## 5 Conclusions  
Pollen may become a seasonal, local-scale soiling issue affecting the PV performance not only in large agricultural and forest areas, but also in urban areas due to the ornamental vegetation in parks, gardens, tree-lined avenues, etc. Anemophilous (i.e., wind-pollinated) plants have a much higher pollen-related soiling potential than entomophilous (i.e., insect-pollinated) ones. According to the literature and local observations, PV modules soiled by pollen can be effectively washed off by rainfall excepting light rain events and photocatalytic anti-soiling coatings could improve their performance. Pollen blooms can be predicted or detected by consulting the local pollen calendar or real-time data from local pollen monitoring stations, respectively. In recent times, airborne pollen monitoring has been generalized in many populated areas through the air quality control policies. It should be noted that there is not a straightforward correlation between the airborne pollen concentration and the degree of soiling by pollen in order to forecast the energy output. This might be attempted case-by-case through statistical analyses when having enough long-term data concerning both the PV performance and the local aerobiological information. Still, local pollen calendars and local pollen monitoring data can help to mitigate soiling by pollen by planning the maintenance operations of PV installations accordingly. Regarding the rooftop PV system installed in a gardened semi-urban area, significant soiling by cypress pollen was noticed from late winter to early spring. Pollen deposition caused both uniform and non-uniform soiling patterns: while homogeneous soiling was observed during rainless conditions, pollen became accumulated along the bottom edge of the PV modules after a light rain event. Based on our observations, the PV installation was more affected by the presence of a nearby pollen source (i.e., cypresses located at meters or few tens of meters away) than by the vegetation causing record airborne pollen levels at local scale. In addition, the results of an artificial soiling test carried out with pollen have been presented. Pollen samples were prepared by using soda lime float glass coupons as substrates and fresh cypress (_Cupressus_ sp.) pollen as the soiling agent. The hemispheric transmittance measurements of the soiled samples were normalized by the hemispheric transmittance spectra of the clean substrates and subsequently averaged over a wide wavelength range (300-1200 nm) coincident with the spectral response of most PV technologies. Among the results, the averaged transmittance (\(T_{\text{AVE}}\)) decreased gradually with the increase of the pollen mass density within the whole experimental range, following the linear relationship \(T_{\text{AVE}}\) (%) = 99.7-3.8-\(\rho_{\text{A}}\) (g/m\({}^{2}\)) up to a value of \(T_{\text{AVE}}\) = 65.1 % for \(\rho_{\text{A}}\)= 9.1 g/m\({}^{2}\). This experimental soiling curve is expected to help into the interpretation of the PV energy loss in areas seasonally impacted by pollen sedimentation. Interestingly, for a given pollen mass density (\(\rho_{\text{A}}\)), while the glass transmittance remained constant within the VIS range due to non-selective light scattering, a higher transmittance loss occurred in the UV range, seemingly due to intense UV light absorption by the sporopolenin content of the pollen grain exine. This effect, which is extensive to all relevant pollens, might be particularly detrimental for wide-bandgap PV technologies (i.e., with narrow spectral responses). Achieving artificial soiling tests with pollen of more vegetal species, especially from others which also cause high airborne pollen levels, would be desirable to better understand the role of pollen as a soiling agent.