Investigation of outdoor BTEX: Concentration, variations, sources, spatial distribution, and risk assessment

Highlights

• Seasonal variation, spatial mapping, photochemical aging, and risk assessment of BTEX in the ambient air of Tehran were investigated. .

• The maximum and minimum concentrations of BTEX were observed for toluene and ethylbenzene, respectively.

• Spatial distribution of BTEX pollution showed that the highest concentrations were found along the major roads because of heavy traffic.

• The cancer risk of benzene and noncarcinogenic risk of BTEX were in the acceptable range.

Abstract

The aim of this study was to measure BTEX (benzene, toluene, ethylbenzene, and xylenes) concentrations in the ambient air of Tehran, the capital of Iran, and investigate their seasonal variations, probable sources, spatial mapping, and risk assessment. The concentrations of BTEX were measured using a continuous monitoring device installed in seven stations around the city. Spatial mapping procedure was conducted using the inverse distance weighting (IDW) method. Monte Carlo simulation was used to assess the carcinogenic and noncarcinogenic risks imposed by BTEX. The highest and lowest annual mean concentrations of toluene and ethylbenzene were recorded as 16.25 and 3.63 μg m⁻³, respectively. The maximum (6.434) and minimum (3.209) toluene/benzene (T/B) ratio was observed in summer and winter, respectively. The spatial distribution of BTEX pollution indicated that the highest concentrations were found along the major roads because of heavy traffic. Spearman's rank correlation coefficients and concentration ratios showed that BTEX were produced by the multiemission sources. The mean of inhalation lifetime cancer risk (LTCR) for benzene was 3.93 × 10⁻⁷, which is lower than the limits recommended by the United States Environmental Protection Agency (US EPA) and the World Health Organization (WHO). The hazard quotient (HQ), noncarcinogenic risk index, for all BTEX compounds was <1. The obtained results showed no threat of BTEX concentrations to human health. However, as the concentrations of BTEX will increase due to the rapid growth of vehicles and industrial activities, much effort is required to control and manage the levels of these compounds in the future.

Introduction

Traffic and industrial activities together with natural emissions are the major sources of air pollution in urban areas. A large number of pollutants such as sulfur oxides (SOx), nitrogen oxides (NOx), particulate matter (PM), carbon oxides (COs), ozone (O₃), radioactive pollutants, and volatile organic compounds (VOCs) are released by these sources into the ambient air. In recent years, there has been an increasing interest in research on VOCs not only for concerns about their carcinogenic and noncarcinogenic effects on human population but also for their photochemical reactions, which can produce secondary pollutants such as O₃ and proxy acetyl nitrate (PAN) (Ghozikali et al., 2016, Guimarães et al., 2014, Lin et al., 2016, Pekey and Yilmaz, 2011).

Over the past two decades, a considerable number of studies have been conducted on benzene, toluene, ethylbenzene, and ortho (o), para (p), and meta (m)-xylene (BTEX) as the most important VOCs because of their high potential to exert carcinogenic effects and relatively high abundance in ambient air. The most abundant and hazardous BTEX compound is benzene, which has been categorized by the International Agency for Research on Cancer (IARC) as a known carcinogenic to humans (Group 1) (Demirel et al., 2014).

Two important characteristics of BTEX are their presence in the ambient air and the correlations with their atmospheric concentrations (Bruno et al., 2008, Hoque et al., 2008, Khoder, 2007, Miller et al., 2010). The latter, in the BTEX studies, is used to determine the sources and photochemical ages of these compounds.

Toluene-to-benzene (T/B) ratio has been widely used by many researchers worldwide as an emission source index of BTEX (Buczynska et al., 2009, Hoque et al., 2008, Miller et al., 2011). The T/B ratio in the range of 1.5–4.0 indicates that transportation is the original source of BTEX in the region concerned. In addition, the ratio of (m + p)-xylene to ethylbenzene (m + p-X/E) has been known as a photochemical age index (Zhang et al., 2008). Although emission rate of (m + p)-xylene is 3.6 times more than ethylbenzene (Monod et al., 2001), the (m + p)-xylene is removed by chemical reactions about 3 times faster than ethylbenzene. Therefore, the (m + p)-X/E ratio is reduced by photochemical aging (Nelson and Quigley, 1983). Consequently, this ratio can be used to determine the period of time in which BTEX entered the ambient air.

Although extensive studies have been carried out on BTEX, they have been mostly restricted in terms of intermittent measurement frequency and low number of sampling sites.

Most studies have been carried out only in maximum 12 sites, for 1 or 2 weeks and in one season (Brocco et al., 1997, Bruno et al., 2008, Ho et al., 2004, Ho et al., 2009, Khoder, 2007, Moriarty, 1988, Smith et al., 2007). Such approaches have failed to address the seasonal trend of BTEX concentrations and their exposure assessments as the atmospheric reactions of these compounds are dependent on time (Miller et al., 2012). In the present study, however, the BTEX concentrations were obtained through continuous measurement device, which can record the BTEX concentrations throughout the year. To the best of our knowledge, this study is unique in this field and can fill this research gap. Therefore, it was designed to investigate the seasonal variations, spatial mapping, photochemical aging, and risk assessment of BTEX in the ambient air of Tehran, the capital of Iran.