Résumé
In the present investigation, dust fall rates over Cairo urban area and the industrial district of Helwan were examined. Chemical constituents of the collected samples were determined and related to their sources and other factors affecting their existence in the atmosphere of the urban and industrial districts. Results of this work clearly indicate that both urban and industrial districts of Cairo received rates of dust fall exceed the air quality standards set to protect man and his properties from air pollution. However, the rates of dust fall over the industrial area of Helwan icrribly exceed those found over the industrial urban districts. Furthermore, the highest concentrations of tarry matters, which are products of incomplete combustion processes, were found in dustfall samples collected from the atmosphere of Cairo city centre. A good correlation was found between NH[4+] and sulphates concentrations in dust fall over the urban districts, possibly due to the atmospheric reactions leading to the formation of ammonium sulphate in Cairo, urban atmosphere. On the other hand, sulphate concentrations were correlated with calcium concentration, in the atmosphere of Helwan industrial district. This is due to the emissions from cement industries where kilns dust contains high concentration of calcium sulphate. Results also indicate that the sources of chlorides in Cairo urban atmosphere are mainly combustion processes and natural causes. However, in the industrial district cement and other industries are significant sources of chlorides found in the atmosphere of Helwan industrial district. Moreover, these industries are major sources of calcium and free silica emitted to the atmosphere of Helwan. Several other conclusions and recommendations arc included