TRAJECTORIES FOR SAHARAN DUST TRANSPORTED TO BARBADOS USING STOKESS LAW TO DESCRIBE GRAVITATIONAL SETTLING

Atmospheric chemistry programs often make use of retrospective back tr ajectories to determine the source regions of substances sampled at a particular site. Isentropic trajectories, which depict motion on hypot hetical surfaces of constant potential temperature, have been used as part of the Atmosphere/Ocean Chemistry Experiment to understand the so urces of aerosols and gases sampled over the North Atlantic Ocean. How ever, 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 w as the source region for 12% of the samples with significant aluminum (Al) concentrations (> 1.0 mu g m(-3)). Upper-level isentropic traject ories (310 and 315 K() indicated transport from northern Africa for ap proximately half of the samples with Al concentrations greater than 1. 0 mu g m(-3). However, at the location of the sampling site, the upper -level trajectories were well above the boundary layer, where the mine ral aerosol samples were collected. Stokes's law was used in the calcu lation of nonisentropic trajectories that incorporated the gravitation al settling of aerosol particles. These trajectories, which began in t he boundary layer, were calculated from a combination of lower level a nd upper-level wind fields, and more accurately represented the wind f ields that transported Saharan dust to Barbados. The Stokes's law traj ectories for 4-mu m aerosol particles reached a maximum in transport f rom northern Africa during the summer, which was also when the highest Al concentrations were observed. The Stokes's law trajectories for 4- mu m aerosol particles indicated northern Africa as the source region for 55% of the high Al samples, while the equivalant value was only 9% for the 2-mu m aerosols trajectories. Dust samples collected at Barba dos have a smaller mean radius than the calculation required, indicati ng that other vertical motions are important during transport in addit ion to gravitational settling. However, up to 20% of the dust sampled at Barbados is in the size range used(4 mu m).