Measurement of BTEX (benzene, toluene, ethybenzene, and xylene) levels at urban and semirural areas of Algiers City using passive air samplers

Abstract

The study presents the levels of air pollution by aromatic organic compounds BTEX (benzene, toluene, ethylbenzene, o-, m-, and p-xylenes) in the city of Algiers. The sampling was carried out using Radiello passive sampler. Three sampling campaigns were carried out in roadside, tunnel, urban background, and semirural sites in Algiers. In order to determine the diurnal mean levels of air pollution by BTEX to which people are exposed, a modified passive sampler was used for the first time. In addition, monitoring of pollution inside vehicles was also made. In the spring of 2009, more than 27 samplings were carried out. In the background and road traffic sites the Radiello sampler was exposed for 7 days, whereas the time exposure was reduced to 1 day in the case of the vehicle as well as the tunnel. The results indicate that average benzene concentrations in the roadside and inside vehicle exceed largely the limit value of 5 μg m⁻³ established by the European Community (EC). On the other hand, it has been noticed that the concentration levels of other BTEX are relatively high. Also, in order to identify the origin of emission sources, ratios and correlations between the BTEX species have been highlighted. This study shows that road traffic remains the main source of many local emission in Algiers.

Implications

The vehicle fleet in Algeria is growing rapidly since the 1990s following economic growth and is responsible for the increasing air pollution in large cities. Because there are no data collection of BTEX carried out by national air quality network, all environmental and transportation policies are based on European emissions standards, but national emission standards are currently not in place. This work will contribute to the analysis of real emissions of BTEX in Algiers, for the development of management and for assessment of population exposure variation depending on the location in the city of Algiers.

Introduction

Algiers is considered to be a relatively big city, with a population higher than 3.5 million and a large car fleet of approximately 1 million vehicles (2009), which represents nearly 25% of the total national car fleet. According to the last information given by the National Office of Statistics (NOS, 2009), Algeria, the land travel passengers exceeded 88% in 2007.

The car use ratio in Algeria is about 35 vehicles per 100 people. This is considered to be one of the highest rate in developing countries. In addition, this automobile fleet is fairly old and has a weak maintenance. Indeed, the average age of a car is around 11 years. Vehicles gas exhaust is considered to be the main source of air pollution in urban sites in Algiers, as reported by previous works (Kerbachi et al., 2006, 2010; Moussaoui et al., 2010).

Algeria has adopted the same program as the other Mediterranean countries to reduce the lead content in gasoline (0.15 g L⁻¹). According to refineries' officials, the gasoline aromatic content varies between 35% and 40%, where benzene represents 2.5–3.0% of the total aromatics (Kerbachi et al., 2006). The substitution of lead by benzene and other aromatic compounds (European Council [EC] Directive 85/210) in gasoline has been the object of several studies, which clearly proved the increase of aromatic emissions in the atmosphere (Brown et al., 1996; Gee and Sollars, 1998). In order to limit benzene emissions in the atmosphere, the EC Directive has fixed the maximum benzene content in petrol at 1% (Directive 2003/17/EC).

The monoaromatic hydrocarbons benzene, toluene, ethybenzene, and xylenes, also known as BTEX, are the sources of a variety of adverse health effects such as asthma, dizziness, fatigue, and eye, nose, and throat irritation. Moreover, nausea and similar nonspecific symptoms have been also associated with BTEX (U.S. Environmental Protection Agency [EPA], 1987, 1991).

Among BTEX compounds, xylenes are highly reactive and contribute to ozone formation and hence to climate change (Finlayson-Pitts and Pitts, 1993). BTEX have been extensively measured worldwide in the last decade. Two major sources can be attributed to BTEX emissions in the air. The former is a stationery source such as fuel or gas stations, garages of vehicular maintenance, and many small factories and refineries, and the latter is closely related to the road traffic (Edgerton et al., 1989).

