Particle size distribution and characteristics of polycyclic aromatic hydrocarbons during a heavy haze episode in Nanjing, China
Graphical abstract
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental pollutants that are ubiquitous in urban and suburban atmospheres. Atmospheric PAHs are partitioned between the gaseous phase and the particulate phase; the latter are called particle-bound polycyclic aromatic hydrocarbons (pPAHs). Although atmospheric PAHs are partitioned between two phases, PAHs are predominantly associated with particulate matter. pPAHs are a significant danger to human health through inhalation (Chetwittayachan, Shimazaki, & Yamamoto, 2002). Many PAHs have mutagenic and carcinogenic properties, and the hazard posed by these compounds increases as their molecular weight increases (Tsai et al., 2004). PAHs are primarily derived from the imperfect combustion processes of mobile sources (vehicular emissions) and of stationary sources (industries) (Ravindra, Sokhi, & van Grieken, 2008).
Haze is defined by relative humidity <80%, by visibility <10 km and by the concentration of particles when the relative humidity is between 80% and 95%. Haze is called as a heavy haze episode when the visibility is lower than 2 km (Wu et al., 2010). Haze is generally believed to be related to atmospheric particulate matter (Chan & Yao, 2008). Recent studies also showed that haze not only damages human health but also is important for the global energy balance (Huang et al., 2011, Menon et al., 2002, Tao et al., 2009). There have been many studies concerning pPAHs in large cities in China, including Guangzhou (Tan et al., 2006, Tan et al., 2009a, Tan et al., 2009b), Shanghai (Cheng et al., 2007), Dalian (Kong et al., 2011, Tian et al., 2009), Hong Kong (Guo, Lee, Ho, Wang, & Zou, 2003), Fuzhou (Zhang et al., 2013), Xiamen (Hong, Yin, Wang, & Ye, 2007), Beijing (Liu, Gao, & An, 2008), and Nanjing (Wang, Huang, Zhao, Niu, & Dai, 2006). These authors found that the concentrations of PAHs were closely related to the sizes of atmospheric particles. Concentrated in the respirable size range of 0.1–1 μm, the burning of fossil fuels is the most important emission source of PAHs. Some studies also found that the main influencing factors of the concentration of pPAHs were emission sources and meteorological factors, such as temperature, radiation, rainfall, wind speed and direction, boundary layer characteristics and so on (Hien et al., 2007, Ravindra et al., 2006, Tham et al., 2008). To understand the distribution characteristics of PAHs under specific weather conditions, Fan et al. (2009) and Gu et al. (2011) analyzed the influence of foggy weather conditions. These authors found that fog could aggravate the pollution of PM2.5 and PM10 near the ground layer and that size distributions of PM10 and the total PAHs in PM10 were greatly affected by the daytime fog and also by the duration of each fog event. Duan, Bi, Tan, Sheng, and Fu (2006) and Tan et al., 2009a, Tan et al., 2009b, Tan et al., 2011 performed research on pPAHs during haze periods in Guangzhou. These authors discovered differences between seasons, with low-ring PAHs displaying greater changes than high-ring PAHs.
Agricultural burning is a serious environmental problem in China, particularly in the middle and lower Yangtze River and in the southern part of China during autumn. Some scholars have found that the atmospheric particles of Nanjing were seriously affected by agricultural burning from the Jiangsu, Anhui, Hubei, and Hunan provinces, and agricultural burning was an important source of pPAHs (Keshtkar & Ashbaugh, 2007). Although there are several studies concerning the influence of haze episodes on atmospheric PAHs concentrations, less is known concerning PAHs size distributions, particularly under the influence of agricultural burning. The purpose of this study is to examine pPAHs concentrations and size distributions in an autumn haze episode in city and suburban sites around Nanjing, including periods with a strong influence from agricultural burning.
Section snippets
Filter sampling
Filter samples were collected at two sites, as shown in Fig. 1. One site was in the suburban campus of the Nanjing University of Information Science and Technology (NUIST), approximately 17 km away from the downtown area. The sampler was set up on the roof at approximately 4–6 m above ground. There is a residential area between the campus and downtown, with a busy road and an industrial district east of the sampling site. The industrial district has many factories, such as petrochemical plants,
Meteorological conditions
Nanjing experiences a subtropical monsoon climate. The average temperature in autumn is approximately 16 °C, with a larger fluctuation than in other seasons. The dominant wind directions are northerly and southerly.
As shown in Fig. 2, we found that the heavy haze episode formed during the morning of November 7, 2009 and persisted until the morning of November 9, when it rained during the sampling period. Because there was persistent precipitation from November 9 to November 12, there was higher
Conclusions
In this study, we analyzed the concentrations of airborne particles and PAHs in both fine and coarse particle fractions during a heavy haze episode in Nanjing. Both backward trajectory and the diagnostic ratio values during the sampling period indicated that the haze was caused by agricultural burning. The results showed that the particle concentrations during the haze episode were as high as 579.55 and 573.43 μg/m3 at the suburban and city sites, respectively, and that the proportion of fine
Acknowledgments
This study was funded by the National Natural Science Foundation of China (Nos. 41275151, 41375138 and 71341029) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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