A heavy dust event in the sub-Sahel during January
2000 was documented from observations made at the University of Ilorin, Ilorin
(08° 19’ N, 04° 20’ E), Nigeria, in cooperation with the Aerosol Robotic
Network (AERONET) (Holben et al., 1998; Pinker et al., 2001).
Analysis of the observations in seven wavelengths revealed that during
the dust outbreak event, the optical properties of the dust aerosols were
much different from what is assumed in aerosol climatalogies in desert
areas, or from observations preceding the dust event. Aerosol optical
depths at all seven wavelengths showed a sharp increase when compared to
the average for the season, reaching values up to 3.5 at 500 nm (Figure
The daily mean size distributions observed during the dust outbreak
of January 29–February 1, when large amounts of coarse and fine particles
were transported to the site, are presented in Figure 2 showing an order of magnitude
increase in volume size distribution can be seen.The Angstrom exponent was
reduced from 1.2 to 0.3 as seen in Figure 3.
The volume size distribution was retrieved
from the direct solar and diffuse sky radiance measurements as discussed
in Dubovik and King (2000). Temporal variation of single scattering
albedo as observed on a typical day when biomass burning was reported in
areas surrounding the experimental site is illustrated in (Figure 4).
During the morning hours, before the presence of aerosols from
biomass burning, wo increases with increasing wavelength. When biomass
burning started (reported at 1 p.m. by local observers), the single scattering
albedo values dropped and the spectral dependence reversed.
Using a two-year record of continuous ground-based measurements
at the Ilorin site, desert aerosol models as presented in the literature
are augmented, to better characterize the prolonged dust outbreak season
in West Africa, which overlaps with the biomass-burning season.
In Table 1, observed average values
of aerosol optical depth are compared with those from other sources.
In Table 2, observed average values
of single scattering albedos during the Harmattan and non-Harmattan seasons
are compared with those from other sources
Comparisons were done with aerosol optical depths as derived from
satellite observations, such as the TOMS instrument. To compare
with the TOMS aerosol index, which is derived mainly from the 340 and
380 nm reflectances, CIMEL observed aerosol optical depths at 340 nm were
grouped into daily means for two years (from April 1998–March 2000).
TOMS aerosol index over Ilorin, Nigeria was computed by taking weighted
means from four neighboring grids of 1-degree spatial resolution.
A comparison of daily mean AOD at 340 nm and TOMS aerosol index is shown
in Figure 5, illustrating good agreement
in the detection of the dust outbreak. Precipitable water vapor
that was retrieved using direct sun measurements at 940 nm (Bruegge et
al., 1992), exhibits significant seasonal variation and agrees well with
the NCEP reanalysis values (Figure 6).
Seasonal means of AODs are negatively correlated with precipitable water
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