## 3 The Influence of Soiling on Photovoltaic Energy  
### Soiling by Pollen on PV Modules: A Review  
Pollen must be considered as a seasonal, local-scale soiling agent. At certain times of the year, airborne pollen deposition may leave a yellowish blanket over the ground and the outdoor surfaces. Certainly, pollen can build up on PV modules, particularly near the lower edge of the framed modules, affecting the energy conversion process [79]. Pollen typically causes soiling issues in springtime, which is the main blossoming season in most temperate world regions. That is to say, soiling by pollen is a consequence of reaching record-peak levels in the air pollen concentration (\"pollen blooms\"), which mostly occurs during spring, although it can also happen during summer or autumn [7, 12, 54, 80]. Since the production and emission of pollen are governed by interacting environmental factors subjected to fluctuations, the onset, duration and intensity of the period when pollen is significantly present in the air varies from year to year [68]. Further, recent climatic change trends point towards an increasing soiling potential for pollen: in the case of Europe, the airborne pollen concentrations seem to increase and, compared to 50 years ago, the main pollen season currently starts earlier and lasts longer [17, 69]. Particularly, pollen becomes a concern for PV systems installed in geographical areas where high airborne pollen concentrations are periodically reached, such as in the Eastern United States [81, 82]. Regarding Portugal, not only the Saharan dust events but also the pollen blooms were identified as atmospheric intrusions significantly impacting the PV performance [12]. Soiling mostly due to pollen and causing a power reduction of 4.1% was reported during a rainless spring period in southern Portugal (Csa climate) [54]. Pollen is usually mentioned among the soiling agents, particularly in agricultural areas [6, 83, 84, 85]. Some authors have managed to identify the pollen grains of the soiling layers [86, 87, 88, 12, 80]. The determination of the pollen type (and, consequently, also of the pollen source) can be achieved based on the morphological features of the pollen grains through microscopy imaging techniques [7, 12, 54]. In this regard, reference information can be found in pathological databases such as PalDat [34, 89], which is the world's largest one and a free-access online resource. In general terms, soiling losses are reversible: most soiling layers can be removed either by rainfall or by cleaning the PV modules [80, 83, 86]. Concerning rainfall, there is not a well-defined precipitation threshold which guarantees a full restoration of the efficiency of the PV module. For instance, in the Southwestern United States, while [PERSON] et al. [83] reported that 2.5 mm would be enough, [PERSON] et al. [90] suggested the need for more rainfall, being roughly delimited between 5 mm and 20 mm. In addition, it must not be forgotten that stubborn dirt (e.g., bird droppings, exhaust fumes...) can resist heavy rainfall, namely requiring scrubbing, detergents, etc. [91]. In the particular case of soiling by pollen, rainfall has been reported to effectively clean the PV modules, although the registered precipitation was not mentioned [7, 54]. In any case, provided that pollen deposition may cause a significant soiling loss under prolonged rainless conditions, the maintenance regimen of the affected PV systems should include, at least, a cleaning operation of the PV modules after the end of the main pollen season [92]. In addition, an outdoor soiling test was performed in Belgium (Cfb climate) during springtime by placing a series of glass coupons, both sheltered and unshetletered from rain, and by measuring the evolution of their transmittances over time [7]. Concurring with high airborne pollen levels, a significant transmittance loss happened in the sheltered glasses, where large-sized pollen grains (\(\sim\)60 \(\upmu\)m in size) could be observed by scanning electron microscope.