Trajectories for Saharan Dust Transported to Barbados Using Stokes's Law to Describe Gravitational Settling

Abstract

Atmospheric chemistry programs often make use of retrospective back trajectories to determine the source regions of substances sampled at a particular site. Isentropic trajectories, which depict motion on hypothetical surfaces of constant potential temperature, have been used as part of the Atmosphere/Ocean Chemistry Experiment to understand the sources of aerosols and gases sampled over the North Atlantic Ocean. However, isentropic trajectories typically do not adequately describe the transport of mineral aerosol, for example, from the Sahara Desert to Barbados. Boundary layer trajectories indicated that northern Africa was the source region for 12% of the samples with significant aluminum (Al) concentrations (> 1.0 µg m?3). Upper-level isentropic trajectories (310 and 315 K) indicated transport from northern Africa for approximately half of the samples with Al concentrations greater than 1.0 µg m?3. However, at the location of the sampling site, the upper-level trajectories were well above the boundary layer, where the mineral aerosol samples were collected. Stokes's law was used in the calculation of nonisentropic trajectories that incorporated the gravitational settling of aerosol particles. These trajectories, which began in the boundary layer, were calculated from a combination of lower-level and upper-level wind fields, and more accurately represented the wind fields that transported Saharan dust to Barbados. The Stokes's law trajectories for 4-µm aerosol particles reached a maximum in transport from northern Africa during the summer, which was also when the highest Al concentrations were observed. The Stokes's law trajectories for 4-µm aerosol particles indicated northern Africa as the source region for 55% of the high Al samples, while the equivalent value was only 9% for the 2-µm aerosols trajectories. Dust samples collected at Barbados have a smaller mean radius than the calculation required, indicating that other vertical motions are important during transport in addition to gravitational settling. However, up to 20% of the dust sampled at Barbados is in the size range used (4 µm).