Origin of black carbon concentration peaks in Cairo (Egypt)

The concentration in black carbon (BC) has been monitored in the megacity of Cairo (Egypt) during the autumn 2004 and spring 2005 intensive observation periods of the Cairo Aerosol CHaracterization Experiment (CACHE). As expected for a species released by human activities, hourly mean of this concentration is found to be large at all times. It is also significantly larger in autumn than in spring (9.9 ± 6.6 and 6.9 ± 4.8 µgC/m3, respectively) and quite variable at shorter (diurnal) time scales. Indeed, sharp concentration peaks larger than 25 µgC/m3 are frequently detected during both observation periods. In order to apportion the roles played by emission intensity and meteorological conditions in the development of these peaks, a simple model is developed that allows derivation of the hourly mean BC emissions by the part of town located upwind of the measurement site. The analysis of the time dependence of these emissions indicates that traffic is by far the major source of BC in Cairo during daytime and this even in autumn when biomass burning takes place in the Nile delta. It is only between 03:00 and 05:00 in the night, at a time when traffic emissions are quite reduced, that the influence of this particular source on BC concentration can become significant. This study also indicates that BC emissions by motorized traffic remain important from the morning rush hour until late in the night. During the day, and particularly in spring, the dilution effect resulting from the development of the planetary boundary layer prevents BC concentrations from becoming very large. This is no longer the case just before sunrise and after sunset, when the combination of dense traffic and low boundary layer is responsible for the observed sharp increase in BC concentration.