Atmospheric Deposition

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The coastal ocean receives nutrients and trace metals from continental runoff – water that runs over land and rivers that drain into the ocean carry essential elements that support marine productivity. In the open ocean where we currently sit hundreds of miles from the coast, the main source of metals and nutrients is atmospheric deposition. Atmospheric deposition can be “dry” as dust falls into the ocean, or “wet” as rain falls delivering dissolved elements.

Pictured above are sampling devices to capture both wet and dry deposition. The white bucket on the left holds a sample bottle and funnel and is equipped with a sensor to open only during rain events. The four house-like structures to the right are aerosol samplers that use a vacuum to pull air through filters that collect fine grain dust particles invisible to the naked eye.

Studying atmospheric deposition gives us a chance to determine how much mercury entering the ocean is from natural sources versus anthropogenic (man-made) sources. Mercury is a naturally occurring element found in earth’s crust in constant proportion to other naturally occurring elements such as iron and aluminum. By looking at the ratio of mercury to say aluminum (Al), we can determine if the aerosol source is natural or anthropogenic. The average crustal ratio of Hg/Al is known, so if the aerosol ratio is greater than the crustal ratio, then the aerosols likely contain mercury from an anthropogenic source. Combustion of coal for energy production releases tons of mercury into the air that remains for up to one year, allowing for mercury released in the United States or China to transverse the world and be deposited over remote and seemingly pristine open ocean regions.

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