# Contribution of Atmospheric Fallout to the Soil-Root-Leaf Transfer of PAHs in Higher Plants
## 1 Introduction  
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous persistent organic pollutants. In rural areas, main anthropogenic sources include biomass burning [1] and traffic-related emissions [2]. While biomass burning for heating purposes deteriorates air quality in the cold seasons, traffic is a year-round pollution source, impacting plants in the vegetation period. It is generally assumed that foliar uptake is the major PAH exposure pathway for higher plants [3]. However, PAHs from the atmosphere can be transported to the soil, providing an additional exposure route for plants [4]. [PERSON] et al. [5] used source appointment calculations to reveal that PAHs sedimented in Chongming wetland (China) came mainly from long-distance atmospheric transportation. While the uptake of PAHs from the soil or the atmosphere is often discussed, these studies mostly address exposure routes separately. Relatively few test systems exist which have been designed to follow PAH uptake simultaneously from the atmosphere and the soil. [PERSON] et al., for example, used a combination of soil-cultured and hydroponic plants to identifying the contributions of root and foliage gaseous/particle uptakes of organic pollutants in indoor environments [6]. As such, oneof the main aims of this study was to present a test system where both pathways can be followed and evaluated. The main question was to quantify how atmospheric PAHs move in the soil-plant system after wet deposition, with special regard to heavier compounds. The accumulation of PAHs in the soil has been widely addressed, as a significant proportion of PAHs remain in the soil, depending on the type of the compound [7]. Being lipophilic molecules, PAHs generally tend to bind strongly to soil particles [8]; soil will then provide a sink to these compounds. The behavior of low molecular weight (LMW) and high molecular weight (HMW) PAHs differs: 4-6-ring PAHs are most likely deposited close to emission sources, while the lower molecular weight compounds are more capable of long-range transport [9]. Also, after reaching the ground, LMW PAHs will be degraded or returned to the atmosphere due to volatilization or microbial biodegradation [10], while HMW PAHs will remain in the soil bound to particles [11]. As PAHs are poorly soluble in water, once deposited in the soil, their downwards migration is limited. They mostly accumulate in the upper layers--according to [PERSON] et al., the upper 10 cm layer is the most affected [12]. However, in the case of highly polluted soils, they might also occur in the deeper soil layers [13]. Slow downwards migration has been experimentally confirmed in the study of [PERSON] et al., based on migration behavior of \({}^{137}\)Cs and \({}^{210}\)Pb\({}_{\text{xs}}\)[14]. In Europe, PAH content of arable soil moves within a very wide range. Levels of 16 EPA PAHs were measured in Polish arable soils with concentrations between 80 and 7264 \(\upmu\)g/kg, although 75% of soil samples showed concentrations below 695 \(\upmu\)g/kg [15]. [PERSON] et al. reported total concentration of 16 PAHs in the range of 55 and 4584 \(\upmu\)g/kg in agricultural soils in Serbia [16]. The no-contamination benchmark of 200 \(\upmu\)g/kg was suggested by [PERSON] [17]. [PERSON] et al. showed a close relationship between PAHs in dustfall and surface soil [18]. In the study of [PERSON] et al., soil absorption contributed to 9.4% of total PAH uptake in tested vegetables, demonstrating that atmospheric dustfall serves as an important source of increasing soil PAH concentrations [19]. Once uptaken, compounds are assumed to be transported within the plant through the plant vascular system, or by diffusion through the cells [20]. Aerial tissues of winter wheat samples were analyzed in the study by [PERSON] et al. [21]. Samples were collected in areas affected by coal combustion and PAH concentrations were measured. The results showed that HMW (6-ring) PAHs originated from atmosphere, while LMW (3-4-ring) PAHs were taken up by the roots. [PERSON] et al.