Abstract
Observations of reactive gaseous mercury (RGM) in marine air show a consistent diurnal cycle with minimum at night, rapid increase at sunrise, maximum at midday, and rapid decline in afternoon. We use a box model for the marine boundary layer (MBL) to interpret these observations in terms of RGM sources and sinks. The morning rise and midday maximum are consistent with oxidation of elemental mercury (Hg 0) by Br atoms, requiring <2 ppt BrO in most conditions. Oxidation of Hg 0 by Br accounts for 35–60% of the RGM source in our model MBL, with most of the remainder contributed by oxidation of Hg 0 by ozone (5–20%) and entrainment of RGM-rich air from the free troposphere (25–40%). Oxidation of Hg 0 by Cl is minor (3–7%), and oxidation by OH cannot reproduce the observed RGM diurnal cycle, suggesting that it is unimportant. Fitting the RGM observations could be achieved in the model without oxidation of Hg 0 by ozone (leaving Br as the only significant oxidant) by increasing the entrainment flux from the free troposphere. The large relative diurnal amplitude of RGM concentrations implies rapid loss with a lifetime of only a few hours. We show that this can be quantitatively explained by rapid, mass-transfer-limited uptake of RGM into sea-salt aerosols as HgCl 3 − and HgCl 4 2−. Our results suggest that 80–95% of Hg II in the MBL should be present in sea-salt aerosol rather than gas-phase, and that deposition of sea-salt aerosols is the major pathway delivering Hg II to the ocean.
Highlights
Atmospheric deposition is the principal source of mercury to the ocean (Lindberg et al, 2007)
A critical step towards understanding the deposition of mercury to the ocean is quantifying the supply of HgII to the atmospheric marine boundary layer (MBL), which extends about 1 km above the ocean surface and is in turbulent contact with it
While we have included the gas-phase Hg0 þ O3 reaction (Hall, 1995) in our simulations, several investigators have challenged its mechanism or atmospheric relevance (Shepler and Peterson, 2003; Calvert and Lindberg, 2005; Hynes et al, 2008). This reaction is a small source of MBL reactive gaseous mercury (RGM) in our model (5–20%), and it has little diurnal variability, so its contribution could be replaced with a larger entrainment flux from the free troposphere
Summary
Atmospheric deposition is the principal source of mercury to the ocean (Lindberg et al, 2007). The Hg þ Br reaction kinetics that they used (Ariya et al, 2002) seem too fast in light of recent data (Goodsite et al, 2004; Balabanov et al, 2005; Donohoue et al, 2006; Ariya et al, 2008) As a result, their model required a diurnal cycle in marine Hg0 emission to balance oxidation and account for the observed lack of diurnal variation in atmospheric Hg0 concentrations (Hedgecock and Pirrone, 2004). We will show that these shortcomings can be corrected while preserving Hedgecock’s central result that Br is a major Hg0 oxidant in the MBL
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