Transport and dispersion patterns of aerosols over the east African region

Abstract

This study used aerosol optical depth (AOD) from satellite based Moderate Resolution Imaging Spectroradiometer (MODIS) observations at 550nm in 1° X 1° grid resolution to investigate the transport and dispersion patterns of aerosols over the East African region. The specific objectives of the study was to perform spatial analysis using monthly means of MODIS Terra AOD data, evaluate monthly means for a five year MODIS Terra AOD data, examine diurnal patterns between MODIS Terra and MODIS Aqua AOD data, and simulate transport and dispersion of the AOD during the North Easterly Trade winds and the South Easterly Trade winds to identify the possible sources and sinks of the aerosols. The region of study lies within 28.5°E-42.5°E, 4.5°N-12.5°S. Several analyses were performed. To begin with, time series analysis was performed on the data which revealed the seasonality and a bimodal annual pattern of the MODIS Terra AOD data. The Hovrnoller analysis revealed spatial and temporal patterns of the aerosols hence clearly showing areas of higher AOD values in space and time; an indication of high aerosol concentrations. Backward air trajectory analysis was performed to identify the possible sources of aerosols for a period of forty eight (48 hrs) hours of air transport over three selected locations namely: 2°S, 300E; 2°S, 400E and 4°N, 36°E. These three locations had indicated higher AOD values by performing spatial and Hovmoller analysis. High AOD values symbolize high aerosol concentrations. Through backward air trajectory analysis, the possible sources of aerosols in the Northern hemisphere were found to be the Middle East, Sahara and Arabian deserts during the month of February, whereas in the Southern hemisphere, the possible sources were found to be the Congo rain forest, Kalahari and Namibian deserts, Southern Atlantic Ocean, South west Indian Ocean, Madagascar Island and South African region during the month of July. Also, computed was the forward air trajectory analysis over the same locations. These were necessary in identifying whether there were possible transport and dispersion of aerosols within the region of study. Through the forward air trajectory analysis, it was discovered that aerosols are greatly dispersed over the Eastern African region. This dispersion is depended on the prevailing wind flow patterns. The findings of this study established that long distance transport of aerosols and their dispersion through low level winds is responsible for the aerosols affecting the East African region. The transport of aerosols and their dispersion patterns greatly depend on the season of the year together with the prevailing atmospheric conditions