Air-soil diffusive exchange of PAHs in an urban park of Shanghai based on polyethylene passive sampling: Vertical distribution, vegetation influence and diffusive flux

Ying Liu,Shuya Xie,Lirong Zheng,Tongtong Li,Yajie Sun,Limin Ma,Zhifen Lin,Peter Grathwohl,Rainer Lohmann

doi:10.1016/j.scitotenv.2019.06.500

Abstract

Compared with dry and wet deposition rates, air-soil exchange fluxes cannot be directly measured experimentally. Polyethylene passive sampling was applied to assess transport directions and to measure concentration gradients in order to calculate diffusive fluxes of polycyclic aromatic hydrocarbons (PAHs) across the air-soil interface in an urban park of Shanghai, China. Seven campaigns with high spatial resolution sampling at 18 heights between 0 and 200 cm above the ground were conducted in 2017–2018. Air-to-soil deposition was observed, e.g. for phenanthrene, and soil-to-air volatilization for high molecular weight compounds, such as benzo[b]fluoranthene. Significant linear correlations between gaseous PAH concentration and log-transformed height were observed. Influence of vegetation on vertical concentration gradients of gaseous PAHs was insignificant in most cases except during the growing season. Local micrometeorological conditions resulted in a directional eddy diffusion in air and then influenced vertical diffusion of gaseous PAHs. Furthermore, the vertical eddy diffusivity was estimated as a function of distance to the air-soil surface. Air-soil exchange fluxes based on the Mackay's fugacity approach were calculated and confirmed by diffusive fluxes within air layer based on vertical concentration gradient of PAHs and eddy/molecular diffusion. Polyethylene passive sampling technology provides a useful tool to investigate air-soil exchange process.

Highlights

Polycyclic aromatic hydrocarbons (PAHs) are a ubiquitous class of semi-volatile organic pollutants mainly originating from the combustion of organic matter and fuels (Liu et al, 2015), and are widespread all over the world (Casal et al, 2018).the earth surface becomes a critical sink of these hydrophobic pollutants via atmospheric deposition and direct application e.g. by sewage sludge (Feng et al, 2017; Zheng et al, 2015)
To assess air-soil exchange, many studies quantified gradients in air and soil fugacities based on model predictions or field measurements. The latter include active air sampling (Degrendele et al, 2016; Kurt-Karakus et al, 2006; Tasdemir et al, 2012; Wang et al, 2015) and more recently passive sampling technologies to measure the air fugacity based on polyurethane foam, low-density polyethylene (LDPE) and other materials (Donald and Anderson, 2017; Dumanoglu et al, 2017; Wang et al, 2017; Zhang et al, 2011)
Turbulence varied with height, which affected both uptake of PAHs and release of PRCs, and in general, the sampling rate increased with height

Summary

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are a ubiquitous class of semi-volatile organic pollutants mainly originating from the combustion of organic matter and fuels (Liu et al, 2015), and are widespread all over the world (Casal et al, 2018).the earth surface (e.g. soil and vegetation) becomes a critical sink of these hydrophobic pollutants via atmospheric deposition and direct application e.g. by sewage sludge (Feng et al, 2017; Zheng et al, 2015). After primary pollutant sources were reduced or eliminated in air, soils may become secondary sources of accumulated semi-volatile organic compounds depending on air-surface exchange rates (Bao et al, 2015; Degrendele et al, 2016). To assess air-soil exchange, many studies quantified gradients in air and soil fugacities based on model predictions or field measurements. The latter include active air sampling (e.g. low-volume air samplers) Active fugacity samplers were designed to sample the air close to the soil surface (Cabrerizo et al, 2009; Degrendele et al, 2016; Wang et al, 2015)

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