The harmattan haze in three Nigerian cities: concentration, size and settling velocity

Abstract

During two harmattan seasons of 1999/2000 and 2000/2001, harmattan dust particles were collected by suction on pre-weighted filter papers in Jos (9.9°N, 8.9°E). In January to March 2002, the dust particles were collected by gravitational settling on microscope slides placed 2 metres above the ground at the Northern Nigerian cities of Bauchi (10.4°N, 9.8°E), Jos (9.9°N, 8.9°E) and Makurdi (7.7°N, 8.5°E). The masses of the aerosols collected were determined by weighing while the sizes were measured using optical microscope. The average mass concentration of aerosols was 1.6±0.4mgm^-3 at Jos while estimates for Bauchi and Makurdi were 1.7mgm^-3 and 1.4mgm^-3 respectively. These values are far above acceptable ambient air quality standards for particulate matter. The dust fall concentration in the three cities was modelled by a decreasing exponential function, which showed a decrease in concentration as the distance from the source (Sahara Desert) in the direction of the prevailing wind, increased. For the entire duration of the harmattan, estimated amounts of particulate matter deposited on one square metre of land per year are 3.1kg, 1.7kg and 1.2kg at Bauchi, Jos and Makurdi respectively. The sizes of harmattan haze particles in the cities had average diameters of 7.5±0.6mm with a range of 2 to 40mm at Bauchi, 3.9 ±0.3mm with a range of 0.5 - 20mm at Jos, and 2.8 ±0.5mm with a range of 0.5 - 10mm at Makurdi. The corresponding settling velocities and residence times showed decreased magnitudes as the particles were entrained southwards from the latitude of Bauchi to Makurdi. The size distribution of the particles could generally be described by the Junge-distribution curve having an average slope 2.1 ±0.4 confirming the continental origin of the particles.

Keywords: mass concentration, size distribution, settling velocity and residence time

Nigerian Journal of Physics Vol. 17, 2005: 92-98