Abstract
A decadal survey of atmospheric aerosols over Egypt and selected cities and regions is presented using daily aerosol optical depth (AOD) data from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) at 550 nm wavelength onboard the Aqua satellite. We explore the AOD spatio-temporal variations over Egypt during a 12-year record (2003 to 2014) using the MODIS high-resolution (10 km) Level 2 data product. Five cities and two geographic regions that feature different landscape and human activities were selected for detailed analysis. For most of the examined areas, AOD is found to be most frequent in the 0.2–0.3 range, and the highest mean AOD was found to be over Cairo, Alexandria, and the Nile Delta region. Severe events are identified based on AOD higher than a selected threshold. Most of these events are engendered by sand and dust storms that originate from the Western Desert during January–April. Spatial analysis indicates that they cover the Nile Delta region, including cities of Cairo and Alexandria, on the same day. Examination of the spatial gradient of AOD along the four cardinal directions originating from the city’s center reveals seasonally dependent gradients in some cases. The gradients have been linked to locations of industrial activity. No trend of AOD has been observed in the studied areas during the study period, though data from Cairo and Asyut reveal a slight linear increase of AOD. Considering Cairo is commonly perceived as a city of poor air quality, the results show that local events are fairly constrained. The study highlights spatial and seasonal distributions of AOD and links them to geographic and climatic conditions across the country.
Highlights
Atmospheric aerosols are liquid or solid particles suspended in the atmosphere [1]
Statistics of aerosol optical depth (AOD) over the entire country and each one of the selected sites are presented in terms of background AOD and severe air pollution episodes (SAPE) AOD
AOD represents the normal level triggered by local emissions or by aerosols transported from other regions
Summary
Atmospheric aerosols ( known in the meteorological field as particulate matter) are liquid or solid particles suspended in the atmosphere [1] They are emitted from the Earth’s surface both naturally (e.g., dust, sea salt, biogenic emissions, etc.) and as a result of human activities (products of combustion such as smoke and ashes). The primary parameters that determine the environmental and health impacts of the aerosols include their concentration, size distribution, chemical composition, and optical properties The latter includes aerosol optical depth (AOD), which is a metric for the total extinction of incoming solar radiation at the surface due to the presence of aerosols and clouds. Other NASA sensors are used to derive AOD, include the Multi-angle Imaging SpectroRadiometer (MISR) with its multispectral and multi-angle capabilities that are used to retrieve AOD over ocean and land [13].
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