Dust concentrations and particle-size characteristics of an intense dust haze event: Inland Delta region, Mali, West Africa

An Intense dust plume was monitored in the Inland Delta region of Mall during the period 27-30 April 1990. The plume was generated by thunder cell downdraughts 500 km from the study site and subsequently carried by low-velocity winds to arrive approximately 28 h after its generation. Peak atmospheric dust concentrations (13,735 mu g m(-3)) measured during the event and under very low wind shear conditions exceeded any previously reported in the literature for dust haze events. Particle-size features of the plume dust indicated a distant source. The distribution of dust-particle sizes was uni-modal with a mean of 3 mu m at 10 m. Calculated vertical dust fluxes indicated that, upon arrival of the plume, the vertical flux of particles was not in equilibrium with the local wind shear. This was the result of the very high mass concentrations of dust in the atmosphere. After the peak levels of concentration had passed, the relationship between vertical dust flux and wind shear velocity tended towards a relation which predicts dust moving upwards through the atmosphere, varying with wind shear velocity to the fourth power.