## 2 Soiling-Related Pollen Topics
### Pollen Particle Size  
Pollen grains exhibit a large interspecific variability concerning their morphological features: size, shape, polarity, apertures, pollen wall structure, ornamentation, coating, etc. [25]. For our purpose, which is considering pollen as a soiling agent, the pollen particle size is the primary morphological characteristic, although more factors will affect the area susceptible to soiling by pollen (by influencing the pollen dispersion, transport, and deposition) and the properties of the resulting soiling layer. These factors, which will be addressed later, are pollen aggregation and disaggregation, pollen fragmentation, pollen hygroscopicity and pollen surface chemistry.  
The pollen particle size depends on the vegetal families, genres, and species. The first consideration is that the individual pollen grains cover a broad size interval. As the pollen grain size fluctuates because of its natural variation, its degree of hydration and its preparation method, it is recommended to categorize it by \"size classes\" according to the length of its longest axis [33]. [PERSON] (1969) [25] stablished the following size classes for sporomorphs (i.e., pollen and spores): very small (\(<\)10 \(\upmu\)m), small (10-24 \(\upmu\)m), medium (25-49 \(\upmu\)m), large (50-99 \(\upmu\)m), very large (100-200 \(\upmu\)m), and gigantic (\(>\)200 \(\upmu\)m). A more recent criteria classifies the pollen size as very small (\(<\)10 \(\upmu\)m), small (10-25 \(\upmu\)m), medium (26-50 \(\upmu\)m), large (51-100 \(\upmu\)m), and very large (\(>\)100 \(\upmu\)m) [33; 34]. Airborne pollen grains are mainly small- or medium-sized, although some of them have large or very large sizes (Figure 1). These exceptions usually correspond to \"saccate pollen\" (i.e., pollen grains having one to three air-filled bladders), which is exclusive of three confer families (i.e., Pinaceae, Podocarpaceae, and Phyllocladaceae) including several conifer genera (e.g., _Pinus_, _Cedrus_, _Picea_, _Abies_, etc.). Such pollen grains can manage wind dispersal because their morphological adaptation compensates for a larger particle size through enhanced aerodynamics [35; 36].  
The second consideration is that not all the vegetal species disperse pollen grains individually. There exists a variety of pollen dispersal units (PDUs) ranging from solitary pollen grains (\"monads\") to massive pollen clumps (\"pollinariums\"), which affects the pollen particle size, shape, mass... Sometimes several pollen grains have common walls: dyads (two grains), pseudomonads (three grains), tetrads (four grains), and multiples of tetrads such as polyads (up to 32 grains), massulae and polinium. In addition, monads, tetrads and other pollen units may further group together through exines coated by viscous fluids (e.g., pollenkitt, primevine matrix, trypphine, elastoviscine) or exines with entangling filaments (i.e., viscin threads) [24; 39]. That said, anemophilous plants, whether _Gymnosperms_ or Angiosperms, are known to disperse solitary monads (e.g., Betulaceae, Urticaceae, Poaceae, etc.) with few exceptions (for instance, Cistaceae and Myrtaceae disperse monads clumped by pollenkitt) [40]. In contrast, most entomophilous plants produce pollen clumps and some of them disperse a range of PDUs [39].  
A rough idea of the relative abundance of the pollen size classes and the PDU classes can be inferred from Figure 2. Among the included vegetal species, the two main PDUs are monads (92.2%) and tetrads (3.4%). In addition, some Angiosperm families have been classified according to their representative PDU classes [41].