The collection of BTEX from air can be obtained by active or passive sampling techniques. In addition to the low cost of the passive sampling, this technique provides access not only to the time-weighted average but also to the air level concentration at large scale (Van Aalst et al., 1998; Wright et al., 1998; Bertoni and Allegrini, 2000; Krupa and Legge, 2000; Angiuli et al., 2003; Wideqvist et al., 2003; De Santis et al., 2004).

However, the use of passive samplers presents some limitations; among which one can name the need of determining the virtual aspiration rate of the device itself. In fact, it depends on the nature of the compounds and the exposure time, according to the consistency of the tests.

A diffusive sampler such as Radiello can determine the average content over a time range of 8–14 days, at different concentration levels. However, in the case of traces or very low concentration levels, this technique is far from reaching the performance of an active sampling (Fondazione Salvatore Maugeri, Italy, 2008).

The results of three monitoring campaigns, carried out in the urban and semirural areas of Algiers, using Radiello diffusive samplers, are reported in the present study. These campaigns have allowed us to evaluate the gas emissions generated by car traffic in different areas of the city, and to focus on the most critical areas of the urban territory. So, it was possible for the first time to evaluate accurately the diurnal variation of the concentration of BTEX using modified Radiello diffusive samplers. In addition, measurements of air pollution by BTEX inside tunnel and inside vehicles have been also carried out.

Methodology

Reagents

The chemical compounds, namely, benzene, toluene, ethylbenzene, m-xylene, p-xylene, and o-xylene, were purchased from Supelco (Bellefonte, PA, USA). 1-Chlorooctane, supplied by Supelco, was adopted as internal standard compound. The passive samplers (Radiello, purchased from Supelco) were just unsealed from the twin-layer aluminum envelops and used as received. Carbon disulfide (CS₂) was purchased from Fluka (reference 84713; low in benzene, ≥99.5%).

Sampling sites and periods

The collection of BTEX was carried out at seven different sampling sites. Five urban sites, one semirural site, and one tunnel located in the suburb of Algiers were investigated during the spring of 2009. The first sampling site (S1) is located at the roadside of a busy canyon street, Didouche Mourad Street (Algiers center), where the road traffic is very heavy and frequent. The second sampling site (S2) is located at roadside of Basta Ali Avenue (Algiers), with moderate and frequent road traffic. The third sampling site (S3) is located at the crossroad of Hassan Badi Street (El-Harrach, Algiers). Beside its very high and frequent vehicle congestion, this site is located close to a gas station.

The fourth (S4) and the fifth sites (S5) are located inside the department of Bachdjerrah and on the campus of the National Polytechnic School (El-Harrach), respectively. Both S4 and S5 sites are considered to be the urban background sites in Algiers.

The sixth sampling site (S6) is situated 25 km southeast of Algiers next to a city called Cherarba. According to environmental features, it is considered to be a semirural area and very appropriate for detecting the influence of vehicle exhaust emissions.

The seventh site (S7) is located inside the Oued Ouchayeh (1.5 km length) tunnel, 12 km far from the city center. Figure 1 shows the detailed positions of all sampling sites.

Figure 1. Locations of sampling sites locations in Algiers City. S1, S2, S3, and S7 represent roadside sites, S4 and S5 urban sites, and S6 a semirural site.

In addition to these seven sampling sites, the indoor air of a given car (Peugeot model 206, year of release 2000) was investigated three times 8 hr a day. The sampler was hung on the front mirror of the car while the vehicle was used every day in heavy urban traffic and operating in different modes in real driving conditions.

Three sampling campaigns were carried out at S1, S2, S3, S4, S5, and S6 during the spring of 2009. The first campaign was carried out between 15 and 21 March 2009, the second between 6 and 13 April 2009, and the third between 11 and 15 May 2009. The air tunnel sampling was investigated for 24 hr during March, April, and May 2009.

