Transoceanic transport of metals and deposition in the Southeastern United States

Saharan dust is persistently being transported and deposited in ecosystems of the western Atlantic Ocean. Satellite photos reveal that this dust is transported in tropospheric low-pressure waves that cross the central Atlantic Ocean. This dust is an aggregate of clay and quartz particles cemented with iron oxides. Analysis of dust samples collected from Mali (central Africa), the Azores, the Caribbean and the Eastern United States show that metal concentrations are significantly higher than average crustal rocks. Over the past decade, there has been a significant effort to understand the cycling of mercury in south Florida, but other metals has received very little attention. Trace metal measurements on the ombrogeneous sediment formed during the last decade in south Florida indicates that metals can be correlated with aluminum, which is considered a proxy for dust. The largest available aerosol data set is provided by the IMPROVE (Interagency Monitoring of Protected Visual Environments) program. Focusing on arsenic as an example, the average concentration in aerosols collected during this program range from 17 mg/kg in the Virgin Islands to 79 mg/kg at Chassahowitzka, Florida. At Chassahowitzka, most of the arsenic appears to be associated with organic carbon. If it is assumed that the concentrations in Mali dust and in the aerosols in the Virgin Islands are indicative of soil dust, then the higher values at Chassahowitzka are most likely derived from local or regional sources. A simple calculation indicates that African dust supplies about 25 % of the arsenic deposited from aerosols in the southeastern United States. Comparison of the average yearly arsenic concentrations measured in the Virgin Islands and Everglades shows a negative relationship with the North Atlantic Oscillation Index (NAO). This relationship demonstrates the influence of climate on the transport and deposition of aerosols with associated metals to the southeastern United States.