## 4 Artificial Soiling Test with Pollen  
### Methodology  
Artificial soiling tests are regarded as a helpful experimental methodology to understand and quantify the soiling issues. Despite having some constraints to replicate real soiling processes,  
Figure 8: Performance of two identical mc-Si modules at tilt angle = 8\({}^{\circ}\); one module was cleaned weekly (“reference PV module”) and the other was kept uncleaned (“soiled PV module”). (**a**) performance ratios calculated from short-circuit measurements and daily airborne pollen load and (**b**) soiling ratios derived from the performance ratios and daily precipitation regime. artificial soiling tests performed under laboratory conditions can provide broad information much faster and under much more controlled experimental parameters than outdoor soiling tests performed under environmental conditions. Up to date, artificial soiling tests performed on glass samples or PV modules have been reported for a large number of pollutants: sand, clay, limestone, red soil, silica, calcium carbonate, carbon, ash, and cement, among others [107]. Nevertheless, the present work is the first publication concerning an artificial soiling test carried out with pollen. Soda lime float glass coupons (approx. \(4\times 4\times 3\) mm) were used as substrates. Their surface areas were accurately measured with a caliper gauge. Sample weighting was performed with a microbalance Sartorius TE64 with a precision of 0.1 mg. Hemispheric transmittance spectra at near-normal incidence were obtained from 300 nm to 1800 nm by means of a Perkin Elmer Lambda(r) 900 spectrophotometer equipped with a Spectralon(r)-coated LabSphere(r) 150 mm integrating sphere. The photometric accuracy of the instrument in transmission mode is \(\pm\)0.08% at absorbance equal to 1 and transmission \(\pm\)0.05% at absorbance equal to 0.5. The hemispherical transmittance was measured instead of the normal transmittance to better replicate the incident-light scattering of the soiled glass cover of a PV module. Hence, the glass coupons were cleaned, measured, and weighted and their transmittance spectra were obtained. Next, they were used as substrates to collect fresh cypress (_Cupressus_ sp.) pollen samples from CIEMAT's gardens. After that, the soiled substrates were weighted and analyzed optically again. Finally, the corresponding pollen mass densities (\(\rho_{\text{A}}\)) were calculated as \(\rho_{\text{A}}\) = \(\Delta\)_M_/_A_, where \(\Delta\)_M_ is the deposited pollen mass and \(A\) is the area of the glass coupon. To improve the sampling representativeness, the set of pollen samples was not prepared by collecting the cypress pollen from a single tree on a particular day, but by collecting it from various trees in different sunny days during the blossoming season. The pollen samples were gathered by selecting cypress specimens with abundant male cones and shaking them manually to release the mature pollen while placing the clean glass coupons close below them, allowing the deposition by sedimentation of soiling layers containing different amounts of pollen. Some of the resulting pollen samples are shown in Figure 9. This sampling procedure imitates, although more vigorously, the natural dispersion of cypress pollen under the wind action. Artificial pollen aggregation was discarded due to the small size, spheroidal shape and non-clumping characteristics of the cypress pollen grains. Next, the pollen samples were carefully transported to the laboratory inside a hermetic box and were analyzed within the first 48 h after collection. In total, 25 pollen samples were selected for the artificial soiling test, while others were discarded because the resulting pollen layer was inhomogeneous or included some vegetal debris.