Ground-based measurement of Aerosol Optical Thickness and identification of meteorological influencing factors over two tropical urban sites, Abidjan and Korhogo, Côte d’Ivoire

Résumé

Abstract

Continuous measurements of aerosol optical thickness (AOT) at two urban sites (Abidjan in south and Korhogo in north) are used to examine aerosols characteristics as part of the PASMU (Pollution de l’Air et Santé dans les Milieux Urbains de Côte d’Ivoire) project. The specific objective of this work is to provide an approach in determining the variability of the aerosol optical thickness and Ångström exponent (Alpha) over a period of 1 year (from October 2018 to December 2019) using sun photometer data. The maximum relative frequency of AOT is found between 0–0.5 (>60%) and 0.5–1 (20<AOT<40%).The maximum relative frequency of Alpha is found in the following ranges: 0–0.5, 0.5-1 and 1-1.5 (20<Alpha<40%).The findings indicate that the local meteorological pattern over the two urban cities, mainly driven by the regional monsoon, has a great effect on the concentration and size distribution of the aerosols. The monthly average AOT values are observed to be generally higher in dry season than in wet season with some high values during the onset of the monsoon season (March) and the rainy period (July) at Korhogo. In general, the Ångström exponent values are, higher when AOT values are low and lower when AOT values are high. Moreover, the results reveal significant month-to-month variability in both AOT (ranging from 0.22 (AOT540) to 0.86 (AOT465) and from 0.24 (AOT619) to 0.75 (AOT465) at Abidjan and Korhogo, respectively) and Alpha (ranging from 0.33 to 1.05 and from 0.55 to 1.51 at Abidjan and Korhogo, respectively).The later highlights the influence of varying aerosol types. A link was established between the back-trajectories and the aerosol optical thickness. An important conclusion of this approach has been to identify one of the main aerosol sources, influencing the atmospheric optical properties over Abidjan and Korhogo.