Soil-Atmosphere Exchange of PAHs: The Determination of Concentration Gradients with Passive Samplers

Abstract

Polycyclic aromatic hydrocarbons (PAHs) as a group of ubiquitously occurring organic pollutants are released into the atmosphere by combustion processes of organic materials. Parallel to the beginning of industrialisation (1870), PAH accumulation in soils is clearly evident. With increasing environmental protection, about one century later, the emission and thus concentrations of PAH in the atmosphere decrease. Variations in the atmosphere significantly influence the relationship and equilibrium conditions of PAH between soil and air. Accordingly, low emissions can induce a change in the function of soils as pollutant sinks towards a secondary source. The aim of this study is to identify the current pollutant flux of PAHs between atmosphere and soil as well as potential temporal variations. Passive samplers serve as chemometers, i.e. equilibrium concentrations of PAHs in the samplers reflect their activity in the respective sampled medium. In this way, concentrations on the samplers can be directly compared to identify the current direction of PAH flow between atmosphere and soil. As a control on equilibrium concentrations on the passive samplers, active sampling in air was conducted and sorption behaviour of PAHs in the examined soils was measured. At three sites in the rural surroundings of Tübingen, soil profiles were sampled and seasonal monitoring of PAH concentrations in the atmosphere in the gas phase and bulk deposition was carried out. The individual sites are very comparable in both soil characteristics and in PAH concentrations in atmosphere, deposition and soil. Differences result mainly from the agricultural use at two sites and the associated regular mixing of the plough horizon. Seasonal monitoring in the atmosphere shows a clear correlation of (up to 5-fold) higher PAH concentrations during the indoor heating period in autumn and winter compared to spring and summer. In the atmospheric bulk deposition this seasonal variation is even more pronounced with a factor of 30. Temperature-dependent sorption and lower summer emissions imply temporary fluctuations with short-term outgassing of semi-volatile components (up to pyrene) from soils. Stable PAH distribution patterns at the three sites as well as for the different monitorings indicate a good air mixing and constant source profiles in the investigated region. Constant concentration ratios among all analyzed substances allow the use of a single substance as a reference compound for the calculation of concentrations of the other target substances. Identical distribution patterns were determined for atmospheric particles and within the bulk soil confirming particle-bound deposition of PAHs as the main input into the soil. This results in a continuous flux of PAHs from the atmosphere into the soil, and consequently defines soils overall as long-term sinks for this group of substances.