Air-soil diffusive exchange of PAHs in an urban park of Shanghai based on polyethylene passive sampling: Vertical...

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.