## 4 Artificial Soiling Test with Pollen
### Results
Besides, the cypress pollen exhibited an excellent linear correlation between the mass accumulation and the optical loss, as it could be expected from the near-ideal geometrical characteristics of their PDUs, which are solitary spheroidal monads [34]. Strictly speaking, this empirical soiling curve would be representative of cypress pollen (i.e., the genus _Cupressus_), but due to their botanical similarities, it could presumably represent Cupressaceae pollen in general (e.g., also the genus _Juniperus_, _Thuja_, _Tetraclini_, _Taxodium_, _Callitropsis_, etc.). More significant deviations might arise for other plants dispersing pollen with larger differences on its size, shape, or density. Regarding the particle shape, given the same pollen mass density for a soiling layer, pollen grains with irregular shapes could lead to a different optical behavior due to the formation of a heterogeneous soiling layer (i.e., affecting the particle settling orientation, the particle packing density, etc.). For instance, saccate pollen grains of the genera _Pinus_ and _Cedrus_ underwent preferential orientations during settling experiments [20]. Concerning the diameter (d) and the density (\(\rho\)) of the pollen grains, these differences can be observed when comparing pollen of _Juniperus_ sp. (d = 18-24 \(\mu\)m; \(\rho\)\(\sim\) 640 kg/m\({}^{3}\)), _Pinus_ sp. (d\(\sim\) 50-60 \(\mu\)m; \(\rho\) = 450-560 kg/m\({}^{3}\)), _Platanus_ sp. (d = 19 \(\mu\)m, \(\rho\) = 920 kg/m\({}^{3}\)), and maize (d\(\sim\)75-90 \(\mu\)m; \(\rho\) = 935-1450 kg/m\({}^{3}\)) [13, 113, 114]. In the case of _Cupressus_ sp., pollen grain sizes are known (d = 29-38 \(\mu\)m) [78] but pollen density values have not been published. Beyond estimating the optical loss from a soiling curve based on a \"generic\" pollen mass density, estimating its dependence on the grain size, the grain shape and the grain density would provide a deeper insight into this kind of soiling. Hence, additional artificial soiling tests performed with pollen would be valuable to assess the influence of the pollen type. There also remains the question whether the properties of a soiling layer made of pollen would change significantly in case of a long-term outdoor permanence (e.g., due to moisture, UV radiation, etc.). In any case, we expect that this experimental soiling curve will be helpful in order to interpret the energy loss of the PV systems seasonally soiled by pollen in general. When comparing this soiling curve (Figure 12) with the results of other artificial soiling tests accomplished with different soiling agents, the cypress pollen was found to cause a relatively higher transmittance reduction. For instance, at a dust density of 12 g/m\({}^{2}\), a soda lime glass soiled with desert dust (mostly below 30 \(\mu\)m in size; composed by SiO\({}_{2}\), CaCO\({}_{3}\), etc.) retained a transmittance around 90% (300-1200 nm) and showed a weak wavelength dependence [97]. In contrast, a pollen  
Figure 12: Linear fitting of the averaged transmittance (\(T_{\rm AVE}\)) as a function of the pollen mass density (\(\rho_{\rm A}\)). density of 8.6 g/m\({}^{2}\) reduced the transmittance to around 70% and 40% in the VIS and UV ranges, respectively (Figure 10). Other authors reported that dust mass densities of 10 g/m\({}^{2}\) reduced the transmittance to 99% (white sand, 250 \(\upmu\)m), 90% (clay, 68 \(\upmu\)m), and 81% (cement, 10 \(\upmu\)m) [7]. Interestingly, cypress pollen shows a higher soiling capacity (by extrapolation to \(\rho_{\text{A}}\) = 10 g/m\({}^{2}\), \(T_{\text{AVE}}\) = 61.7%) than cement dust despite a smaller particle size and a stronger VIS absorption were expected for the latter. These transmittance differences made us speculate about other influencing factors. Some technical testing aspects might contribute to such discrepancies (e.g., differences on the optical instrument and the light source, on the measured wavelength range, on the sample manipulation, etc.).