Aerosol properties and associated radiative effects over Cairo (Egypt) M. El-Metwally a, ⁎, S.C. Alfaro b , M.M. Abdel Wahab c , O. Favez d , Z. Mohamed a , B. Chatenet b a Physics Department, Faculty of Science, Port Said University, Port Said, Egypt b LISA-UMR CNRS 7583, Universités de Paris 12 et de Paris 7, Créteil, France c Astronomy and Meteorology Department, Faculty of Science, Cairo University, Giza, Egypt d LSCE/IPSL, Laboratoire CEA-CNRS-UVSQ, CEA Orme des Merisiers, 91191 Gif-sur-Yvette, France article info abstract Article history: Received 26 December 2009 Received in revised form 12 October 2010 Accepted 13 October 2010 Cairo is one of the largest megacities in the World and the particle load of its atmosphere is known to be particularly important. In this work we aim at assessing the temporal variability of the aerosol's characteristics and the magnitude of its impacts on the transfer of solar radiation. For this we use the level 2 quality assured products obtained by inversion of the instantaneous AERONET sunphotometer measurements performed in Cairo during the Cairo Aerosol CHaracterization Experiment (CACHE), which lasted from the end of October 2004 to the end of March 2006. The analysis of the temporal variation of the aerosol's optical depth (AOD) and spectral dependence suggests that the aerosol is generally a mixture of at least 3 main components differing in composition and size. This is confirmed by the detailed analysis of the monthly-averaged size distributions and associated optical properties (single scattering albedo and asymmetry parameter). The components of the aerosol are found to be 1) a highly absorbing background aerosol produced by daily activities (traffic, industry), 2) an additional, ‘pollution’ component produced by the burning of agricultural wastes in the Nile delta, and 3) a coarse desert dust component. In July, an enhancement of the accumulation mode is observed due to the atmospheric stability favoring its building up and possibly to secondary aerosols being produced by active photochemistry. More generally, the time variability of the aerosol's characteristics is due to the combined effects of meteorological factors and seasonal production processes. Because of the large values of the AOD achieved during the desert dust and biomass burning episodes, the instantaneous aerosol radiative forcing (RF) at both the top (TOA) and bottom (BOA) of the atmosphere is maximal during these events. For instance, during the desert dust storm of April 8, 2005 RF BOA , RF TOA , and the corresponding atmospheric heating rate peaked at -161.7 W/ m 2 , -65.8 W/m 2 , and 4.0 K/d, respectively. Outside these extreme events, the distributions of the radiative forcing values at BOA and TOA are Gaussian with means and standard deviations of -58 (±27), and -19(±11)W/m 2 , respectively. These two negative values indicate a cooling effect at the 2 atmospheric levels but the largest absolute value at BOA corresponds to a trapping of solar radiation inside the atmosphere. The averages of the instantaneous forcing efficiencies (FE) derived from measurements performed at solar zenith angles between 50 and 76° are -195 (±42) and -62(±17)W/m 2 .AOD 550 for BOA and TOA, respectively. The value at TOA is larger than in other urban environments, which could be due to the desert dust component backscattering more solar radiation to space than absorbing urban aerosols. The lower absorption of solar light by desert dust also explains qualitatively the lower than usual value of FE BOA . A more precise study of the effects of the desert dust and biomass burning aerosols shows that fluctuations of their monthly-averaged concentrations explain the departures of the TOA and BOA radiative forcings from the background situation. In April, the contributions of DD to the month averages of the instantaneous radiative forcing are as high as 53% at BOA, and 66% at TOA. In October, the biomass burning mode contributes 33 and 27% of these forcings, respectively. Noteworthy is that Keywords: Aerosol radiative forcing Forcing efficiency Aerosol optical depth Aerosol size distribution Heating rate CACHE AERONET Cairo Atmospheric Research 99 (2011) 263–276 ⁎ Corresponding author. E-mail address: melmetwally@yahoo.com (M. El-Metwally). 0169-8095/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2010.10.017 Contents lists available at ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmos