Measurement of the marine mercury emissions in the Atlantic (DFG)

Joachim Kuss, Hildegard Kubsch

Mercury mobilized by anthropogenic activity is introduced into the sea by wet and dry deposition and by river drainage. The major emissions are caused by fossil fuel burning and industrial processes. But also the sea may become a significant source for mercury by elemental mercury (Hg⁰) sea-air gas exchange. Transformation of ionic mercury to volatile Hg⁰ is possibly caused by direct photon-induced reactions and/or by biotic processes in surface waters. The spatial and seasonal variability of the Hg⁰ emission were shown for the Baltic Sea in 2006 (Kuss and Schneider, 2007) and will be investigated for the Atlantic ocean in the frame of this project. During two transatlantic transects on R/V “Polarstern”, the elemental mercury (Hg⁰) sea-air gas exchange has been investigated in various geochemical provinces of the Atlantic Ocean. The transect Bremerhaven - Cape Town in November 2008 has been complemented by a transect Punta-Arenas – Bremerhaven in April/May 2009 as a seasonal intercomparison.

The Hg⁰ measurement is based on a new method for the high-resolution determination of Hg⁰ (about every 15 nautical miles) in surface water using an equilibrator coupled to a cold-vapour atomic fluorescence spectrometer (Description of the method, see below). Alternately Hg⁰ is measured in the marine atmosphere (Hg⁰_atm). The Hg⁰ sea-air flux (F_Hg⁰) is then calculated based on the measured concentration difference and the gas exchange transfer velocity k, with Henry’s law constant H that depends on the temperature, by using

The variability of the flux will be investigated in relation to prevailing biological conditions and physical forcing factors like wind and solar radiation. In this context analyses will be carried out for the determination of dissolved and particulate organic carbon (DOC/POC), Chlorophyll a, and phaeopigments. Also the partial pressure of CO₂ will be continuously measured to identify the state of the algae bloom. Insight into the surface water mercury budget will be obtained by additional determinations of reactive Hg and total Hg. Moreover ship data of solar radiation, temperature and salinity of the surface water (thermosalinograph) and of wind speed will be recorded for calculations and interpretation of the data set.

References
Kuss, J. and Schneider, B., 2007. Variability of the gaseous elemental mercury sea-air flux of the Baltic Sea. Environmental Science and Technology, 41(23): 8018–8023.

Description of the method

The new method that rely on the determination of Hg⁰ in air that is in equilibrium with surface seawater (Hg⁰_equ). During the transect seawater is continuously pumped by a clean-seawater supply system and dispersed into the headspace of a 20 L gas bottle by a shower head and drained afterwards. The equilibrium is achieved in about one hour. The supply of the headspace gas to the analytical instrument without dilution or contamination by laboratory air is achieved by a raising water level in the equilibrator simply after closing the water drainage of the equilibrator. Subsequently Hg⁰ is measured in the equilibrated air using cold-vapour atomic fluorescence spectrometry after pre-concentration by gold amalgamation (Tekran 2537A). During times of equilibration, thus between the measurements of Hg⁰_equ, air is supplied to the TEKRAN from an upper deck for the determination of the Hg⁰ concentration in the marine atmosphere (Hg⁰_atm).