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

Abstract

[1] 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 yr À1 and a TGM lifetime of 0.8 years 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 midlatitudes 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.

Global lifetime of elemental mercury against oxidation by atomic bromine in the free troposphere

“…This is similar to current estimates of the lifetime of Hg 0 against oxidation by ozone [e.g., Selin et al, 2006] and could account for a large part of the Hg 0 loss within the 0.5-2 yr observational constraint on the atmospheric lifetime of total mercury. We find that $85% of the Hg(II) formed is HgBrOH.…”

“…This would be an important, and possibly dominant, global pathway for oxidation and deposition of atmospheric mercury. It could reconcile the atmospheric evidence that Hg 0 oxidation is photochemically mediated [Bergan and Rodhe, 2001;Selin et al, 2006] with the evidence against a major role for oxidation by OH [Calvert and Lindberg, 2005]. The mechanism appears qualitatively consistent with mercury observations -the seasonal cycle of Hg 0 ; airborne particulate mercury measurements; and gaseous Hg(II) in Antarctic summer -but global CTMs are necessary for more stringent quantitative tests.…”

Recent Advances in Atmospheric Chemistry of Mercury

“…Previous studies have assumed O 3 and OH to be the major Hg 0 oxidants in the atmosphere [51][52][53][54][55][56]. Despite theoretical doubts concerning the thermal stability of gaseous HgOH and HgO products, many modeling results using O 3 /OH as the main atmospheric Hg oxidants showed good agreement with the observed Hg 0 concentration and wet deposition flux.…”

Characteristics and potential sources of atmospheric mercury at a subtropical near‐coastal site in East China

“…Natural emissions, including the emission from geologic sources and the reemission of previously deposited Hg, make up approximately two thirds of global Hg emission [Schroeder and Munthe, 1998]. Human activities emit 1960 Mg of Hg into the atmosphere per year with Asia contributing >50% of the release (777 Mg yr À1 from East and Southeast Asia) [Pirrone et al, 2010;Selin et al, 2007;United Nations Environment Programme, 2013]. East China (including Shanghai, Anhui, Jiangsu, Zhejiang, Fujian, Jiangxi, and Shandong Provinces) is an important source region of atmospheric Hg in China, constituting approximately 20% of the total emissions [Wu et al, 2006].…”