The design and construction of an improved optical counter and cell were made for the monitoring of particulates in situations involving higher concentration such as those in chemical vapor phase deposition (CVD) reactor, in polluted urban environments and in earlier stages of fires. The modification of Jaenicke' s equation was also made for the correction of coincidence loss. Particularly, the quick estimation of in-situ polynuclear aromatic hydrocarbons (PAH) concentration in urban air was achieved by monitoring particulates on the basis of the established linear relations between PAH concentration and the total number of smaller particulates in unit volume instead of the total volume of particulates.