Chemical and Temporal Characteristics of Saharan dust Episodes Inferred from Aerosols Collected over the Subtropical North Atlantic - A Link to the North Atlantic Oscillation Revisited

We studied temporal patterns of Saharan dust transported from source regions in North Africa to three sites in the North Atlantic Ocean: Izania (Canary Islands), Barbados (West Indies), and Bermuda. Samples representing dust episodes were selected from daily air filters based on a semi-quantitative color-based method and validated by comparing their elemental composition with the previously obtained data for Saharan dust and aerosols. Out of more than 6000 aerosol samples collected for the Atmosphere/Ocean Chemistry Experiment (AEROCE) over the period of ten years (1989-1998), about 2000 air filters displayed brown color deposit indicative to the presence of mineral dust. The absolute concentrations (ppm) of 33 elements (Na, Mg, Al, Cl, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, As, Se, Br, Rb, Sb, I, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Hf, Ta, and Th) in Saharan aerosol sampled at Barbados and Izania were calculated using the measured Al to ash ratio of 0.104 in brown color samples. For Izania and Barbados, annual concentrations of aluminum, an indicator of atmospheric dust, revealed weak but significant correlation with the annual North Atlantic Oscillation Index (NOA). On a more detail scale, however, a time series of average monthly dust concentrations (1989-1998) for these two sites was generally out of phase with NAO index but become correlated when the dust time series were lagged by 2 to 6 months. This finding brings a new insight to the relationship between dust production and climate modes especially in assessing the contribution of soil condition and transport variations to dust concentration over the North Atlantic Ocean.