Sources and sinks evaluation of PAHs in leaves of Cinnamomum camphora in megacity: From the perspective of land-use types

https://doi.org/10.1016/j.jclepro.2020.123444Get rights and content

Highlights

  • 1. The content of PAHs and land-use types of 84 sampling sites were calculated.

  • 2. PAHs in leaves were determined to characterize the content in the atmosphere.

  • 3. For MMW PAHs, industry served as main source, water and woodland served as sinks.

  • 4. The main source of HMW PAHs was road with the main sink being farmland.

  • 5. Land-use adjustment should be different in the urban stock land and incremental land.

Abstract

Direct and indirect effects of complex urban land-use types on polycyclic aromatic hydrocarbons (PAHs), including their total amount, composition, sources and sinks, were evaluated from 84 sampling points in the megacity of Shanghai. We measured the content and composition of PAHs adsorbed by the leaves of Cinnamomum camphora and established buffers of different radii at each point. The different land-use types areas were counted therewith to investigate their effects on the sources and sinks of PAHs in leaves. Results showed that the average value of PAHs in the leaves was 3014.06 ± 1879.713 ng g−1. The substance profiles were dominated by medium-molecular-weight (MMW, 54%) and high-molecular-weight (HMW, 45%) PAHs, and these fractions were much higher than that of low-molecular-weight (LMW, 1%) PAHs. Through correlation analysis, the land-use type of industry, road, business, and residential areas were initially assessed as the sources of PAHs, whereas water, farmland, and woodland areas were sinks. Lasso regression proved that road and residential areas have proven to be the main source of PAHs in Shanghai, rather than industry and business areas. Woodland was found to be a better sink for PAHs than water and farmland. Sources and sinks of PAHs with different benzene rings also differed greatly: the main sources of MMW PAHs were industrial emissions and commercial activities, with the main sinks being water and woodland; the primary sources of HMW PAHs were traffic emissions and residential activities, with the main sink being farmland. This study assessed the sources and sinks of PAHs in cities, from the perspective of varied effects of different urban land-use types on PAH composition and spatial distribution, and bridged the gap between urban PAHs alleviation and the land-use planning in megacity, with instructive significance for planning policy development.

Introduction

Polycyclic aromatic hydrocarbons (PAHs), a type of trace toxic organic pollutant, are widely distributed in the atmosphere worldwide (Capozzi et al., 2017; Kargar et al., 2017). In particular, high-molecular-weight (HMW) PAHs are concerning due to their teratogenic, carcinogenic, and mutagenic effects on living organisms. PAHs are derived from a broad range of natural and anthropogenic sources, such as traffic and industrial activity involving the combustion of oil, wood, and other organic materials (Keshavarzifard et al., 2017; Krzebietke et al., 2018; MacAskill et al., 2016). A large majority of them are adsorbed on particulate matter, especially respirable particulates with a radius of less than 5 μm, which poses a great threat to human health (Tretiach et al., 2011).

Cities contain complex PAH sources and sinks. Once produced, PAHs can transform into one another within the atmosphere, water, and soil in an urban environment through various complex physical and chemical biological pathways (MacAskill et al., 2016). PAHs are discharged into the urban atmospheric environment through incomplete combustion in transportation and industry. They enter the vegetation and soil through dry and wet deposition, adsorption, and diffusion and enter the water through infiltration and surface runoff (Andersson et al., 2014; Keshavarzifard et al., 2017). Some PAHs are subsequently released from the soil and water, and they return to the atmosphere and participate in the geochemical cycle (MacAskill et al., 2016). Therefore, the complex land-use types in the city directly or indirectly affect the content and distribution of PAHs in the environment, becoming the potential sources or sinks of PAHs.

Plants play a crucial role throughout the cycle of PAHs and can obtain PAHs from the atmospheric environment through leaf absorption (Fellet et al., 2016; J. Wang et al., 2017). Domingos et al., 2015 summarized the search for bioindicator tree species in southeast Brazil, and specifically indicated Piptadenia gonoacantha, a native Brazilian plant, was the most efficient accumulator of PAHs and may be recommended for future assessments of the levels of these hydrocarbons in particulate matter. By focusing on PAH accumulation in the foliage of dominant tree species, Dias et al. (2016) found that the analysis of PAH accumulation in the leaves of native plant species is a viable tool for inventorying emission sources and estimating the contamination level in the polluted ecosystem. Zhao et al. (2018) defined that the measured plant/air partition coefficients have been found to be exponentially proportional to the reciprocal temperature, indicating there is a significant correlation between PAH concentration in the air and leaves. Therefore, the native plant species growing in a polluted ecosystem may be sentinels of the atmospheric deposition of such toxic compounds. The content of PAHs in plant leaves characterizes the content of PAHs in local atmosphere to some extent.

