Abstract
Abstract. Since 1999 Cairo and the Nile delta region have suffered from air pollution episodes called the "black cloud" during the fall season. These have been attributed to either burning of agriculture waste or long-range transport of desert dust. Here we present a detailed analysis of the optical and microphysical aerosol properties, based on satellite data. Monthly mean values of Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) at 550 nm were examined for the 10 yr period from 2000–2009. Significant monthly variability is observed in the AOD with maxima in April or May (~0.5) and October (~0.45), and a minimum in December and January (~0.2). Monthly mean values of UV Aerosol Index (UVAI) retrieved by the Ozone Monitoring Instrument (OMI) for 4 yr (2005–2008) exhibit the same AOD pattern. The carbonaceous aerosols during the black cloud periods are confined to the planetary boundary layer (PBL), while dust aerosols exist over a wider range of altitudes, as shown by Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) aerosol profiles. The monthly climatology of Multi-angle Imaging SpectroRadiometer (MISR) data show that the aerosols during the black cloud periods are spherical with a higher percentage of small and medium size particles, whereas the spring aerosols are mostly large non-spherical particles. All of the results show that the air quality in Cairo and the Nile delta region is subject to a complex mixture of air pollution types, especially in the fall season, when biomass burning contributes to a background of urban pollution and desert dust.
Highlights
The characterization of aerosol optical and microphysical properties is crucial to the understanding of their effect on the Earth-atmosphere radiation budget and its climate (e.g. Haywood et al, 1999; Horvath et al, 2002; Meloni et al, 2005; Moorthy et al, 2009)
The Moderate Resolution Imaging Spectroradiometer (MODIS) record length is different for Terra and Aqua: the monthly mean values from the Terra satellite are shown for the period February 2000–December 2009, and the Aqua values are from January 2003–December 2009
To derive aerosol optical depth (AOD) climatology the MODIS Collection 5 monthly AOD550 product derived for 10 yr from Terra data and for 7 yr from Aqua data has been employed
Summary
The characterization of aerosol optical and microphysical properties is crucial to the understanding of their effect on the Earth-atmosphere radiation budget and its climate (e.g. Haywood et al, 1999; Horvath et al, 2002; Meloni et al, 2005; Moorthy et al, 2009). Favez et al (2008) analyzed the chemical composition during fall (i.e. the time of the black cloud) of bulk aerosols over 1.5–3 yr at two urban sites in Cairo These analyses indicated that emissions of rice straw burning account for 50 % of the water soluble organic carbon (WSOC). Prasad et al (2010) suggest a different reason for the fall pollution (black cloud season) They showed that long range transport of dust at high altitude (2.5–6 km) from the Western Sahara and its deposition over the Nile Delta region is the major contributor to air pollution episodes at this season. All satellite data used for this study were obtained from the Atmospheric Data Center of the NASA Langley Research Center (LARC) (http://eosweb. larc.nasa.gov/)
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