Mercury in the global troposphere: a three-dimensional model study

The global distributions of elemental mercury (Hg-0) and divalent mercury c ompounds (HgII) were estimated with a climatological transport model (MOGUN TIA). Natural and man-made sources, including re-emission of previously dep osited mercury (of man-made origin), oxidation of Hg-0 to HgII and wet and dry deposition of HgII were explicitly treated. Comparisons with observatio ns of Hg-0 in surface air, HgII in precipitation and trends in mercury depo sited in lake sediments and peat bogs show a reasonable agreement if the ox idation rate of Hg-0 was chosen to be 1.0/yr. An oxidation time scale outsi de the range 0.5-1.5 yr is difficult to reconcile with observations. A rece ntly measured large decrease in the concentrations of Hg-0 over the Atlanti c is difficult to explain only by a decrease in man-made emissions in Europ e and North America. This latter difficulty indicates either that the man-m ade emissions have been underestimated or that there are large temporal var iations in natural emissions (or re-emissions). We conclude that direct glo bal man-made mercury emissions are likely to be at least 30% as large as th e natural emissions, implying that the deposition rate, averaged over the g lobe, has increased by at least 50% since pre-industrial times. To the exte nt that re-emission of previously deposited mercury of man-made origin is i mportant, the average deposition rate may well have tripled. In and around the most industrial regions (Europe, North America, Southeastern China) the deposition rate has increased by a factor 2-10 during the past two hundred years. (C) 1999 Elsevier Science Ltd. All rights reserved.