Sampling protocols

Radiello as passive samplers were used to collect air in all the investigated sites. The sampling system is made of a cylindrical adsorbing cartridge that is housed coaxially inside a cylindrical diffusive body of polycarbonate and microporous polyethylene. The cartridge is a stainless steel net cylinder mesh 3 × 8 μm, 4.8 mm external diameter, and it holds 530 ± 30 mg of 35–50 mesh activated carbon (Figure 2a). A mountable polypropylene shelter protects the sampler from bad weather and direct sunlight. In all sampling sites, the samplers were placed 3 m above ground level; except in the tunnel where the sampler was placed 1.5 m above the ground level. The temperature recordings, during sampling operations, were obtained from the National Office of Meteorology of Algiers (NOM, 2009).

Figure 2. (a) Diffusive sampling principle. (b) Modified diffusive sampling.

Moreover, to assess the highest diurnal levels of air pollution to which people are daily exposed, the S3 sampling site was selected for diurnal sampling using a modified passive sampler that was sealed only during night hours. The modification was done by installing a cylindrical polypropylene insulating cover 1.9 mm thick and 75 mm long. The cylinder was placed coaxially with the diffusive body, using a screw-threaded cap of 21 mm diameter, fixed to the supporting plate (Figure 2b). This technique allows the measurement of BTEX only during daytime from 7:00 a.m. to 8:00 p.m. To evaluate the differences between the diurnal and daily BTEX means, a second nonmodified sampler was placed concurrently with the modified sampler and to check the seal, another sampler was placed throughout the sampling period.

Analysis

The cartridge charcoal used during sampling was transferred into a vial and first fortified with 100 µL of 1-chlorooctane solution (internal standard with concentration of 10 µg mL⁻¹), then 2 mL carbon disulfide was added, and the vial was immediately sealed with a septum cap. The sample was slightly shaken for 30 min at room temperature. Five microliters of the extract were analyzed per run by means of gas chromatography with flame ionization detection (GC-FID) (Shimadzu GC-17A; Noisiel, France). The chromatographic column was Bentone 34/DNDP SCOT type (0.5 mm × 15 m; Supelco; Bellefonte, PA) and nitrogen was used as carrier gas. The injector and detector temperatures were held at 250 °C. The column was kept at constant temperature of 90 °C for 45 min. The compounds identification was carried out by comparing the features of peaks of the eluted sample with those of authentic analyte standards and GC retention times. Each sample was injected twice and the mean values were reported. Blank tubes were analyzed in the same way as the samples and all results were corrected. The detection limit (DL) of the method for each BTEX was determined as equal to 3 times the standard deviation of the signal obtained from duplicate measurements of blanks, divided by the slope of the calibration curve. DLs as low as 0.5 μg m⁻³ were obtained for the BTEX series.

The daily mean concentration C (µg m⁻³) of a specific BTEX during the exposure time t is calculated by the following equation:

1

where m (μg) is the amount of analytes adsorbed on the cartridge, Q (mL min⁻¹) is the uptake rate at 298 K, and t (min) is the sampling period. The Q values are given by the Radiello manual; at the normal conditions as defined by EC directives at T = 293 K and P = 101.3 kPa, the Q values are 80 mL min⁻¹ for benzene; 74 mL min⁻¹ for toluene; 68 mL min⁻¹ for ethylbenzene; 70 mL min⁻¹ for (m+p)-xylenes; and 68 mL min⁻¹ for o-xylene.

Because the temperature and pressure were different from the normal conditions, the Q values must therefore be corrected using eq 2, as follows:

2

where Q_T is the sampling rate at the temperature T, and Q ₂₉₈ is the Q value at the normal condition. Relative humidity between 15% and 90% as well as wind speed between 0.1 and 10 m sec⁻¹ have no influence on sampling rates.

Results

BTEX levels in Algiers

The average concentrations of BTEX at the seven sites during the sampling period are reported in Table 1. Benzene and toluene have been detected in all the investigated sites; however, toluene was the most abundant compound. The average concentration of benzene and toluene varied from 1.1 to 26.8 µg m⁻³ and from 3.5 to 63.3 µg m⁻³, respectively, the total concentrations of xylenes (m-, p-, and o-isomers) varied from 8.9 to 61.5 μgm⁻³, with the highest level recorded for m-xylene in all the investigated sites; the ethylbenzene concentration varied from 2 to 12 µg m⁻³. Xylenes (m-, p-, and o-isomers) and ethylbenzene were not detected in the semirural site (S6); this result can be explained by the fact that the main removal mechanism of atmospheric higher-molecular-weight aromatics is by the reaction with OH^• radical (Singh et al., 1985). It happened that during spring and summer, this removal process can be faster due to a higher OH^• radical atmospheric content.

