Aerosol characteristics over urban Cairo: Seasonal variations as retrieved from Sun photometer measurements

[ 1] During the Cairo Aerosol Characterization Experiment an automated Sun photometer belonging to the NASA Aerosol Robotic Network has been implemented for the first time in the megacity of Cairo, Egypt. The inversion of the measurements performed by this instrument several times a day and over a duration of more than 1 year ( from the end of October 2004 to the end of January 2006) provides a way of determining the temporal variability of aerosol characteristics such as size distribution, complex refractive index, single- scattering albedo, and asymmetry parameter. The analysis of the results reveals that Cairo's aerosol is a mixture of three individual components produced by different mechanisms: `` background pollution'' aerosol produced by local urban activities, `` pollution- like'' aerosol resulting from biomass burning in the Nile delta, and `` dust- like'' aerosol released by wind erosion in the Sahara. It is also shown that the variations in the overall aerosol properties are in fact due to changes in the proportions of this mixture. In particular, short- duration dust storms and biomass- burning episodes explain the largest observed aerosol optical depths ( AOD) ( AOD > 0.7) through the extreme enhancements of concentrations in dust- like aerosols characterized by low A ngstro " m's exponent values ( a < 0.5) and in `` biomass- burning'' aerosols ( 1.0 < a < 1.5). When averaged over longer ( monthly and yearly) time periods, the effects of these high- frequency modifications are smoothed. In particular, an average `` mixed aerosol'' type is defined for the whole duration of the measurements period. The low single- scattering albedo ( SSA) of this average aerosol and its marked spectral dependence clearly indicate that, at least on a yearly basis, the aerosol is dominated by its two light absorbing pollution components ( background pollution and pollution- like) and to such an extent that it compares well with values obtained in other polluted megacities ( e. g., Mexico City). This general dominance of the absorbing components can be challenged at shorter timescales. Indeed, the occurrence of several dust storms in springtime, and particularly in April, causes a significant increase in SSA and a parallel decrease in spectral dependence during this month. Conversely, the October biomass- burning events are not able to cause such important deviations from the yearly averaged mixed aerosol model that its optical properties can no longer be used for this month.