## 1 Introduction
in urban areas, wild trees (_Quercus_ sp., _Frazinus_ sp.) in forestry-linked areas, and extensive olive crops (_Olea europaea_) in the southern region has been emphasized [18]. The spatial-temporal scale of the aerial transport of pollen is difficult to ascertain. [PERSON] [19] differentiated between short-range transport (i.e., pollen produced at local level which lasts no more than 1 hour in the air and reaches from 100 m to 1 km); medium-range transport (i.e., pollen produced at regional level which lasts about 1 day in the air and reaches from 1 to 100 km) and long-range transport (i.e., pollen produced in remote regions which remains in the air during several days and can travel even continental or intercontinental distances). In any case, as the pollen grains are coarse airborne particles, gravitational settling becomes their major deposition mode [20], which usually limits their atmospheric lifetime to a few hours [21]. As a consequence, from a quantitative point of view, pollen deposition only becomes relevant over the first kilometer(s) from the source [15; 22]. Pollen grains are the male reproductive cells of the seed-bearing plants, which include both _Gymnosperms_ (e.g., pine, fir, cedar, cypress, etc.) and Angiosperms (e.g., plane tree, olive, oak, birch, etc.). The pollen grain protects the inner cytoplasm with an external wall (\"sporoderm\") which usually combines a pectinulose-rich inner layer (\"intine\") and a complex sporopolenin-rich outer layer (\"exine\"), although the latter may be reduced or even absent in some vegetal species [23; 24]. Besides, the outermost part of the exine, which is sometimes referred to as \"sexine\", displays a submicronic sculpturing pattern (\"ornamentation\") which varies depending on the vegetal species: smooth, reticulate (grooved), echinate (spiked), etc. [11; 25]. After mature pollen grains are released by the floral organ, their aerial transport and deposition may include a series of processes: wind dispersion, several dry/wet deposition mechanisms (e.g., sedimentation, turbulent deposition, impaction, rain scavenging, etc. ), and resuspension of dry deposited pollen. Meanwhile, apart from interacting with moisture, wind, and UV radiation, airborne pollen is affected by atmospheric pollution [26]. Pollen can adsorb suspended particulate matter, either inorganic or organic, but mostly belonging to the PM2.5 fraction [27]. With that said,the most common alteration experienced by pollen is the harmmegathic effect: pollen grains undergo minor volume and/or shape changes due to cytoplasmic water uptake/loss before, during and after pollen dispersal. Concerning the aerial transport stage, such changes happen depending on the air humidity, as the water content of the pollen grains fluctuates towards equilibrium [16; 23]. A good number of knowledge areas (e.g., botany, ecology, allergology, atmospheric sciences, etc.) have provided extensive but disperse information about pollen. Here, rather than presenting a global review of the science of pollen, a reference framework for the research on soiling by pollen will be presented. Hence, some particularities of pollen will be introduced because of their implications for soiling in Section 2. Next, the literature published so far reporting PV-related soiling issues caused by pollen will be summarized in Section 3. Moreover, the influence of soiling on PV energy will be introduced in Section 3, where both a review of the reported soiling issues due to pollen and an experimental assessment of PV soiling losses are presented. Then, a preliminary artificial soiling test performed with pollen is described in Section 4. The test, which has been carried out using fresh cypress pollen as soiling agent, has provided an empirical correlation between the mass density of the deposited pollen (\(\rho_{\text{A}}\)) and the remaining optical transmittance of the glass substrate (\(T_{\text{AVE}}\)). This relationship could help to interpret the soiling loss on PV sites undergoing significant pollen deposition.