Discrimination of aerosol types over nairobi, Skukuza and ilorin using AOD-AE clusters

Abstract EN

Climatology of aerosols, their trends and classification based on the long-term Moderate Resolution Imaging Spectroradiometer (MODIS) measurements (from February 2000 to July 2015) of aerosol optical depths at 550 nm (τ550) and Angstrom exponent (α470-660) using the wavelengths of 470 and 660nm in Nairobi, Skukuza and Ilorin AERONET stations were analyzed in this work.

The level-2 collection-6 Deep Blue (L2 C006 DB) of the parameters listed above from the aqua- (MYD04) and terra- (MOD04) MODIS of the study area were statistically analyzed using SPSS.

To be able to understand the temporal variation in the characteristics of aerosols in the three stations and during each season separately, MODIS measurements of τ, retrieved for the study area, were compared with AERONET τ.

Overall, aqua-MODIS τ corroborate the AERONET measurements well in Nairobi and Ilorin stations with underestimation of 29.80 % and overestimation of 2.90 % respectively, whereas Skukuza station has terra-MODIS τ as the best representation of the AERONET measurements with underestimation of 1.90 %.

On seasonal bases, MODIS τ agree well with AERONET measurements during rainy season in Nairobi and Skukuza, with underestimation ranging between 17.20 % and 29.70 % and Ilorin station had overestimation of 7.20 % during dry seasons.

Coupled with the fact that the coefficient of determination (R2) > 0.50, the results suggest that both aqua- and terra-MODIS give a good estimate of τ.

τ records presented a unique peak in Nairobi (June), Skukuza (October) and Ilorin (February), while the α monthly mean variability indicated a more complicated behavior with minimum values in Nairobi (June), Skukuza (June) and Ilorin (August).

The maximum τ values occurred during dry seasons, while their minimum values occurred during rainy seasons or transition months.

The α value showed an opposite behavior to that of τ such that the highest α values occurred during rainy seasons (or transition months) and the lowest values were observed during dry seasons.

This behavior may be attributed to the domination of single-mode aerosol particles in each station.

Lastly, τ and α values have been employed to identify several aerosol types seasonally and overall.