Is oxidation of atmospheric mercury controlled by different mechanisms in the polluted continental boundary layer vs. remote marine boundary layer?

Abstract. Atmospheric mercury (Hg) is a global pollutant and thought to be the main source of mercury in oceanic and remote terrestrial systems, where it becomes methylated and bioavailable; hence, atmospheric mercury pollution has global consequences for both human and ecosystem health. Understanding of spatial and temporal variations of atmospheric speciated mercury can advance our knowledge of mercury cycling in various environments. This review summarized spatiotemporal variations of total gaseous mercury or gaseous elemental mercury (TGM/GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM) in various environments including oceans, continents, high elevation, the free troposphere, and low to high latitudes. In the marine boundary layer (MBL), the oxidation of GEM was generally thought to drive the diurnal and seasonal variations of TGM/GEM and GOM in most oceanic regions, leading to lower GEM and higher GOM from noon to afternoon and higher GEM during winter and higher GOM during spring–summer. At continental sites, the driving mechanisms of TGM/GEM diurnal patterns included surface and local emissions, boundary layer dynamics, GEM oxidation, and for high-elevation sites mountain–valley winds, while oxidation of GEM and entrainment of free tropospheric air appeared to control the diurnal patterns of GOM. No pronounced diurnal variation was found for Tekran measured PBM at MBL and continental sites. Seasonal variations in TGM/GEM at continental sites were attributed to increased winter combustion and summertime surface emissions, and monsoons in Asia, while those in GOM were controlled by GEM oxidation, free tropospheric transport, anthropogenic emissions, and wet deposition. Increased PBM at continental sites during winter was primarily due to local/regional coal and wood combustion emissions. Long-term TGM measurements from the MBL and continental sites indicated an overall declining trend. Limited measurements suggested TGM/GEM increasing from the Southern Hemisphere (SH) to the Northern Hemisphere (NH) due largely to the vast majority of mercury emissions in the NH, and the latitudinal gradient was insignificant in summer probably as a result of stronger meridional mixing. Aircraft measurements showed no significant vertical variation in GEM over the field campaign regions; however, depletion of GEM was observed in stratospherically influenced air masses. In examining the remaining questions and issues, recommendations for future research needs were provided, and among them is the most imminent need for GOM speciation measurements and fundamental understanding of multiphase redox kinetics.

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Speciated atmospheric mercury in the marine boundary layer of the Bohai Sea and Yellow Sea

Atmospheric Environment ◽

10.1016/j.atmosenv.2016.02.021 ◽

2016 ◽

Vol 131 ◽

pp. 360-370 ◽

Cited By ~ 18

Author(s):

Chunjie Wang ◽

Zhijia Ci ◽

Zhangwei Wang ◽

Xiaoshan Zhang ◽

Jia Guo

Keyword(s):

Boundary Layer ◽

Yellow Sea ◽

Bohai Sea ◽

Marine Boundary Layer ◽

Atmospheric Mercury ◽

The Bohai Sea

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New evidence for atmospheric mercury transformations in the marine boundary layer

10.5194/acp-2020-397 ◽

2020 ◽

Author(s):

Ben Yu ◽

Lin Yang ◽

Linlin Wang ◽

Hongwei Liu ◽

Cailing Xiao ◽

...

Keyword(s):

Boundary Layer ◽

Marine Boundary Layer ◽

Volume Effect ◽

Atmospheric Mercury ◽

Isotopic Signatures ◽

Global Transport ◽

New Evidence ◽

Transformation Processes ◽

Cl Atoms ◽

Observation Results

Abstract. Marine boundary layer (MBL) is the largest transport place and reaction vessel of atmospheric mercury (Hg). The transformations of atmospheric Hg in MBL are crucial for the global transport and deposition of Hg. Herein, Hg isotopic signatures in total gaseous mercury (TGM) and particulate bound Hg (PBM) collected during three cruises to Chinese seas in summer and winter were measured to reveal the transformation processes of atmospheric Hg in the MBL. Unlike the observation results at inland sites, isotopic compositions in TGM from MBL were shaped not only by mixing continental emissions, but also largely by the oxidation of Hg0 primarily derived by Br atoms. Lower air temperature could promote the positive MIF in TGM in summer, while the relative processes might be weak in winter. In contrast, the positive Δ199Hg and high ratios of Δ199Hg / Δ201Hg in PBM indicated that alternative oxidants other than Br or Cl atoms played a major role in the formation of Hg(II) in PBM, likely following the nuclear volume effect. Our results suggested the importance of local Hg environmental behaviours caused by an abundance of highly reactive species, and provided new evidence for understanding the complicated transformations of atmospheric Hg in the MBL.

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Enhanced production of oxidised mercury over the tropical Pacific Ocean: a key missing oxidation pathway

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-21541-2013 ◽

2013 ◽

Vol 13 (8) ◽

pp. 21541-21572

Author(s):

F. Wang ◽

A. Saiz-Lopez ◽

A. S. Mahajan ◽

J. C. Gómez Martín ◽

D. Armstrong ◽

...

