Diurnal, Seasonal And Weekdays-weekends Variations Of Ground Level Ozone Concentrations In An Urban Area In Greater Cairo

摘要

Ground level ozone (O_3) concentration was monitored during the period of December 2004 to November 2005 in an urban area in Greater Cairo (Haram, Giza). During the winter and summer seasons, nitrogen dioxide (NO_2) and nitric oxide (NO) concentrations and meteorological parameters were also measured. The mean values of O_3 were 43.89, 65.30, 91.30 and 58.10 ppb in daytime and 29.69, 47.80, 64.00 and 42.70 ppb in whole day (daily) during the winter, spring, summer and autumn seasons, respectively. The diurnal cycles of O_3 concentrations during the four seasons revealed a uni-modal peak in the mid-day time, with highest O_3 levels in summer due to the local photochemical production. The diurnal variations in NO and NO_2 concentrations during the winter and summer showed two daily peaks linked to traffic density. The highest levels of NOx were found in winter. Nearly, 75%, 100%, 34.78% and 52.63% of the mean daytime concentrations of O_3 during spring, summer, autumn and the whole year, respectively, exceeded the Egyptian and European Union air quality standards (60 ppb) for daytime (8-h) O_3 concentration. About, 41.14% and 10.39% of the daytime hours concentrations and 14.93% and 3.77% of the daily hour concentrations in summer and the whole year, respectively, exceeded the Egyptian standard (100 ppb) for maximum hourly O_3 concentration, and photochemical smog is formed in the study area (Haram) during a periods represented by the same percentages. This was based on the fact that photochemical smog usually occurs when O_3 concentration exceeds 100 ppb. The concentrations of O_3 precursors (NO and NO_2) in weekends were lower than those found in weekdays, whereas the O_3 levels during the weekends were high compared with weekdays. This finding phenomenon is known as the "weekend effect". Significant positive correlation coefficients were found between O_3 and temperature in both seasons and between O_3 and relative humidity in summer season, indicating that high temperature and high relative humidity besides the intense solar radiation (in summer) are responsible for the formation of high O_3 concentrations.