Significant formation of water-insoluble secondary organic aerosols in semi-arid urban environment

[1] Diurnal variations of submicron elemental carbon ( EC), water- soluble organic carbon ( WSOC) and water- insoluble organic carbon ( WIOC) were investigated at an urban background site in Cairo ( Egypt) from 23 March to 14 April 2005. Well- marked diurnal patterns, with minima during the traffic- influenced morning period ( 6: 00 - 9: 00) and maxima during the intense photochemical period ( 12: 00 - 15: 00), were observed for the WSOC/ EC ratio and, more surprisingly, for the WIOC/ EC ratio. Such diurnal variations suggest significant formation of both water- soluble and waterinsoluble secondary organic aerosols during the afternoon. Applying the EC- tracer method, freshly- formed secondary organic carbon ( fresh SOC) was found to possibly account for more than 50% of OC concentrations measured during the 12: 00 - 15: 00 period, and this fresh SOC was calculated to be mainly ( similar to 60%) composed of water- insoluble species. The latter ( unexpected) result could be due to low ambient relative humidity as well as to the importance of anthropogenic volatile organic compounds in Cairo.