Aerosol concentration statistics for the Northern Tropical Atlantic

Abstract

During July, August, and September of 1974, daily aerosol measurements were made at three stations in the tropical North Atlantic: Sal Island, Cape Verde Islands; Barbados, West Indies; and Miami, Florida. The two major components at all stations were mineral aerosol, composed of Saharan dust, and sea-salt aerosol. The distribution of the mass concentration of each component is accurately described as the composite of two log normal distributions, usually with one mode decidedly dominant. At Sal Island and Barbados, which experience winds of nearly constant direction, the dominant modes for sea-salt aerosol were similar: for Sal Island, geometric mean GM = 24.9 μg m−3, standard geometric deviation SGD = 1.24 and for Barbados, GM = 17.3 μg m−3, SGD = 1.31. The lower GM at Barbados is attributable to a lower mean wind speed. At Miami the highly variable wind and air mass characteristics result in a unimodal distribution with a substantially lower GM of 5.94 μg m−3 and a significantly higher SGD of 1.77. At Barbados the secondary mode is very high (GM = 51.3, SGD = 1.23) and is mainly attributable to the long-range effects of two tropical storms and a hurricane, which passed well to the east and north of the island but which did not result in any increase in wind speeds at the island. The dominant mode in the mineral aerosol distributions at Sal, Barbados, and Miami is attributable to Saharan dust. The SGD for this mode was approximately 2.4 at each location, while the GM decreased from 21 μg m−3 at Sal to 19 at Barbados and 10 at Miami. The secondary modes may be due to the influence of background aerosols and local inputs.