## 1 Introduction  
According to the increasing number of scientific and technical papers published in recent times [1], the analysis of the soiling loss (i.e., the energy production loss caused by soiling) in photovoltaic (PV) systems has become an important topic of interest within the solar energy community. In particular, there are many works dealing with soiling by mineral dust in arid world regions with abundant solar resource [2; 3]. soiling (i.e., dirt accumulation) is a very complex issue due to the large number of influencing parameters: the geographical location, the PV plant design, the PV module type, the surrounding environment, the specific particles causing soiling, and their seasonal atmospheric loads, etc. All these factors determine the local severity of soiling as well as the maintenance operations required to deal with it [4]. Owing to the outdoor operating conditions, the front glass cover of the PV modules will gradually accumulate dirt at a variable rate [5]. As a result, the built-up soiling layer reduces the glass transmittance by partially absorbing and reflecting the incident sunlight and, consequently, it decreases the PV energy output leading to a soiling loss [5]. Depending upon the location of the PV system, the soiling layer may consist on a variable mixture of organic and inorganic matter (e.g., soot, dust, sand, sea salt, bird droppings, leaves, pollen, etc.) coming from a variety of sources (e.g., air pollution, mineral dust aerosol, marine aerosol, microbiological colonization, etc.). Although several works have analyzed soiling in detail from different perspectives (e.g., efficiency loss, regional and climate dependence, physicochemical mechanisms, involved pollutants,monitoring methods, mitigation strategies, etc.) [5; 6; 7; 8; 9], little attention has been paid to the role of pollen. However, pollen deposition may seasonally become a concern for many PV installations (whether ground-mounted, roof-mounted, or building-integrated systems), potentially causing a significant increase on the local soiling loss. Primary biological aerosols represent a significant fraction of the total atmospheric aerosol load. They arise when some biological processes, being exposed to air movements, generate airborne organisms or propagules. Roughly speaking, these biogenic airborne particles have diameters of less than 0.3 \(\upmu\)m for viruses, 0.25-8 \(\upmu\)m for bacteria, 1-30 \(\upmu\)m for fungal spores, 6-30 \(\upmu\)m for moss spores, 20-60 \(\upmu\)m for fern spores, 17-58 \(\upmu\)m for anemophilous pollen, etc. [10; 11]. Hence, as pollen aerosols are composed by rather large particles, the \"pollen blooms\" causing soiling would not be properly detected through the atmospheric parameters PM2.5 and PM10, requiring alternative monitoring approaches instead, such as consulting aerosol optical depth (AOD) measurements [12; 13] or, even better, consulting real-time local data from aerobiological monitoring stations [14]. The airborne pollen spectrum depends on several factors which influence the production, the release, and the dispersion of pollen. Basically, these factors are related to the geographical location, the vegetation in the area of influence, the time of the year and the weather conditions [15]. In more detail, some important factors are the height of pollen release, the pollen grain aerodynamics (e.g., size, shape, density, surface roughness), the phenology of the local vegetation (i.e., its annual blossoming period) and the local meteorology (e.g., solar radiation, temperature, relative humidity, precipitation, wind direction, wind velocity, air turbulence) [10; 15; 16]. Concerning Europe, recent decades have witnessed a general trend towards rising airborne pollen concentrations; such phenomenon becomes more pronounced in urban areas next to parks, gardens, avenues, etc., being mainly caused by the abundance of tree pollen [17]. With regard to the Iberian Peninsula, the contribution of ornamental trees (_Platanus_ sp., Cupressaceae, _Frazinus_ sp.)