Weekday/weekend differences in ambient aerosol level and chemical characteristics of water-soluble components in the city centre

Weekday and weekend ambient aerosol samples were collected from the city centre of Cairo, namely “Ramsis” during the summer season of the year 2006, and have been analyzed for water-soluble ionic species. The average concentrations of the total suspended particulate matter (TSP) and their water-soluble components were higher during weekdays than on weekends, indicating that the decreased traffic density on weekends leads to a decrease in the levels of the TSP and their water-soluble ionic species. The average concentrations of the TSP were 454 μg m−3 on weekdays and 298 μg m−3 on weekends. The weekday/weekend concentration ratios were 1.52 for TSP, 1.27 for SO42−, 1.64 for Cl−, 1.54 for NO3−, 1.17 for NH4+, 1.67 for Ca2+, 1.83 for Na+, 1.75 for K+ and 1.73 for Mg2+. City centre of Cairo has high levels of the TSP and their water-soluble ionic species compared with many polluted cities in the world. Among all of the measured water-soluble components, SO42− was the most abundant species followed by Ca2+ on weekdays and weekends. The average mass ratios of NO3−/SO42− in the TSP were 0.41 on weekdays and 0.34 on weekends, suggesting that the stationary source emissions were more predominant. The NH4+/SO42− molar ratios and its relation with the concentrations of TSP and Ca2+ during the weekdays and weekends indicate that the chemical form of sulfate and ammonium in aerosol particles varies with TSP and Ca2+ levels. At high TSP and Ca2+ levels, and NH4+/SO42− molar ratios less than one, SO42− in aerosol particles may be present as CaSO4 and (NH4)2SO4·CaSO4·2H2O, whereas it is expected to be present as (NH4)2SO4, (NH4)2SO4·CaSO4·2H2O and CaSO4 at low levels of TSP and Ca2+, and NH4+/SO42− molar ratios between 1 and 2. The mean pH values of the TSP were 7.65 on weekdays and 6.97 on weekends, indicating that aerosol particles brought a large amount of crustal species, and might alleviate the tendency of acidification. The relationships between the concentrations of acidic components (NO3− and SO42−) and basic components (NH4+, Ca2+ and Mg2+) on weekdays and weekends indicate that the acidity of aerosol particles is neutralized. Ca2+ and NH4+ are the most dominant neutralization substances in Cairo atmosphere.