## 3 Results and Discussion
In the collector plates exposed for 15 days, we observed BAF values greater than 1 for nitrate, phosphate, Mg, Ca, Mn, Co, Ni, Cu, and Zn at the MM site; for ammonium, formate, chlorides, nitrate, phosphate, K, Mg, Mn, Co, Ni, Cu, and Zn at the PG site; and finally, for formate, MSA, chlorides, sulfates, bromide, nitrate, phosphate, Na, Mg, K, Cr, Mn, Fe, Co, Cu, Zn, Pb, Ba, and Sr at the TR site. In this case, the highest bioaccumulation values were noted for phosphate (BAF = 7.4) at the MM and PG sites (BAF = 19.6) and for K at the TR site (BAF = 10.1) (Table 3, Figure S2). If the organism is taken from external sources, the constituent elements previously accumulated in its tissues will be added to those that are present in the ecosystem investigated. Therefore, the use of the BAF index could lead to mistakes in the interpretation of the results and a different index should be applied [44]. Since olive pollen presents some non-zero elements and ion concentrations (Table 1), in 2021, we also estimated the bioaccumulation index over time (BAIt) [44]. The BAIT index allowed us to determine the relative increase in the concentration of a given element or ion with respect to its initial concentration in pollen and interpret the temporal dependence of the pollen exposure to pollutants (Table 4, Figure S3). BAIt values between 0 and 1, which indicate a low bioaccumulation level, were detected after 7 days for Ca, sulphate, and Cr at the MM site; for ammonium, Ca, sulphate, and Cr at the PG site; and sulphate and nitrate at the TR site. Instead, after 15 days, we observed BAIt values between 0 and 1 for Na at the MM and PG sites; for chloride at the PG site; for sulphate at the PG and TR sites; for nitrate at the TR site; and finally, for Cr at the PG site. BAIt values greater than 1 indicate that the relative concentration of the given element in pollen grains increased during the experiment and therefore bioaccumulation occurred. We observed BAIt > 1 for nitrate and Mn at the MM site; for Ca, nitrate, and Mn at the PG site; and Ca, Cr, and Mn at the TR site after 7 days of exposure. On the other hand, after 15 days of exposure, we found BAIT values greater than 1 for Ca and nitrate and Mn at the MM and PG sites and for Ca, Cr, and Mn at the TR site. We also observed negative BAIt values for all the other analytes, which indicated that the final concentration of a given element or ion in the pollen was lower than the initial value, meaning that a given element or ion was transferred from the pollen to the atmosphere. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Monte Martano**} & \multicolumn{5}{c}{**Perugia**} & \multicolumn{5}{c}{**Terni**} \\ \hline
**Days** & **7** & **15** & **Mean** & **SD** & **7** & **15** & **Mean** & **SD** & **7** & **15** & **Mean** & **SD** \\ \hline
**Na\({}^{*}\)** & \(-0.2\) & \(0.5\) & \(\mathbf{0.1}\) & \(\mathbf{0.3}\) & \(0.1\) & \(0.6\) & \(\mathbf{0.3}\) & \(\mathbf{0.3}\) & \(-0.2\) & \(-0.0  
In summary, we observed BAIt values between 0 and 1 or greater than 1, which indicated bioaccumulation in pollen samples, only for some ions and element among those already present in pollen. These were sulphates, nitrates, ammonium (typical urban pollutants), Cr and Mn, (industrial and steel mill contaminants), and the crustal elements Na and Ca. We did not detect any bioaccumulation of potassium--the main element in freshly collected pollen that is considered a co-inductor or adjuvant of allergenicity [11; 12], nor chloride, which is different from the results of previous studies [12]. Furthermore, significant differences were found between the mean BAIT values observed in the three sites for Ca (\(F\) = 231.6, 2, \(p\) < 0.001), Cr (\(F\) = 227.1, 2, \(p\) < 0,001), and Mn (\(F\) = 26.5, 2, \(p\) = 0.012).