JULY 2006: Chemical cycling and deposition of atmospheric mercury: Global constraints from observations

We use a global 3-D model of atmospheric mercury (GEOS-Chem) to interpret worldwide observations of total gaseous mercury (TGM) and reactive gaseous mercury (RGM) in terms of the constraints they provide on the chemical cycling and deposition of mercury. Our simulation including a global mercury source of 7000 Mg y-1 and a TGM lifetime of 0.8 y reproduces the magnitude and large-scale variability of TGM observations at land sites. However, it cannot capture observations of high TGM from ship cruises, implying a problem either in the measurements or in our fundamental understanding of mercury sources. Observed TGM seasonal variation at northern mid-latitudes is consistent with a photochemical oxidation for Hg(0) partly balanced by photochemical reduction of Hg(II). Observations of increasing RGM with altitude imply a long lifetime of Hg(II) in the free troposphere. We find in the model that Hg(II) dominates over Hg(0) in the upper troposphere and stratosphere, and that subsidence is the principal source of Hg(II) at remote surface sites. RGM observations at Okinawa Island (Japan) show large diurnal variability implying fast deposition, which we propose is due to RGM uptake by sea-salt aerosols. Observed mercury wet deposition fluxes in the United States show a maximum in the southeast, which we attribute to photochemical oxidation of the global Hg(0) pool. They also show a secondary maximum in the industrial Midwest due to regional emissions that is underestimated in the model, possibly because of excessive dry deposition relative to wet (dry deposition accounts for 68% of total mercury deposition in the United States in the model, but this is sensitive to the assumed phase of Hg(II)). We estimate that North American anthropogenic emissions contribute on average 20% to U.S. mercury deposition.

The figure shows annual average mercury concentrations in surface air. Model results (background, for year 2003) are compared to observations (circles) from long-term surface sites (Tables 2 and 3). Also shown are observations from ship cruises in the tropical Atlantic in May-June 1996 [Lamborg et al., 1999], across the Atlantic (north-south transect) in December 1999-January 2000 from Temme et al. [2003b], and across the NW Pacific in May-June 2002 from Laurier et al. [2003]. The figure shows total gaseous mercury (TGM) in the observations, and the sum of Hg(0) and Hg(II) in the model. Color scales are saturated at the maximum values indicated in the legend. A full description is given in Selin et al. [2006].