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

Abstract

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 SO 4 2−, 1.64 for Cl −, 1.54 for NO 3 −, 1.17 for NH 4 +, 1.67 for Ca 2+, 1.83 for Na +, 1.75 for K + and 1.73 for Mg 2+. 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, SO 4 2− was the most abundant species followed by Ca 2+ on weekdays and weekends. The average mass ratios of NO 3 −/SO 4 2− in the TSP were 0.41 on weekdays and 0.34 on weekends, suggesting that the stationary source emissions were more predominant. The NH 4 +/SO 4 2− molar ratios and its relation with the concentrations of TSP and Ca 2+ during the weekdays and weekends indicate that the chemical form of sulfate and ammonium in aerosol particles varies with TSP and Ca 2+ levels. At high TSP and Ca 2+ levels, and NH 4 +/SO 4 2− molar ratios less than one, SO 4 2− in aerosol particles may be present as CaSO 4 and (NH 4) 2SO 4·CaSO 4·2H 2O, whereas it is expected to be present as (NH 4) 2SO 4, (NH 4) 2SO 4·CaSO 4·2H 2O and CaSO 4 at low levels of TSP and Ca 2+, and NH 4 +/SO 4 2− 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 (NO 3 − and SO 4 2−) and basic components (NH 4 +, Ca 2+ and Mg 2+) on weekdays and weekends indicate that the acidity of aerosol particles is neutralized. Ca 2+ and NH 4 + are the most dominant neutralization substances in Cairo atmosphere.