Particle-Bound Mercury Driven by Water-Soluble Organic Carbon during Severe Haze

“…Most physical and chemical processes lead to MDF, but odd-MIF is mainly produced by photochemical reactions (e.g. photoreduction of aqueous Hg­(II), photodemethylation of methylmercury). − Even-MIF is mainly observed in atmospheric samples, and its mechanism may be attributed to atmospheric redox processes. − Previous studies have reported Hg isotopes of PBM in different regions (e.g., urban, rural, and coastal areas) ,− and seasonal even diurnal variation of odd-MIF ,, and indicate that the isotopic compositions of PBM emitted from primary sources would be modified during atmospheric transformation or transportation. However, the mechanism that drives Hg isotope variations of PBM is still poorly understood, especially during the transformations of PBM in severe haze events.…”