RELATIONSHIPS BETWEEN MERCURY AND SEA SALT ION CONCENTRATIONS IN RAINWATER FROM A MARINE SITE

The ocean-atmosphere interface is important in the global Hg cycle. Gaseous evasion of Hg from the oceans is the largest worldwide natural source of Hg to the atmosphere. The ocean also serves as a sink for atmospheric Hg that is deposited to its surface via wet and dry mechanisms. The atmospheric cycling of gas phase mercury is largely dependant on its oxidation state. The elemental gas phase (Hg⁰) is less soluble than the oxidized form, Hg²⁺, operationally defined as reactive gaseous mercury (RGM). As a result of their differences in solubility, the average atmospheric lifetime of Hg⁰ (~1 year) is much longer than RGM (~ 2 weeks). Hg⁰ can become oxidized in the atmosphere however the primary atmospheric oxidizing agent remains uncertain. Recent studies indicate that brominated compounds, which are typically found in the marine environment, may facilitate oxidation of Hg⁰ and contribute to the rapid cycling of Hg in the coastal and marine atmosphere. Additionally, sea salt aerosol may serve as a sink for RGM in the marine atmosphere.

During summer 2009 rain water samples were collected from Appledore Island, Maine located approximately 10 km off the New Hampshire coast. Samplers were co-located for event based collection and analysis of total aqueous Hg, dissolved organic carbon, and major ions including Cl^-, SO₄^2-, NO₃^- Na⁺, K⁺, Ca²⁺, Mg²⁺, and NH₄⁺. The major ions were used to calculate the amount of sea salt present in the samples and interesting relationships were found when compared to Hg concentrations. Samples containing a mid-range percentage of sea salt have the highest Hg concentrations indicating that Hg may be most associated with aged sea salt aerosols. This relationship might also be explained by a mixing of marine air with polluted continental air. DOC concentrations were also examined due to the correlation between Hg and DOC in fresh water ecosystems. This relationship remains relatively unexamined in rain water and our results indicate a positive correlation between DOC and Hg concentrations (r = 0.54, p=0.06) at this site.