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Mercury is a contaminant of global concern. It is transported in theatmosphere primarily as gaseous elemental mercury, but its reactivity anddeposition to the surface environment, through which it enters the aquaticfood chain, is greatly enhanced following oxidation. Measurements andmodelling studies of oxidised mercury in the polar to sub-tropical marineboundary layer (MBL) have suggested that photolytically produced bromine atoms arethe primary oxidant of mercury. We report year-round measurements ofelemental and oxidised mercury, along with ozone, halogen oxides (IO and BrO)and nitrogen oxides (NO2), in the MBL over theGalápagos Islands in the equatorial Pacific. Elemental mercuryconcentration remained low throughout the year, while higher than expectedlevels of oxidised mercury occurred around midday. Our results show that theproduction of oxidised mercury in the tropical MBL cannotbe accounted for by bromine oxidation only, or by the inclusion of ozone and hydroxyl. As a two-step oxidationmechanism, where the HgBr intermediate is further oxidised to Hg(II), dependscritically on the stability of HgBr, an additional oxidant is needed to reactwith 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 missingoxidant, though their relative importance cannot be determined explicitly atthis time due to the uncertainties associated with mercury oxidationkinetics. We conclude that the key pathway that significantly enhancesatmospheric mercury oxidation and deposition to the tropical oceans ismissing from the current understanding of atmospheric mercury oxidation

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