Table 1. Ambient air mean of BTEX concentrations (µg m^{− 3}) measured in Algiers at the seven sites

	BTEX
Sites	Benzene	Toluene	Ethylbenzene	p-Xylene	m-Xylene	o-Xylene	Total BTEX
Roadside
S1	26.8	63.3	12.0	14.4	32.4	14.7	163.7
S2	9.3	27.5	4.0	3.7	8.7	4.9	58.1
S3	13.9	30.6	4.3	3.9	9.9	4.2	66.9
Average	16.7	40.5	6.8	7.4	17.0	7.9	96.2
Urban background
S4	1.1	10.3	2.0	1.7	3.2	4.0	22.3
S5	4.2	9.1	3.0	2.4	4.8	2.2	25.6
Average	2.7	9.7	2.5	2.1	4.0	3.1	23.8
Semirural
S6
Average	2.2	3.5	ND	ND	ND	ND	5.76
Tunnel
S7
Average	13.9	32.9	4.3	3.4	19.5	5.1	79.1
Inside vehicle
Average	58.9	107.9	12.7	13.0	31.1	15.3	238.9
Note: ND = not detected.

The highest concentrations of the individual aromatic compounds were observed along the road traffic sites (S1, S2, and S3), when the samplers were very close to the road traffic. Indeed, the average concentration of benzene and toluene were 16.7 and 40.5 µg m⁻³, respectively. The total average concentration of BTEX at the road traffic sites was 96.2 µg m⁻³. The BTEX species detected at the different sites are generally related to the automobile exhaust gas emissions in the urban area. This result is in fact consistent with other studies (Zielinska et al., 1996; Sexton and Westberg, 1984; Löfgren and Petersson, 1992; Lee et al., 2002; Grosjean et al., 1998). In urban areas, these BTEX constitute up to 60% of main nonmethanic volatile organic compounds (NMVOCs) and hence can be considered to be an efficient indicator of organic compound pollution from road traffic (Lee et al., 2002).

The variation of the BTEX concentration recorded in this study is in good agreement with accord with previously studies. It was shown in fact that in the road traffic sites, the concentration levels of atmospheric pollutants in urban areas at respiratory height (1.5–2.0 m) vary strongly from road to road and also in the same road depending on the distance from the kerb or the distance from road intersections (Kaur et al., 2005; Vardoulakis et al., 2002).

The benzene, toluene, ethylbenzene, and total xylene (m-, p-, and o-isomer) levels measured in this study at urban (background, roadside, and tunnel) and semirural sites in Algiers were lower than those measured in Rome (Italy), in Guangzhou, Nanhai, and Macau (south China), and at Cross Harbor Tunnel (Hong Kong). However, they were higher than those measured in Algiers (winter and summer 2006–2007), in Helsinki (Finland), in Yokohama (Japan), in Berlin (Germany), in London (England), in the United Kingdom, in Bombay (India), in eight states in USA, in La Coruña and Pamplona (Spain), in Toulouse (France), and in Seoul (South Korea).

The benzene concentration recorded in this study was quite similar to those recorded at Athens (Greece) and Naples (Italy), but higher than those recorded at urban and roadside in Sao Paulo (Brazil). Also the toluene, ethylbenzene, and total xylene (m-, p-, and o-isomer) concentrations were quite similar to those recorded at Sao Paulo (Brazil), but lower than those recorded at Naples (Italy) (Table 2).

