Saharan dust as a mercury transport vector

Abstract

Mercury is a potent toxicant whose concentration in the environment has increased markedly due to human activity. However, uncertainties exist on sources and sinks of mercury, including those associated with mineral dust, with the Sahara Desert being the largest aeolian dust source worldwide. To characterize mercury associated with Saharan dust export to the Atlantic Ocean, aeolian dust (>2.2 µm in size) and soil samples were collected from the Canary Islands and Cabo Verde, ~400 and 800 km off the west coast of Africa. The sources of the dust samples were determined by back-trajectory analysis, and mercury content was characterized via direct mercury analysis. Mercury concentrations differed significantly (p < 0.05) between Atlantic-origin dust and Saharan-origin dust. Atlantic-origin dust, defined as dust with a back trajectory which is predominantly (>70%) over the Atlantic Ocean, had a mean mercury concentration of 29.8±31.3 ng/g (w/w), or 1.40±1.34 pg/m3 (w/v); this result is in the lower end of the data reported for remote coastal locations on a w/v basis, but lower than any on a w/w basis. Saharan-origin dust, defined as dust with a back trajectory predominantly (>70%) over the Sahara Desert, had a higher mean mercury concentration of 72.8±7.43 ng/g, or 4.64±2.15 pg/m3. Concentrations of mercury in dust samples were found to increase as back trajectories passed over a larger extent of the western African land surface relative to the Atlantic Ocean water surface. Finally, particulate-bound mercury (PBM) concentrations measured at Cabo Verde were used to estimate the flux of particulate mercury to major dust-impacted regions of the Atlantic Ocean, an area comprising 21.3 million km2. The PBM flux to this region is estimated at 6.5±2.2 t/y if all dust originates from the Sahara, or 3.6±1.6 t/y if the dust is of mixed Saharan and Atlantic origins. These numbers represent only ~1% to 3% of the total mercury deposition to the region estimated by current global mercury models. Surface soil mercury concentrations measured in the Canary Islands further support minimal mercury inputs from Saharan dust. Our study thus suggests the current models may have considerably overestimated Saharan dust contribution to mercury deposition.