Extreme variability of aerosol optical properties: The Cairo Aerosol CHaracterization Experiment case study

Because they scatter and absorb solar and terrestrial radiation, aerosol plumes can easily be detected on satellite images. Thus, the time-dependent spatial extension of the aerosol clouds can be derived from space-borne observations. However, using remote observations for estimating particle concentrations, let alone for apportioning aerosol loads between all the potential sources of particulate matter, is less straightforward. Indeed, this apportionment would require perfect knowledge of the scattering and absorbing potential of particles of different origins as well as the spectral dependence of these potentials. Contrary to what can be done with most atmospheric gases, it is usually impossible to reproduce in the laboratory the complexity of natural atmospheric aerosols. As a consequence, measuring their wavelength-dependent optical properties can only be done during specially designed experiments performed in natural conditions. This is the case of the Cairo Aerosol CHaracterization Experiment (CACHE) that was performed in the Egyptian capital from the end of October 2004 to mid April 2005. During this period a wide variety of aerosol conditions have been sampled, but this work is focused on the spring intensive observation period during which several occurrences of mineral dust transport to Cairo were observed. We detail the modifications of optical properties resulting from these inputs of mineral particles into the background 'urban aerosol' and show that scattering and absorption, as well as their spectral dependence are extremely sensitive to the proportions of the "urban pollution/mineral dust" mixtures that form over Cairo during the dust events. Unfortunately, this precludes the use of predefined aerosol models supposed to represent particularly simple aerosol types (e.g., urban pollution, mineral dust,...) for inverting satellite observations over areas where aerosol mixing is known to be the rule rather than the exception (e.g., over the eastern Mediterranean in spring, over or downwind of continental China during the dust season, over west Africa during the biomass burning period,...). In these cases, sophisticated parameterizations of the optical properties must be used for assessing the impact of aerosol mixtures on radiative transfer.