Long-range transport of North African dust to the eastern United States

The long-range transport of North African dust to the Middle East, Europe, South America, and the Caribbean has been well documented during the past 25 years. With the advent of routine collection and analysis of fine aerosoIs at national parks, monuments, and wilderness areas in the continental United States, these North African dust incursions can now be tracked, characterized, and quantified across much of the eastern half of the United States. Identification of the North African source of these dust episodes is confirmed by mass distribution measurements, a characteristic Al/Ca ratio, isentropic backward air mass trajectories, and sequential plots of the spatial distribution of the dust plumes. North African dust incursions into the continental United States persist for similar to 10 days and occurred, an average, 3 times per year from 1992 to 1995. Fine soil mass usually exceeds 10 mu g m(-3) during these dust episodes and dominates local fine soil dust by an order of magnitude or more, even in the so-called ''dust bowl'' states of the central United States. Size-resolved measurements of elemental composition taken during July 1995 indicate that the mass mean diameter of the transported North African dust is < 1 mu m. The high mass scattering efficiency and abundant particle surface area associated with these submicron soil aerosols could have important consequences for both the radiative balance of the region and the chemistry of the local aerosols during summer when the long-range transport of North African dust to the United States is most common.