PHOTOCHEMICAL FORMATION OF OZONE IN THE ATMOSPHERE OF GREATER CAIRO

Abstract

The present study deals with the concentration of surface ozone at three contrasting sites, selected to represent various activities characterizing the different sudied areas. The first was Rarnsis street which represented the city centre of Cairo, the second was Coke Factory representing a heavy industrial area and Shebin El-Kom (Menoufeia) as a third site in a rural area (background area). Surface ozone concentrations were continuously monitored at 12 meters above the ground level, using Dasibi ozone monitor. The results can be summarized as follows :

Background of ozone; principally of stratospheric origin; is present in lower troposphere. Naturally occuring tropospheric ozone exhibits a seasonal and diurnal variation in concentration. Mean surface ozone concentration at rural area, which representing the background ozone concentration, was 16.5 ppb in Winter and was 21 ppb in Spring seasons. An unexpected jump in the value of mean ozone concentration during Summer (56 ppb) may be due to the photochemical reactions of the organic pesticides sprayed for crops in that area during Summer season which were dispersed in the atmosphere . - Diurnal variation of mean hourly ozone concentrations at the city centre area was similar to that observed at industrial area, with maximum concentration during late morning and early afternoon hours which are the hours of the day characterized by relative warm and high solar radiation intensity while minimum concentrations occured during late night and early morning hours.