Mechanisms Of Atmosphere-to-soil Migration And Transformation Of Polycyclic Aromatic Hydrocarbons Based On Passive Sampling Techniques

Polycyclic Aromatic Hydrocarbons（PAHs）are widely present in various environmental media such as the atmosphere,water and soil.PAHs are emitted into the atmosphere through various combustion processes and are redistributed between the gaseous and particulate phases depending on the properties of the compounds,meteorological conditions and the properties of the particulate matter.Dry and wet deposition is the main pathway for transferring PAHs from the atmosphere to the soil.Soil is an important sink for PAHs in the environment and represents a stable and long-term indicator of PAHs contamination levels.The main loss of PAHs from soil is due to microbial degradation processes.Therefore,in-depth studies on the variation of PAHs concentrations in the atmosphere,dry and wet deposition and soil,source correlation and microbial degradation of PAHs in soil are the focus of a comprehensive understanding of the processes and mechanisms of PAHs transport and transformation in environmental multi-media.In addition,in this study,low-density polyethylene（PE）sheet was selected as a passive sampler to monitor low concentrations of contaminants in environmental media,which is simple to operate and provides a good response to the time-weighted average concentration of pollutants.In this thesis,a meadow near the city of Tübingen,Germany,was chosen as the study area for the monitoring of gas-phase PAHs concentrations using PE sheets,and different calculation methods were developed for short-term and long-term monitoring,respectively.Also,the PE sheets passive sampling technique was used to monitor the concentration variations of PAHs in soil with biodegradation in laboratory,and a mechanistic model was constructed based on the mechanism of biodegradation of PAHs in soil.Finally,by comparing and analysing the contamination levels and sources of PAHs on different media,the migration and transformation patterns among the various environmental media were explored.Specifically,the following key insights were obtained:（1）PE sheets of two different thicknesses,30μm and 80μm,were used for the study and PAHs were selected as the target pollutants.The sorption/desorption kinetics experiments and sorption isotherm experiments were carried out to analyze the sorption/desorption characteristics of PE sheets for PAHs and to calculate the PE-water partition coefficient K_pe values,which were used in the subsequent soil and atmospheric PAHs monitoring studies respectively.（2）Based on the principle of film diffusion,the variations of PAHs concentrations in the gas phase were estimated using numerical modelling methods and analytical methods for the short and long term（1 year monitoring time）using 80μm thick PE sheets,respectively.In particular,the numerical modelling method takes full account of the correction for temperature-dependent parameters(K_pg and D_g)at each time step,and the results showed that the time required to reach equilibrium increased with increasing molecular weight,the PAHs measurements fitted well with the numerical model,and due to the lower K_pg values,the concentrations of Fln and Phe on the PE sheets varied with the daily temperature variation was also evident.The parameters of the analytical methods were based on temperature-corrected monthly averages,which provided a good response to the time-weighted average concentration of pollutants,and the results showed seasonal fluctuations in the concentrations of gas-phase PAHs,with lower-ring PAHs showing higher concentrations in winter than in summer,and higher-ring PAHs showing the opposite pattern.Therefore,the application of PE sheets allows for large-scale monitoring of PAHs over any time span.In addition,the feasibility and accuracy of the results of the PE sheet passive sampler was verified by comparing the atmospheric PAHs concentrations monitored by the active sampler,and the results showed that the monitoring results and calculation methods of the passive sampler were reliable,especially for 3-4 ring PAHs.（3）The process of biodegradation of PAHs in soil was monitored indirectly using PE sheets of 30μm thickness,a mechanical model characterising the process was constructed based on the physicochemical parameters of the soil,PE sheet and each individual PAHs,and the microbial removal rate and biodegradation rate of PAHs in soil with biodegradation were calculated.After 147 days,the maximum removal rate was 32%for Phe and the minimum for Dah A at 6.4%;the biodegradation rate for PAHs increased from 1.3×10^-4s^-1 for Phe to 9.0×10^-3s^-1 for Indeno.In addition,the general applicability of the model was confirmed by the substitution of two other sets of data from the literature.The key mechanism for biodegradation of PAHs in soil is desorption of PAHs from soil particles into the aqueous phase and biodegradation of PAHs in a water-soluble state,with the main influencing factors being properties of the contaminants and soil properties,such as the fraction of soil particles with strong sorption.（4）Atmospheric deposition fluxes of PAHs also showed significant seasonal variation,with dry and wet deposition fluxes of low ring being 1.5-6 times higher in winter than in summer,while dry and wet deposition fluxes of high ring PAHs were similar across seasons,possibly due to more low ring PAHs being emitted from home heating in winter.PAHs concentrations in PE sheets,atmospheric deposition and the gas phase were compared with historical data to verify the seasonal variability of PAHs concentrations and the reproducibility of results across media at different times in the study area.In addition,the source analysis of PAHs on different environmental media showed a consistency in the sources of PAHs in the study area for atmospheric,wet and dry deposition and soil.The transport pattern of PAHs between atmospheric and soil media was discussed using atmospheric deposition of PAHs as a medium,and it was found that the dry deposition of atmospheric particulate matter into soil"diluted"PAHs by about three orders of magnitude and that PAHs were highly persistent in soil,but the fraction of low ring PAHs in the soil environment was reduced due to,for example,microbial degradation.