Keyword(s):

Boundary Layer ◽

Marine Boundary Layer ◽

Thermal Dissociation ◽

Oxidation Mechanism ◽

Elemental Mercury ◽

Atmospheric Mercury ◽

Mercury Oxidation ◽

Enhanced Production ◽

Daily Variability

Abstract. Mercury is a contaminant of global concern. It is transported in the atmosphere primarily as gaseous elemental mercury, but its reactivity and deposition to the surface environment, through which it enters the aquatic food chain, is greatly enhanced following oxidation. Measurements of oxidised mercury in the polar to sub-tropical marine boundary layer have suggested that photolytically produced bromine atoms are the primary oxidant of mercury. We report year-round measurements of elemental and oxidised mercury, along with ozone, halogen oxides (IO and BrO) and nitrogen oxides (NO2), in the marine boundary layer over the Galápagos Islands in the Equatorial Pacific. Elemental mercury concentration remained low throughout the year, while considerable concentrations of oxidised mercury occurred around midday. Our results show that the production of oxidised mercury in the tropical marine boundary layer cannot be accounted for by only bromine oxidation, or by the inclusion of ozone and hydroxyl. A two-step oxidation mechanism where the HgBr intermediate is further oxidised to Hg(II) depends critically on the stability of HgBr. If the current paradigm is considered, another oxidant is needed to explain more than 50% of the observed oxidised mercury. We show that atomic iodine could play the role of the missing oxidant, explaining not only the Hg(II) levels observed, but also the daily variability. However, more recent theoretical calculations indicate that the thermal dissociation rate of HgBr is much faster, by an order of magnitude, than previously reported, which implies that only trace gases at relatively high mixing ratios forming stable complexes with HgBr (such as HO2 and NO2) could compete to generate levels of Hg(II) similar to those observed in our study. Nevertheless, the daily variability of oxidised mercury is not well accounted for by using these new theoretically estimated rates. Furthermore, correlation analysis does not support a major role of NO2 or HO2. We conclude that the key pathway that significantly enhances atmospheric mercury oxidation and deposition to the tropical oceans is missing from the current understanding of atmospheric mercury oxidation.

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Gaseous elemental mercury depletion events observed at Cape Point during 2007–2008

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-1121-2010 ◽

2010 ◽

Vol 10 (3) ◽

pp. 1121-1131 ◽

Cited By ~ 46

Author(s):

E.-G. Brunke ◽

C. Labuschagne ◽

R. Ebinghaus ◽

H. H. Kock ◽

F. Slemr

Keyword(s):

Boundary Layer ◽

Marine Boundary Layer ◽

Elemental Mercury ◽

Atmospheric Mercury ◽

Chemical Mechanism ◽

Polar Regions ◽

Gaseous Elemental Mercury ◽

Transport Distance ◽

Gaseous Mercury ◽

Urban Emissions

Abstract. Gaseous mercury in the marine boundary layer has been measured with a 15 min temporal resolution at the Global Atmosphere Watch station Cape Point since March 2007. The most prominent features of the data until July 2008 are the frequent occurrences of pollution (PEs) and depletion events (DEs). Both types of events originate mostly within a short transport distance (up to about 100 km), which are embedded in air masses ranging from marine background to continental. The Hg/CO emission ratios observed during the PEs are within the range reported for biomass burning and industrial/urban emissions. The depletion of gaseous mercury during the DEs is in many cases almost complete and suggests an atmospheric residence time of elemental mercury as short as a few dozens of hours, which is in contrast to the commonly used estimate of approximately 1 year. The DEs observed at Cape Point are not accompanied by simultaneous depletion of ozone which distinguishes them from the halogen driven atmospheric mercury depletion events (AMDEs) observed in Polar Regions. Nonetheless, DEs similar to those observed at Cape Point have also been observed at other places in the marine boundary layer. Additional measurements of mercury speciation and of possible mercury oxidants are hence called for to reveal the chemical mechanism of the newly observed DEs and to assess its importance on larger scales.

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New evidence for atmospheric mercury transformations in the marine boundary layer from stable mercury isotopes

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-9713-2020 ◽

2020 ◽

Vol 20 (16) ◽

pp. 9713-9723 ◽

Cited By ~ 1

Author(s):

Ben Yu ◽

Lin Yang ◽

Linlin Wang ◽

Hongwei Liu ◽

Cailing Xiao ◽

...

Keyword(s):

Boundary Layer ◽

Marine Boundary Layer ◽

Volume Effect ◽

Atmospheric Mercury ◽

Environmental Behaviors ◽

Isotopic Compositions ◽

Mass Independent Fractionation ◽

Global Transport ◽

New Evidence ◽

Transformation Processes

Abstract. The marine boundary layer (MBL) is the largest transport place and reaction vessel of atmospheric mercury (Hg). The transformations of atmospheric Hg in the MBL are crucial for the global transport and deposition of Hg. Herein, Hg isotopic compositions of total gaseous mercury (TGM) and particle-bound Hg (PBM) collected during three cruises to Chinese seas in summer and winter were measured to reveal the transformation processes of atmospheric Hg in the MBL. Unlike the observation results at inland sites, isotopic compositions of TGM in the MBL were affected not only by mixing continental emissions but also largely by the oxidation of Hg0 primarily derived by Br atoms. Δ199Hg values of TGM were significantly positively correlated with air temperature in summer, indicating that processes inducing positive mass-independent fractionation of odd isotopes in TGM could be more active at low temperatures, while the relative processes might be weak in winter. In contrast, the positive Δ199Hg and high ratios of Δ199Hg∕Δ201Hg in PBM indicated that alternative oxidants other than Br or Cl atoms played a major role in the formation of Hg(II) in PBM, likely following the nuclear volume effect. Our results suggest the importance of local Hg environmental behaviors caused by an abundance of highly reactive species and provide new evidence for understanding the complicated transformations of atmospheric Hg in the MBL.

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