Table 2. BTEX levels in ambient air of various cities worldwide (µg m^{− 3})

Cities	Benzene	Toluene	Ethylbenzene	(m+p)-Xylenes	o-Xylene
Algiers, two campaigns; urban ^a	1.94	4.57	1.2	1.07	0.55
Helsinki ^b	2.1	6.6	1.3	4.1	1.6
Rome ^c	35.5	99.7	17.6	54.1	25.1
Yokohama (Japan) ^d	1.7–3.7	4.7–34.3	0.5–3.8	1.0–2.0	0.1–0.8
Berlin (Germany) ^e	6.9	13.8	2.8	7.5	2.9
London (England) ^e	2.7	7.2	1.4	3.7	1.5
United Kingdom ^f	1.23–6.23	2.30–13.8	0.71–3.84	1.68–11.8	0.88–5.73
Bombay (India) ^g	13.7	11.1	0.4	1.3	2.2
Sao Paulo (Brazil); urban and road sites ^h	4.6	44.8	13.3	26.1	6.9
Guangzhou, Nanhai, and Macau (south China); urban and road sites ^i	20–51.5	39.1–85.9	3–24.1	14.2–95.6*	—
13 sites in 8 states (USA) ^j	0.8–3.6	1.5–10.5	0.6–2.4	1.8–5.2	0.6–1.9
La Coruña (Spain) ^k	3.43	23.6	3.34	5.08	2.74
Toulouse (France) ^l	2	6.6	—	1.2	3.7
Athens ^m	13.3–26.0	—	—	—	—
Caio Duilio, Naples (Italy); nearby road traffic tunnel ^n	14.2	180.6	58.3	280.9*	—
Pamplona (northern Spain) ^o	2.84	13.26	2.15	6.01*	—
Seoul (South Korea) ^p	3.2	24.5	3	10	3.5
Cross Harbor Tunnel in Hong Kong ^q	30.51	200.82	15.07	45.67*	—
Algiers; roadside, background, rural ^r	1.1–26.8	3.5–63.3	2–12	4.9–46.8	2.2–14.7
Notes: ^a Kerchich et al. (2010); ^b Hellén et al. (2002); ^c Brocco et al. (1997); ^d Yamamoto et al. (2000); ^e Monod et al. (2001); ^f Derwent et al. (2000); ^g Mohan et al. (1997); ^h Colon et al. (2001); ^i Wang et al. (2002); ^j Pankow et al. (2003); ^k Fernández-Martínez et al. (2001); ^l Simon et al. (2004); ^m Chatzis et al. (2005); ^n Fabio (2007); ^o Parra et al. (2009); ^p Na and Kim (2001); ^q Ho et al. (2004); ^rpresent study. *(o+m+p)-Xylenes.

Diurnal and nocturnal levels of BTEX

Fairly large differences in BTEX levels are shown by the measurements recorded in site S3. The daily and diurnal (07:00 a.m.–8:00 p.m.) average concentrations were 13.9 and 25.4 µg m⁻³ for benzene and 30.6 and 60.7 µg m⁻³ for toluene, respectively (Table 3).

Table 3. Diurnal and daily mean of BTEX concentrations (µg m^{− 3}) measured in road traffic site S3 by modified and nonmodified passive samplers

Sampler Type		Benzene	Toluene	Ethylbenzene	p-Xylene	m-Xylene	o-Xylene	Total BTEX
Daily sampling*
Average		13.9	30.6	4.3	3.9	9.9	4.2	66.9
SD		1.2	1.1	1.2	0.9	1.0	0.7	5.9
Min.		11.3	25.7	3.9	3.7	9.8	3.7	58.0
Max.		17.6	36.5	6.2	5.5	11.6	5.0	82.5
Diurnal sampling**
Average		25.4	60.7	8.4	7.4	24.3	11.0	137.2
SD		2.9	3.1	2.8	2.6	1.0	1.8	10.7
Min.		22.4	58.0	5.8	4.6	23.3	9.2	126.7
Max.		28.2	64.0	11.4	9.8	25.3	12.9	146.1
Diurnal/daily		1.8	2	1.9	1.9	2.5	2.6	2.1
Notes: *Nonmodified sampler. **Modified sampler (07:00 a.m.–8:00 p.m.).

