Oxidation of elemental Hg in anthropogenic and marine airmasses
Mercury (Hg) is a neurotoxin that bioaccumulates in the food chain.\nMercury is emitted to the atmosphere primarily in its elemental form,\nwhich has a long lifetime allowing global transport. It is known that\natmospheric oxidation of gaseous elemental mercury (GEM) generates\nreactive gaseous mercury (RGM) which plays an important role in the\natmospheric mercury cycle by enhancing the rate of mercury deposition to\necosystems. However, the primary GEM oxidants, and the chemical\ncomposition of RGM are poorly known. Using speciated mercury\nmeasurements conducted at the Mt. Bachelor Observatory since 2005 we\npresent two previously unidentified sources of RGM to the free\ntroposphere (FT). Firstly, we observed elevated RGM concentrations,\nlarge RGM/GEM-ratios, and anti-correlation between RGM and GEM during\nAsian long-rang transport events, demonstrating that RGM is formed from\nGEM by in-situ oxidation in some anthropogenic pollution plumes in the\nFT. During the Asian pollution events the measured RGM/GEM-enhancement\nratios reached peak values, up to similar to 0.20, which are\nsignificantly larger than ratios typically measured (RGM/GEM < 0.03) in\nthe Asian source region. Secondly, we observed very high RGM levels -\nthe highest reported in the FT - in clean air masses that were processed\nupwind of Mt. Bachelor Observatory over the Pacific Ocean. The high RGM\nconcentrations (up to 700 pgm(-3)), high RGM/GEM-ratios (up to 1), and\nvery low ozone levels during these events provide observational evidence\nindicating significant GEM oxidation in the lower FT in some conditions.