Seasonality of major aerosol species and their transformations in Cairo megacity

Bulk aerosols sampled oil a weekly basis at two Cairo (Egypt) urban sites from January 2003 to May 2006 were analysed for their chemical composition of major aerosol species (elemental carbon, water soluble/insoluble organic carbon, nitrate, Sulphate, ammonium, Chloride, sodium and calcium). Data subsequently obtained constitute one of the longest and more detailed dataset related to Cairo aerosols, and offer the opportunity to investigate seasonal trends. Dust aerosols (derived from calcium measurements) displayed maximum concentrations in spring and winter, due to frequent dust storms, but also high background concentration levels (similar to 50 mu g m(-3)), all year long. Within these particles, about 40% oil average of Ca(2+) was found to be associated SO(4)(-2), NO(3)(-) and/or Cl(-), pointing out "dust anthropization" processes and their subsequent climatic impact oil a regional scale. Seasonal variations of non-dust aerosols, equally distributed between carbonaceous aerosols and ions, were also observed, with concentrations of the order of 100 mu g m(-3) in autumn and winter, and of 60 mu g m(-3) in spring and summer. High concentration levels of non-sea-salt chloride (up to 15 mu g m(-3) on a monthly basis), likely of industrial origin, were observed in autumn and winter. During the autumn "Black Cloud" event, biomass burning aerosols originating front rice straw burning in the Nile Delta have shown to account for 12%, 35%, and 50% of Cairo EC, WIOC and WSOC mass concentrations, respectively. Finally, relatively low WSOC/OC ratios (similar to 1/3) were obtained all the year long, calling for more investigation oil the water-solubility of organic aerosols originating from the burning of agricultural waste, and oil that of secondary organic aerosols formed in dry urban atmospheres. (C) 2007 Elsevier Ltd. All rights reserved.