El-Metwally, M, Alfaro, SC, Wahab, MMA, Zakey, AS, Chatenet, B (2010). Seasonal and inter-annual variability of the aerosol content in Cairo (Egypt) as deduced from the comparison of MODIS aerosol retrievals with direct AERONET measurements. ATMOSPHERIC RESEARCH, 97(2-Jan), 14-25.
As this is the case in many megacities of the developing countries, the atmospheric aerosol load is usually particularly large over the Cairo (Egypt) conurbation. However, being the result of a combination of meteorological factors and of the activity of various particle sources, some of which are seasonal, this load is variable in time. The objective of this study is to document this variability at the intra- and inter-annual scales. For this we use the qualitative Aerosol Absorption Index (AAI) derived from Aura-OMI (the ultimate version of the Total Ozone Mapping Spectrometer, TOMS) and the Aerosol Optical Depth (ACID) derived from the radiance measurements performed between 2000 and 2008 by the Moderate Resolution Imaging Spectroradiometers (MODIS) implemented aboard either the Terra or the Aqua satellites. In the sense that ADD maxima are always obtained in April and in October at the peaks of the desert dust and biomass burning periods, respectively, the results yielded by these two methods are in good qualitative agreement with those of direct sunphotometer observations performed in Cairo for more than one year (from end of October 2004 to the end of March 2006). However, a quantitative comparison of the MODIS and AERONET products for their common period of measurements reveals that MODIS tends to overestimate systematically the ADD and underestimate the aerosol's Angstrom exponent. We propose an empirical method for correcting the ADD retrieved by MODIS at 550 nm and match it with the sunphotometer values. When applied to the whole MODIS dataset, the effect of this correction is to smooth the inter-annual differences. As a result, the month-to-month variations of the ADD can be described by the same pattern independently of the year in the period of study (from 2000 to 2008). The monthly averaged ADD obtained by this method is minimal (0.24 +/- 0.04 at 550 rim) from December to February because of the washing out of airborne particles by rain events more frequent in winter. Conversely, the ADD increases in summer because particle accumulation is favored in this season by the absence of precipitations and by atmospheric stability. However, ADD maxima are obtained in April (0.38 +/- 0.02) and in October (0.36 +/- 0.03) when particles produced outside of the city by natural processes (wind-erosion of desert surfaces) or by human activities (burning of agricultural wastes in the Nile Delta) are transported by prevailing winds and contribute significantly to the enhancement of the city's aerosol burden. (C) 2010 Elsevier B.V. All rights reserved.