Retrieval of aerosol optical thickness and size distribution from the CIMEL Sun photometer over Inhaca Island, Mozambique

Abstract

Characterization of aerosol optical properties over southern Africa is needed to better understand the impact of aerosols on regional climate change. CIMEL Sun photometer measurements of aerosol optical thickness over Inhaca Island, Mozambique, between April and November 2000 are analyzed. Comparisons with two other sites, Mongu, Zambia, and Bethlehem, South Africa, are made. The aerosol optical thickness observed at Inhaca Island indicates high turbidity. In ∼50% of the measurements, aerosol optical thickness values are above 0.2, with an overall mean of 0.26 ± 0.19. The Angström exponent parameter has a wide range from 0.2 to 2, with a modal value of 1.6. This indicates a wide range in particle sizes and the dominance of fine mode aerosols at this site. Data from all three sites reveal seasonal variability, with a significant increase in aerosol content between August and October. This suggests a strong contribution of biomass burning to the atmospheric aerosols content during this time of year, which corresponds to the period of maximum burning in southern Africa. A north to south gradient in aerosol optical thickness is confirmed. The highest aerosol content is observed over Mongu, while Bethlehem has the lowest. The retrieved aerosol volume size distribution over Inhaca Island demonstrates that at high levels of aerosol optical thickness, accumulation mode aerosols dominate. In contrast, coarse mode aerosols dominate when aerosol optical thickness is very low. It is noted that there is a tendency for decreasing particle size as aerosol optical thickness increases, with the peak in distribution of the accumulation mode volume radius decreasing from 0.19 μm at τa = 0.42 to ∼0.14 μm at τa = 1.12.