Extremely high abundance of polycyclic aromatic hydrocarbons in aerosols from a typical coal-combustion rural site in China: Size distribution, source identification and cancer risk assessment

Highlights

• Size-segregated samples were collected to study the PM₁₀-bond PAHs in a typical coal combustion rural site in north China.

• More than 60% of the PM₁₀-bond PAHs were concentrated on PM_0.4 and the residential coal combustion was the primary source.

• Hopanes and steranes, the molecular biomarkers of coal combustion, were identified in this study.

• The Lifetime cancer risk was about two orders of magnitude higher than the guideline of 10⁻⁶.

Abstract

Air quality in China is tending to improve, especially in urban areas. Nevertheless, rural areas and underdeveloped cities in northern China are still suffering from severe air pollution in winter due to large usage of coal and unfavorable meteorological conditions. In this study, size-segregated aerosol samples were collected from a typical coal-combustion rural site in Linfen, China during an extreme air pollution episode in winter 2017. The polycyclic aromatic hydrocarbons (PAHs), n-alkanes, hopanes and steranes were identified to better understand the emission sources of the organic compounds and the health effects of the PAHs in the ambient air here. The PAHs showed a bimodal mode with the major peak in the Aitken mode (<0.4 μm) and the minor one in the size range of 0.7–2.1 μm. Daily PAHs level ranged from 0.5–1.6 μg m⁻³ (average: 1.1 ± 0.3 μg m⁻³) which was 1–2 orders of magnitude higher than other major cities in China. The plots of log (PAH/PM)-log(Dp) showed the slope near −1 for four-, five- and six-ring PAHs, indicating both the adsorption and absorption mechanisms controlled the size distribution. The deposition efficiency displayed a bimodal mode in alveolar region (AR), and the major peak was within the Aitken mode. The total deposition flux (DF) in AR region was 92.7 ng h⁻¹, and the DF based lifetime cancer risk (LCR) was about 2.7 × 10⁻⁴ for adults and 2.0 × 10⁻⁴ for children which were 2 orders of magnitude higher than the acceptable level of 10⁻⁶ proposed by United States Environmental Protection Agency (US EPA). In summary, the residential coal combustion was the most important source of PAHs, especially they emitted a large amount of PAHs in Aitken mode which led to the terrible air quality and high level of health risk. Therefore, the environmental protection measures should be tightened in such rural areas of north China.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) is a kind of persistent organic pollutants (POPs), and they are mainly emitted by incomplete combustion of fossil fuels, biomass burning and pyrolysis of organic compounds (Mu et al., 2013; Yin and Xu, 2018). Sixteen individual PAHs have been selected as the priority pollutants by the US EPA (National Research Council, US, 1972). Due to the properties of toxicity, carcinogenicity, and mutagenicity of PAHs (Armstrong et al., 2004; Guidi et al., 2012), they have been paid attention intensively in recent years. Considerable literatures reported the health risks caused by PAHs exposure (ACGIH, 2005; Unwin et al., 2006; Diggs et al., 2012; Srogi, 2007; Perera et al., 2005; Bach et al., 2003; Boffetta et al., 1997; Olsson et al., 2010).

The size distribution of PAHs is able to affect the atmospheric transportation, deposition and health risk of aerosol PAHs to a human body (Kawanaka et al., 2004; Kameda et al., 2005; Wu et al., 2006; Kwon et al., 2013). Fine particles can penetrate the body much easier via respiration and circulation (Liu et al., 2015; Sun et al., 2015). Previous studies reported that PAHs emitted from coal combustion prefer partitioning to small size particulate matter (PM), particularly in the superfine PM with aerodynamic diameter no more than 1 μm (Chen et al., 2004; Guidi et al., 2012; Shen et al., 2010; Shen et al., 2014). Specifically, high molecular weight (HMW)-PAHs, which have a higher toxicity (Nisbet and Lagoy, 1992), particularly prefer to condense on fine PM (Wu et al., 2006) due to their lower vapor pressure and volatility compared to the low molecular weight (LMW)-PAHs (Duan et al., 2005; Wu et al., 2006; Oliveira et al., 2011).

Molecular diagnostic ratios have been widely used to trace the specific sources of PAHs (Kavouras et al., 1998). However, the diagnostic ratios of PAHs should be cautiously used, because the degradation products of PAHs molecules can vary over time in the ambient air. Some previous studies identified hopanes and steranes to achieve a more reliable source apportionment of atmospheric particulates (Oros and Simoneit, 2000; Shel-Gayar et al., 2002; Bi et al., 2008; Schnelle-Kreis et al., 2011). The hopanes and steranes are exclusively emitted from fossil fuel combustions (Simoneit, 1984), for example coal combustion (Oros and Simoneit, 2000) and vehicle emissions (Rogge et al., 1994). And they are considered as molecular biomarkers in the emission of coal combustion (Ding et al., 2009; Oros and Simoneit, 2000). The research on hopanes and steranes in the atmosphere have been conducted by others (Oros and Simoneit, 2000; Shel-Gayar et al., 2002; Bi et al., 2008; Schnelle-Kreis et al., 2011). In addition, n-alkanes were identified, which are ubiquitous in the ambient air. They can be emitted from both anthropogenic (fossil fuel combustion, wood and agricultural burning, etc.) and biogenic sources (shed from plant wax and suspension of pollen, micro-organisms and insects) (Simoneit, 1977).

In this study, size-segregated PM samples were collected in Linfen city which is located in center China and adjacent to Beijing-Tianjin-Hebei economic region. Linfen is a typical coal-combustion city, the people there use coal to meet the needs of heating and cooking (Li et al., 2011; Wang et al., 2016a), especially in rural areas during wintertime. However, the existed literatures about PAHs in the ambient air in China are mainly focused on major cities (Feng et al., 2018; Yu et al., 2018; Wang et al., 2016b; Fan et al., 2017). In this work, we firstly studied the size-resolved PAHs, hopanes, steranes and n-alkanes in atmospheric PM at a typical coal-combustion rural site in China. Besides, the PM deposition efficiency (DE) in human body was taken into account, which allowed us to evaluate the health effect in different regions of human respiratory system. Only when the DE was included, the results of health effect could be much closer to the real situation. However, only very limited research took DE into consideration when calculating the health effect caused by PAHs in urban China (Lv et al., 2016), and much fewer in the rural areas. Linfen is one of the numerous coal-combustion cities in China, and the results of this study could be helpful to have some insights of physicochemical properties and sources of organic aerosols, especially the size distributions of PAHs and the health effect caused by the PAHs exposure in coal combustion rural areas.