This publication was prepared by an agency of the United States Government. Although these data have been processed successfully on a computer system at the U.S. Geological Survey, no warranty expressed or implied is made regarding the display or utility of the data on any other system, nor shall the act of distribution imply any such warranty. The U.S. Geological Survey shall not be held liable for improper or incorrect use of the data described and (or) contained herein. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.
Every year, billions of tons of fine particles are eroded from the surface of the Sahara Desert and the Sahel of West Africa, lifted into the atmosphere by convective storms, and transported thousands of kilometers downwind. Most of the dust is carried west to the Americas and the Caribbean in the Saharan Air Layer (SAL). Dust air masses predominately impact northern South America during the Northern Hemisphere winter and the Caribbean and Southeastern United States in summer. Dust concentrations vary considerably temporally and spatially. In a dust source region (Mali), concentrations range from background levels of 575 micrograms per cubic meter (µg per m3) to 13,000 µg per m3 when visibility degrades to a few meters (Gillies and others, 1996). In the Caribbean, concentrations of 200 to 600 µg per m3 in the mid-Atlantic and Barbados (Prospero and others, 1981; Talbot and others, 1986), 3 to 20 µg per m3 in the Caribbean (Prospero and Nees, 1986; Perry and others, 1997); and >100 µg per m3 in the Virgin Islands (this dataset) have been reported during African dust conditions. Mean dust particle size decreases as the SAL traverses from West Africa to the Caribbean and Americas as a result of gravitational settling. Mean particle size reaching the Caribbean is <1 micrometer (µm) (Perry and others, 1997), and even finer particles are carried into Central America, the Southeastern United States, and maritime Canada. Particles less than 2.5 µm diameter (termed PM2.5) can be inhaled deeply into human lungs. A large body of literature has shown that increased PM2.5 concentrations are linked to increased cardiovascular/respiratory morbidity and mortality (for example, Dockery and others, 1993; Penn and others, 2005).
The quantity of dust transported is a function of global ocean-atmosphere interactions, regional meteorology, composition of surface materials, and human activities. The primary sources of dust are depositional features such as river floodplains (for example, Niger River and its inland delta) and ancient lakebeds (for example, the Bodélé Depression in Chad; Bristow and others, 2009). Recently, humans and their livestock have turned the Sahel into a major dust source through land disturbance: agriculture, land clearing, mechanized road traffic, and livestock such as sheep and goats whose sharp hooves break the delicate crust of desert soils. In the past four decades, the quantities of dust transported have increased. The composition of dust has changed. Pesticide use (for agricultural pests and disease vectors), combustion of fossil fuels and the widespread burning of plastic-dominated garbage, other synthetic organic products, and biomass all have increased in the past four to five decades (Garrison and others, 2003).
As part of a larger project investigating the effects of African dust on human health and downwind coral reefs of the Caribbean, the U.S. Geological Survey (USGS) sampled dust air masses (1) in a dust source region (the Niger River Valley, Bamako and Kati, Republic of Mali); (2) near to but off the coast of West Africa (Sal Island, Republic of Cape Verde); and (3) at downwind sites in (a) the southeastern Caribbean, affected by winter air masses (Galera Point, Trinidad, and Flagstaff Hill, Tobago), and (b) the northeastern Caribbean, affected more by summer air masses [St. Croix, U.S. Virgin Islands (USVI)]. Air samples were collected intermittently between December 2001 and August 2008 using high volume samplers. Samples were analyzed for 84 targeted semivolatile organic compounds (SOCs; tables 1 and 2) including current use and historical pesticides and breakdown products, polycyclic aromatic hydrocarbons (PAHs) and breakdown products, and polychlorinated biphenyl (PCB) congeners. Many of the detected pesticides have been banned for use (see Garrison and others, 2006 for summary table). All detected SOCs (table 3) are known to persist in the environment for days to years, to bioaccumulate, and to be toxic to organisms, including humans, via single or multiple mechanisms (endocrine disruption, immunosuppression, neurotoxicity, hepatotoxicity, carcinogenicity, oncogenicity, teratogenicity, mutagenicity). The dataset consists of text files of blank-corrected concentrations of SOCs detected in 68 air (SOCs_in_air_data.txt) and two dry deposition samples (SOCs_in_dry_dep_data.txt). Also included are the SOC detections in 28 associated field and laboratory blanks (SOCs_in_blanks_data.txt) and estimated method detection limits (table 4).
Limitations on Use
Any use of this dataset should acknowledge the U.S. Geological Survey as originator.
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The following individuals and agencies in Africa, the Caribbean, and the United States were central to the project. Deepest thanks to the dedicated people who collected samples: R. Smith, M. Ranneberger, A. Babana, M. Coulibaly, M. Kanta, D. Maiga (Mali); H. Tonnemacher, R. Berey, K. Haines (St. Croix, USVI); D. Beckles, S. Mahabir, L. Thomas (Trinidad); and, staff at Speyside Inn (Tobago). Equally, many government agencies provided essential in-kind support over the years: the U.S. Embassy (Mali); the Institute of Meteorology and Geophysics, International Airport and Security Authority (Sal Island, Cape Verde); the University of the West Indies St. Augustine and the Environmental Management Authority (Trinidad and Tobago). Special thanks go to the individuals and agencies that allowed us to sample from their property: the Ministry of Communications (Republic of Mali) and the American International School of Bamako; P.J.P. Gomes (Sal Island, Cape Verde); R. van Heckmann (St. Croix, USVI); and Maritime Services and Tobago House of Assembly (Trinidad and Tobago). Our deep appreciation to P. Lamothe for careful trace-metals analyses and to J. Schrlau, S. Skaates, F. Wiebe, M. Stroppel, S. Smith, and G. Wilson for meticulous SOC analyses. We are especially grateful to the Friends of Virgin Islands National Park for providing the first funding for sample analysis, to NASA, and to the USGS (L. Robbins; the Venture Capital, Earth Surface Dynamics, Terrestrial Aquatic and Marine Ecosystems, and Mendenhall Postdoctoral Programs) for funding the project.
Garrison, V.H., Foreman, W.T., Genualdi, S.A., Majewski, M.S., Mohammed, Azad, and Simonich, S.M., 2011, Concentrations of semivolatile organic compounds associated with African dust air masses in Mali, Cape Verde, Trinidad and Tobago, and the U.S. Virgin Islands, 2001-2008: U.S. Geological Survey Data Series 571, 1 DVD.