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Oxidation of elemental Hg in anthropogenic and marine airmasses

by H. Timonen, J. L. Ambrose, D. a. Jaffe
Atmospheric Chemistry and Physics ()


Mercury (Hg) is a neurotoxin that bioaccumulates in the food chain. Mercury is emitted to the atmosphere pri- marily in its elemental form, which has a long lifetime al- lowing global transport. It is known that atmospheric oxi- dation of gaseous elemental mercury (GEM) generates re- active gaseous mercury (RGM) which plays an important role in the atmospheric mercury cycle by enhancing the rate of mercury deposition to ecosystems. However, the pri- mary GEM oxidants, and the chemical composition of RGM are poorly known. Using speciated mercury measurements conducted at the Mt. Bachelor Observatory since 2005 we present two previously unidentified sources of RGM to the free troposphere (FT). Firstly, we observed elevated RGM concentrations, large RGM/GEM-ratios, and anti-correlation between RGM and GEM during Asian long-rang transport events, demonstrating that RGM is formed from GEM by in-situ oxidation in some anthropogenic pollution plumes in the FT. During the Asian pollution events the measured RGM/GEM-enhancement ratios reached peak values, up to ∼0.20, which are significantly larger than ratios typically measured (RGM/GEM <0.03) in the Asian source region. Secondly, we observed very high RGM levels – the high- est reported in the FT – in clean air masses that were pro- cessed upwind of Mt. Bachelor Observatory over the Pacific Ocean. The high RGM concentrations (up to 700 pgm−3), high RGM/GEM-ratios (up to 1), and very low ozone lev- els during these events provide observational evidence indi- cating significant GEM oxidation in the lower FT in some conditions.

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