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

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 water‐insoluble 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 (∼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.