The BTEX concentrations show that the average levels obtained from diurnal measurements are approximately 2X higher than those measured daily. The diurnal concentration/daily concentration ratios for the different BTEX compounds vary from 1.8 to 2.6 (Table 3). This can be explained by the fact that the considered adsorbed mass of BTEX over 13 hr of the day (07:00 a.m.–8:00 p.m.) is practically the same as that obtained during the daytime and nighttime (daily). This result could be possible only if the nocturnal contents of BTEX as well as the nocturnal road traffic emissions are very low, as well as that the lifetime of BTEX is relatively short (a few hours) (Atkinson, 1990). It seems that in both cases, the levels exceed the European standards.

These major diurnal levels of BTEX have been reported in other studies conducted in urban atmospheres (Barletta et al., 2005) showing that the average concentrations of diurnal benzene and toluene (07:00 a.m.–8:00 p.m.) were twice higher than the average daily concentrations. Pilidis and coworkers show a good correlation between the benzene level and the traffic density and/or street intersections subject to important traffic jam (Pilidis et al., 2005).

Measurement inside a tunnel

Tunnels offer also the advantage of providing an accurate appraisal of the traffic composition and the volume into which these emissions are released (Touaty and Bonsang, 2000; Na, 2006; Lai and Pend, 2011). For these reasons, the ambient air tunnel was selected as representative of direct vehicular emissions.

It appears that toluene is the most abundant compound according to the measurements recorded inside the tunnel. The average concentrations of BTEX were 13.9 µg m⁻³ for benzene, 32.9 µg m⁻³ for toluene, 4.3 µg m⁻³ for ethylbenzene, 3.4 µg m⁻³ for p-xylene, 19.48 µg m⁻³ for m-xylene, and 5.1 µg m⁻³ for o-xylene, giving a total BTEX concentration of 79.1 µg m⁻³

Normally, in a confined environment such as a tunnel, a very weak dilution of pollutants (BTEX) in ambient air is observed when the pollutants (BTEX) accumulate and are expected to reach high levels. However, the level of BTEX recorded in the tunnel was the same as the one recorded along the road traffic in sites S1, S2, and S3. These relatively low levels are probably due to the fact that inside the tunnel, air is always in motion. There is a removal of pollutants by the airflow induced by vehicle movement. This moving air does not allow the pollutant to accumulate and reach high levels. It is possible that the requirements for optimal diffusion of pollutants are not fulfilled for a passive sampling. Thus, the results depend not only on sampling conditions but also on the traffic flow and ventilation in the tunnel (McGaughey et al., 2004).

Indeed, the diffusion tube sampling does not follow continuous pollution levels, but reports an average situation in the exposure tubes (1 day). High concentrations and pollution peaks cannot be detected using this sampling method.

Measurement inside of a vehicle

Concentrations recorded inside the vehicle were respectively 58.9 µg m⁻³ for benzene, 107.9 µg m⁻³ for toluene, and 31.1 µg m⁻³ for m-xylene. The total BTEX concentration was below 238 µg m⁻³. This level is probably the highest that a person or a human being may be exposed to in Algiers. The European legislation sets the exposure limit of benzene at 5 µg m⁻³ (Directive 2000/69/EC), which is already considered too high and presents a serious risk to the passengers. Table 1 shows that the measured concentrations are about twice higher than those recorded at roadside. Benzene is shown to be potentially carcinogenic, and its concentration value reported here is about 12 times higher than the limit values fixed by the international standard.

Consequently, all the drivers (cars, buses, and the various companies, administrations, and institutions) are exposed to extreme BTEX levels. If one considers only urban areas of the 10 biggest cities of Algeria, one can estimate the number of drivers exposed to this severe pollution to be about 150,000. Hence, it is important and necessary to conduct epidemiological studies among this class of population.

