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

Wang, F and Saiz-Lopez, A and Mahajan, AS and Gómez Martín, JC and Armstrong, D and Lemes, M and Hay, T and Prados-Roman, C (2014) Enhanced production of oxidised mercury over the tropical Pacific Ocean: A key missing oxidation pathway. Atmospheric Chemistry and Physics, 14 (3). pp. 1323-1335.

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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 and modelling studies of oxidised mercury in the polar to sub-tropical marine boundary layer (MBL) 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 MBL over the Galápagos Islands in the equatorial Pacific. Elemental mercury concentration remained low throughout the year, while higher than expected levels of oxidised mercury occurred around midday. Our results show that the production of oxidised mercury in the tropical MBL cannot be accounted for by bromine oxidation only, or by the inclusion of ozone and hydroxyl. As a two-step oxidation mechanism, where the HgBr intermediate is further oxidised to Hg(II), depends critically on the stability of HgBr, an additional oxidant is needed to react with HgBr to explain more than 50 of the observed oxidised mercury. Based on best available thermodynamic data, we show that atomic iodine, NO2, or HO2 could all play the potential role of the missing oxidant, though their relative importance cannot be determined explicitly at this time due to the uncertainties associated with mercury oxidation kinetics. 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.

Item Type: Article
Additional Information: Copyright of this article belongs to Author(s) 2014. This work is distributed under the Creative Commons Attribution 3.0 License.
Uncontrolled Keywords: atmospheric chemistry; atmospheric deposition; atmospheric modeling; boundary layer; halogen; marine atmosphere; mercury (element); nitrogen oxides; oxidation; ozone, Ecuador; Galapagos Islands; Pacific Ocean; Pacific Ocean (Tropical)
Subjects: Meteorology and Climatology
Depositing User: IITM Library
Date Deposited: 04 Dec 2014 11:04
Last Modified: 04 Dec 2014 11:04

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