## 1 Introduction  
Precipitation plays a key role in the environment's transport and distribution of radionuclides [1]. Along with dry and wet precipitation, contaminants, including radioactive isotopes, are transported to the earth's surface, which then undergo deposition and bioaccumulation in various biotic and abiotic elements of the environment [2; 3]. Knowledge of the dependence of the level of bioaccumulated substances in selected organisms on the amount of deposition is a key element for the possibility of using such organisms as biomonitors, which is particularly applicable in accidental situations but also in areas where deposition studies are difficult or impossible, as is the case in polar regions, where there are logistical and financial problems of providing continuous monitoring. Therefore, optimal in such cases is the use of descriptive quantitative relationships, through the determination of appropriate coefficients. Over the past 30 years, many studies have highlighted the possibility of using lichens as biomonitors of air quality due to their sensitivity to various environmental factors [4; 5; 6]. Lichens are symbiotic organisms of fungi and algae or cyanobacteria without a root system or a waxy cuticle. Their growth and survival depend on water and nutrients delivered mainly from the atmosphere. Lichens bioaccumulate several elements through a combination of mechanisms, including the physical uptake of particulate matter in the intercellular spaces of the core, extracellular accumulation, and intracellular accumulation [7; 8]. The entire surface of the thallus participates in absorption, allowing elements from both the atmosphere and the substrate to penetrate into the lichen bodies [8; 9]. In this way, lichens can accumulate exceedingly above their physiological requirements [10]. This unique trait makes them excellent collectors of substances deposited from the atmosphere [10; 11; 12]. Lichens are diverse organisms found in extreme conditions around the world. They form the dominant biomass in the Arctic and Antarctic regions. Lichens occupy about 8% of the Earth's land surface, giving them a globally important role in plant ecology [13; 14]. The large surface area of lichen thalli, relative to their mass, is one of the main reasons for their relatively high capacity to accumulate radionuclides or other elements, such as heavy metals [15]. Because they can passively absorb and accumulate contaminants and radionuclides from wet and dry atmospheric deposition through the surface of the thallus, they are considered useful biomonitors of air pollution [2; 4; 5; 6; 16].  
Coefficients are used to describe the bioaccumulation capacity of selected organisms. One of these is the aggregated transfer coefficient (Tag), an empirical coefficient used in predictive models for natural and semi-natural ecosystems. It is defined as the ratio of isotope activity in a sample (organism) to the amount of isotope deposited per unit surface area [17]. While the determination of Tag values may be relatively simple to calculate, their application may require extensive knowledge of the ecosystem [18]. It should be noted that if effective radionuclide decay is considered, the Tag value is not constant, and in some circumstances, Tag may even increase with time. This was confirmed by the observed significant increase in \({}^{137}\)Cs activity in lichens at a very low atmospheric deposition of this isotope in the air after the Fukushima-Daiichi nuclear power plant accident [18].  
The aim of the study was to determine the bioaccumulation efficiency of radioactive isotopes in the tissues of the lichen species _Cladonia uncialis_ through the derivation of the Tag, which can serve as a useful tool for assessing contaminant deposition based solely on the activity measured in the lichen. The study was conducted under controlled conditions to find relationship between environmental deposition levels and accumulation within _C. uncialis_.