These results are in good agreement with different studies, which have shown that the BTEX levels measured in cars over 24 hr (µg m⁻³) and indoor ranged from 1.7 to 44, 10 to 145, 0.6 to 45, 0.9 to 81, and 0.2 to 72 for benzene, toluene, ethylbenzene, (m+p)-xylenes, and o-xylene, respectively (Francesc et al., 2007; Moussaoui et al., 2012).

Mass distribution of BTEX

Concerning the BTEX mass distribution at roadside, this study shows that the average contributions of benzene, toluene, and m-xylene are 17.3%, 42%, and 7%, respectively. These three compounds represent about 77% of the total BTEX (Figure 3). A similar distribution is determined in the tunnel and inside the moving car (83.8% and 82.8%, respectively, for the tunnel and inside the car). In urban background sites (S4 and S5), the average distribution is quite different (since it represents 68% only).

Figure 3. Distribution of BTEX mass (%).

The contribution of benzene, toluene, and m-xylene in urban site (roadside) was higher than those of background and semirural sites; this fact was directly related to the influence of the distance of source emission (dilution and photochemical activity).

BTEX ratios and correlations

Table 4 reports the ratios between the concentrations of toluene and benzene (T/B) and (m+p)-xylenes and ethylbenzene [(m+p)X/E] measured in the considered areas. This type of analysis has frequently been used to estimate the pollution sources and the air photochemical reactivity and is based on the assumption that BTEX have different degradation rates in the air (Ho et al., 2004). The evaluation of BTEX ratios in different urban areas shows that the values of toluene/benzene (T/B) are 1.7–9.3 (Lee et al., 2002), 3–12 (Yamamoto et al., 2000), and 1.9–5.4 (Yassaa et al., 2006). It has been shown also that the T/B ratio, measured at vehicle exhaust, is in the range of 2–8 (CONCAWE, 1995; Sjodin et al., 2000), whereas the ratios for the other species are more uncertain because the determination of xylenes and ethylbenzene seems also to be dependent on the sample technique adopted (Joumard et al., 2004). However, it has been reported that the ratios change also in relation to car engine regimes and the presence or absence of a catalytic converter operating on the exhaust gases (Heeb et al., 2000).

Table 4. T/B and (p+m)X/E ratios found at different sites in Algiers

	Roadside + Tunnel				Urban Background		Semirural
Ratio	S1	S2	S3	Tunnel S7	S4	S5	S6
T/B	2.4	2.9	2.2	2.4	2.2	—	1.6
Average T/B	2.4				2.2		1.6
(m+p)X/E	3.9	2.9	2.4	5,3	3.2	2.5	ND
Average (m+p)X/E	4.4				2.4		—
	Note: ND = not detected.

In this present work, the values of the T/B ratio ranged between 2.2 and 2.9 for the road traffic sites S1, S2, S3, and tunnel S7. In these sites, the highest T/B ratios should appear in the canyon site S1 and inside the tunnel S7. At the urban background site S4, the ratio was 2.2. The lowest ratio (T/B = 1.6) was obtained at the semirural site. One can notice that the T/B ratios decrease with increasing distances from pollution source (Table 4). The atmospheric photochemistry kinetics lead to a more rapid decrease of toluene (toluene is about 5 times more reactive than benzene), resulting in a T/B ratio higher for the sites close to emission sources in comparison with the remote or semirural sites. These results are generally in good agreements with those reported by other authors (Brocco et al., 1997; Yamamoto et al., 2000; Lee et al., 2002; Pilidis et al., 2005). The study of the influence of the distance to emission sources on the T/B ratio leads to the same result if one substitutes toluene by m-xylene, known to have a higher photochemical reactivity.

For the (m+p)-xylenes/ethylbenzene ratio, the data found in the literature (Monod et al., 2001) show that the values are relatively constant, ranging from 2.8 to 4.6 with a mean value of 3.5 recorded by traffic exhaust emissions.

