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 thunde r cell downdraughts 500 km from the study site and subsequently carrie d by low-velocity winds to arrive approximately 28 h after its generat ion. Peak atmospheric dust concentrations (13,735 mu g m(-3)) measured during the event and under very low wind shear conditions exceeded an y previously reported in the literature for dust haze events. Particle -size features of the plume dust indicated a distant source. The distr ibution 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 concen trations of dust in the atmosphere. After the peak levels of concentra tion had passed, the relationship between vertical dust flux and wind shear velocity tended towards a relation which predicts dust moving up wards through the atmosphere, varying with wind shear velocity to the fourth power.