However, previous studies have usually used a limited number of physical high- or low-volume samplers (e.g., HVS, LVS) when investigating the environmental behavior of PAHs, and do not fully reflect the PAHs pollution status of megacities (Schifman and Boving, 2015; Urbancok et al., 2017). When discussing the sources and sinks of PAHs, traditional methods (e.g., Source markers, PAH diagnostic ratio and Principal components analysis) were generally used, which have fully considered the transfer characteristics of PAHs, but few literatures explored their environmental behavior from the perspective of land-use types (Athanasios, 2011; Callen et al., 2013; Khaiwal Ravindra et al., 2006). Furthermore, different PAHs with different benzene rings have particular physicochemical properties and thus have significantly different migration and transformation pathways in environmental mediums. Therefore, it is feasible and important to interpret the sources and sinks of different PAHs from the perspective of land-use type, especially in megacities.

In this study, we distributed dots evenly in a grid and established 84 sampling points in Shanghai. Supercritical fluid extraction (SFE) was used to extract the PAHs in the leaves of Cinnamomum camphora in each sampling site, instead of physical monitoring samplers. By establishing buffer regions of different radii for each sample site and counting the different land-use type areas in each buffer, Shanghai’s complex land-use types are interpreted. After a series of rigorous analyses, the sources and sinks of PAHs in urban environment were evaluated from the perspective of land-use.

In particular, we attempted to answer the following questions:

  • 1)

    In the megacity of Shanghai, which land-use types are main sources of PAHs and which ones are main sinks?

  • 2)

    Is there a difference in the source and sink of PAHs with different benzene rings?

We followed the two key questions. From a novel perspective, we established a connection PAHs alleviation and the complex urban land-use by using the leaves of C. camphora. Also, we discussed the difference of PAHs with different benzene rings. The entire manuscript made a good supplement to the existing literature map and paved the way for future research.

Section snippets

Study area

Shanghai is located in the eastern part of China (120°51′E−122°12′E, 30°40′N–31°53′N) on the west coast of the Pacific Ocean and falls within the subtropical to north subtropical transitional zone. It has a north subtropical maritime monsoon climate. The four seasons are clear, with sufficient sunshine and abundant rainfall. As a typical international megacity, Shanghai has the characteristics common to many metropolises, such as densely populated urban areas, dense traffic, and severe

PAH content and composition in C. camphora leaves in Shanghai

Fig. 2 shows the content of 16 individual PAHs in the C. camphora leaves in Shanghai. Some PAHs had the same characteristic ions; however, because their peaks were difficult to distinguish, they were categorized as one group. The average value of PAHs in the leaves in Shanghai was 3014.06 ± 1879.713 ng g−1, with a large difference in composition. In general, the substance profiles were dominated by medium-molecular-weight (MMW) and high-molecular-weight (HMW) PAHs, accounting for

Sources and sinks assessment of PAHs by lasso regression

Fig. 4 can initially determine the main sources and sinks of PAHs in Shanghai. However, as the collinearity between different land-use types and different buffer of the same land-use type is obviously observed, the only use of correlation coefficient between PAHs content and land-use types is difficult to discriminate the land-use type that actually adsorb or release PAHs. Therefore, we used lasso regression to further analyze the correlation between PAHs and land-use types.

After variable

Conclusion

Eighty-four sampling points for PAHs of Shanghai were established in the present study to explore the sources and sinks of PAHs in C. camphora leaves from the perspective of land-use types. Road and residential areas are main sources of PAHs compared to other land-use types. By contrast, compared with water and farmland, woodland has a better adsorption effect on PAHs, which is the most important sink of PAHs in Shanghai. The sources and sinks of PAHs with different benzene rings also differed

CRediT authorship contribution statement

Shan Yin: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Supervision, Project administration. Lu Tian: Methodology, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Yingge Ma: Methodology, Resources. Haoxin Tan: Formal analysis, Investigation, Data curation. Lurong Xu: Methodology, Investigation, Resources. Ningxiao Sun: Investigation. Hengyu Meng: Formal analysis, Investigation. Chunjiang

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was co-funded by the National Key Research and Development Program of China (2017 YFC0505501), National Natural Science Foundation of China(31971719), and Shanghai Landscaping and City Appearance Administrative Bureau (G171206).

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