In the studied sites in Algiers, the (m+p)-xylenes/ethylbenzene ratio was in the range of 2.4–5.3 (Table 4). This ratio is quite similar to those found at Munich (Rappenglueck and Fabian, 1999), Sydney (Nelson and Quigley, 1983), Algiers (Kerbachi et al., 2006), and in different cities of the UK (Derwent et al., 2000). According to the works carried out by Atkinson (1990) and Simpson (1995), the typical life times for BTEX are, respectively, benzene: 56.5 hr; toluene: 11.7 hr; ethylbenzene: 9.8 hr; o-xylene: 5.1 hr; p-xylene: 4.9 hr; and m-xylene: 2.9 hr. This explains why the (m+p)-xylenes/ethylbenzene ratio found in background sites (S4 and S5) was lower than that of the road traffic sites or in tunnel (S7).

The (m+p)-xylenes/ethylbenzene ratio measured in the moving vehicle was 3.49. This value is very close to the mean value found by Monod and coworkers (Monod et al., 2001), and in this case we can attribute the road traffic emissions as the main source of BTEX.

According to the results reported in Tables 5 and 6, the correlation coefficients (R ²) between BTEX concentration levels are quite high. This is a clear indication that all pollutants have the same origin, as expected. These results show that the correlation coefficients obtained are very close to 1, indicating strong positive correlations. The correlation coefficient of benzene with toluene for all sites is 0.96. The same correlation was found for benzene with total BTEX and benzene with toluene at the road traffic sites. This confirms the assumption that the road traffic was the main source of BTEX emissions in urban areas. These correlations coefficients are similar to those reported by other authors (Lee et al., 2002; Iovino et al., 2009).

Table 5. Linear correlation coefficients between average concentrations of BTEX in road traffic sites

	Benzene	Toluene	Ethylbenzene	p-Xylene	m-Xylene	o-Xylene	BTEX
Benzene	1.00	0.96	0.87	0.75	0.89	0.77	0.96
Toluene		1.00	0.88	0.72	0.85	0.83	0.98
Ethylbenzene			1.00	0.93	0.83	0.93	0.94
p-Xylene				1.00	0.8	0.93	0.88
m-Xylene					1.00	0.82	0.94
o-Xylene						1.00	0.90
BTEX							1.00

Table 6. Linear correlation coefficients between average concentrations of BTEX in all sites

	Benzene	Toluene	Ethylbenzene	p-Xylene	m-Xylene	o-Xylene
Benzene	1.00	0.96	0.89	0.73	0.79	0.77
Toluene		1.00	0.89	0.66	0.74	0.78
Ethylbenzene			1.00	0.96	0.98	0.98
p-Xylene				1.00	0.99	0.98
m-Xylene					1.00	0.79
o-Xylene						1.00

Conclusion

The diffusive sampling method was successfully applied to monitor BTEX and to estimate the air quality in urban sites (background, roadside, and tunnel), semirural sites, and indoor air (inside the car) in Algiers City.

High benzene concentrations in roadside, tunnel, and inside the car were observed, which often far exceeded the allowed limits fixed by the EC Directive. It has been shown that benzene and toluene were the predominant monoaromatic compounds in all the sites investigated.

The levels of BTEX measured in the semirural and background sites were quite similar to those reported by the literature and measured in urban or in remote areas worldwide.

Daily and nocturnal measurements showed that the road traffic is the main source of air pollution by aromatic compounds. There is a good correlation between BTEX concentration and road traffic flow.

The study showed a relationship between the BTEX species that is closely related to the nature of the sampling site as well as to the reactivity of the compounds, considering that Algeria is a very sunny country. The study based on the BTEX measurement close to a car driver trapped by the flow of traffic showed that car drivers are strongly exposed to severe benzene levels when they spend the major part of the day in the urban zone. Public health managers are asked to carry out epidemiological studies for this segment of population.

Benzene, toluene, and xylenes exhibit different reactivities with the OH radical; because the Radiello passive sampler requires considerable time (more than 7 days) in order to reach the equilibrium, the most reactive congeners could be partly degraded. At this regard, further investigations seem necessary to estimate new values of BTEX diagnostic ratios that are in line with the Radiello diffusive sampler and the respective application times.