Aerosol Distributions and Transport over Southern Morocco from Ground-Based and Satellite Observations (2004-2020)

The present study investigates the optical properties of aerosols on daily and seasonal scales with the use of the aerosol optical depth (AOD) and Angstrom exponent (AE) data retrieved from AErosol RObotic NETwork (AERONET) and collected at four stations in Southern Morocco-Saada (31.63 degrees N; 8.16 degrees W), Ouarzazate (30.93 degrees N; 6.91 degrees W), Oukaimeden (31.21 degrees N; 7.86 degrees W) and Ras-El-Ain (31.67 degrees N; 7.60 degrees W). An evaluation of the aerosol volumetric size distribution (AVSD) is also obtained for Saada and Ouarzazate. An AOD inter-comparison is performed between AERONET data and satellite sensors (MODerate resolution Imaging Spectroradiometer-MODIS), as well as assimilation products (Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) and Copernicus Atmosphere Monitoring Service (CAMS)), by the means of a linear regression. Regardless of site location and elevation, the results show the prevalence of the annual cycle of AOD, with a maximum in summer and a minimum in winter. In association with this seasonal variation, the variations in AE and AVSD showed an increase in coarse mode over Ouarzazate and Saada during summer (July to August), underlining that Southern Morocco is prone to the regular transport of desert dust on a seasonal basis. The inter-comparison reveals that the MERRA-2 dataset is slightly more appropriate for the study region, since it shows correlation coefficients (r) ranging from 0.758 to 0.844 and intercepts ranging from 0.021 to 0.070, depending on the study site. The statistical analysis of the back-trajectories simulated by the HYbrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model were consistent with the observations and confirmed the dominance of desert dust aerosols during the summer over the study region. On the other hand, the winter season reveals a predominance of anthropogenic and oceanic aerosols originating from the north and the west of the study site.

Automated summary

The present study investigates the optical properties of aerosols on daily and seasonal scales in Southern Morocco. The results show the prevalence of seasonal variations in aerosol optical depth (AOD), with a maximum in summer and a minimum in winter. The study also reveals the regular transport of desert dust aerosols in Southern Morocco during summer. The MERRA-2 dataset is found to be more appropriate for the study region, and both observations and model analysis confirm the dominance of desert dust aerosols in summer and anthropogenic and oceanic aerosols in winter.