Ambient levels of volatile organic compounds in the atmosphere of Greater Cairo
Introduction
High rate of industrialization/urbanization process, especially in developing countries, leads to an increase in the emission of many types of air pollutants due to fossil fuel combustion. Volatile organic compounds (VOCs) are one of the most important groups of air pollutants in the urban atmosphere because they can cause significant risk to human health. Many VOCs have been reported to be toxic, carcinogenic or mutagenic (Duce et al, 1983; Edgerton et al., 1989; Sweet and Vermette, 1992; Kostiainen, 1995; Mukund et al., 1996). In addition, the presence of VOCs in the atmosphere play an important role in the formation of ground level ozone, photochemical oxidants and smog episodes (Monod et al., 2001) and they are harmful to the ecosystem (Derwent, 1995; Kuran and Sojak, 1996; Dewulf and Van Langenhove, 1997; Atkinson, 2000). In addition, ozone and photochemical oxidants lead to an increase in the formation and fate of airborne toxic chemicals and fine particles (Finlayson-Pitts and Pitts Jr., 1997).
The sources of VOCs are both anthropogenic and natural. The major anthropogenic sources in the urban environment are vehicle exhaust, gasoline evaporation, emissions from the use of solvents, and gas leakage from natural gas and liquefied petroleum gas (LPG) (Fujita et al., 1995; Jose et al., 1998; Kourtidis et al., 1999; Derwent et al., 2000; Srivastava et al., 2005). The levels of VOCs in the ambient air are related to the fuels used, vehicle types and ages, flow rates and speeds of traffic as well as road and environmental conditions in the city (Paul, 1997). The increased aromatics, olefins and other organic compounds in fuels used in vehicles lead to an increase in the emission of aromatic VOCs, especially from vehicles which are not supplied with catalytic converters (Singh et al., 1985; Perry and Gee, 1994). Volatile aromatic hydrocarbons represent a significant fraction of gasoline and other fuels as well as automobile exhausts (Yamamoto et al., 2000). Benzene, toluene and xylenes are the most abundant among the aromatic volatile hydrocarbons (Singh et al., 1985; Brocco et al., 1997).
Aromatic VOCs in the atmosphere have a high photochemical ozone creation potential. They take part in photochemical reactions and are major sources of radicals which can oxidize NO–NO2, the precursor of ozone (Finlayson-Pitts and Pitts Jr., 2000; Reis et al., 2000). Among the aromatic volatile hydrocarbons (benzene, toluene, ethylbenzene and xylenes), xylenes are considered the more reactive species with respect to ethylbenzene, whereas benzene has a lower reactivity and more stable in the atmosphere, due to its relatively longer lifetime (Calvert et al., 2000; Hsieh and Tsai, 2003). Xylenes ((m, p)-xylene plus o-xylene) are the most dominant contributor to ozone formation among BTEX (Na et al., 2005). The (m, p)-xylene/ethylbenzene concentration ratio is used to investigate the extent of photochemical reactivity in the atmosphere and to estimate the photochemical age of air mass (Nelson and Quigley, 1984; Monod et al., 2001; Hsieh and Tsai, 2003).
There is a lack of information on VOCs concentrations in Greater Cairo. Previous studies on air pollution in Greater Cairo had been focused on gaseous pollutants like SO2, NO2, NH3, O3 and total oxidants, and suspended particulate matter and its chemical composition (Khoder, 1997, Khoder, 2002; Hassan, 2000). These studies indicate that the photochemical smog occurs in Cairo atmosphere during the summer season, especially around noon. Therefore, it is very important to evaluate the concentration levels and variation of VOCs in Cairo atmosphere. This will help in setting a proper strategy to control the ground level ozone, photochemical oxidants and smog episodes build up in the future.
Therefore, the present study aims to measure the levels of ambient VOCs in three sampling locations, two in urban areas in Greater Cairo and background one in rural area in Menofiya province, and to study the diurnal variation of VOCs concentrations at the most polluted location (Ramsis).
It should be noted that the present study is a part of the plan designed by the Air Pollution Research Department, National Research Centre to investigate the ambient air quality in Greater Cairo.
Section snippets
Sampling sites and periods
Greater Cairo region lies to the south of Delta in the Nile basin. In the last century, urbanization and industrialization have been rapidly increased, causing an increase in the pollution of Cairo atmosphere. Greater Cairo region (Cairo, Giza & Shoubra El-Khiema cities) houses more than 11.30 million inhabitants. About 52% of the industries and about 40% of electrical power stations in Egypt are found in Greater Cairo, besides more than 1.30 million vehicles are running in the streets of the
VOCs concentrations in the urban and rural areas
The minimum, maximum, median and arithmetic mean daytime concentrations of VOCs measured in the urban (Ramsis and Haram) and rural (Kafr El-Akram) areas are summarized in Table 1. From this table, it can be observed that, the maximum concentrations of VOCs were found in the urban areas, whereas the minimum levels were detected in the rural area, and the differences in mean concentrations were statistically significant (p<0.001). This is due to the higher emission of VOCs from traffic emissions
Conclusions and recommendations
Ambient VOCs play a critical role in the atmospheric chemistry. These pollutants lead to an increase of the formation of ozone, photochemical oxidants and smog episodes which are harmful to human health, ecosystem and atmosphere.
The levels of VOCs were determined at three locations, two in the urban areas in Greater Cairo (Ramsis and Haram) and background one in the rural area in Menofiya province (Kafr El-Akram). The mean concentrations of n-hexane, n-heptane, benzene, toluene, ethylbenzene, (m
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