## 4 Discussion  
The complex interplay between environmental and biological factors affects the relations of pollen with meteorological variables [33; 34]. The increase in ultraviolet (UV) radiation and the induction of spatial and temporal changes in temperature and humidity lead to changes in vegetation patterns (plant cover) and plant physiology (including pollination periods and chemical modifications of plant proteins). In addition, an increase in temperature can lead to a higher concentration of pollen in the atmosphere. The urban environment is particularly sensitive to warming due to the tight sealing of the soil, which causes soil drought, especially in the Mediterranean area [13; 35]. The variability in the pollen concentrations in the atmosphere of the six pollen types studied is very high. This may be due to the maximum rates during the pollination season, which are one month in the case of _Platanus_ and up to five months in the case of Cupressaceae and _Pinus_. Outside these periods, concentrations decrease to zero or almost zero during the rest of the year. Similar results are observed for all 11 stations of the CAM Palinocam Network, and the study period is compared with the 1994-1999 period [31]. At the same time, more recent studies show a trend towards a higher concentration of pollen in the atmosphere, in recent years, of the most abundant tree pollen taxa in the city of Madrid, as well as in the rest of the most populous cities in the Community of Madrid, i.e., Cupressaceae, _Platanus_, _Quercus_, _Olea_ and _Pinus_, among others [19]. In addition, given the interannual variability of meteorological parameters, especially in terms of precipitation (which shows differences in the amount and period of occurrence in the Mediterranean region), as evidenced in other studies [12; 36], it would be interesting to carry out a detailed analysis by year. Flowering and pollination periods depend on meteorological conditions, which influence the pollen concentrations in the atmosphere and the duration of these periods [37]. Other studies show significant and positive correlations with temperature (as for _Olea_, _Pinus_, and _Platanus_, with monthly values, as well as _Populus_, with daily values, in this study, and contrary to the other pollen types) and precipitation (as for _Olea_), as well as negative correlations with wind speed (contrary to this study) [38]. In particular, another dehiscence requires overcoming a specific temperature threshold, associated with solar radiation, for each plant. Moreover, anthesis and pollen emission normally occur when temperatures are highest, i.e., in the middle of the day. For these reasons, the positive correlations found in this study, of both meteorological variables, may support these events, especially since convective phenomena, which facilitate the vertical transport of pollen grains and can prolong increases in pollen concentration in the atmosphere, are favored by maximum temperatures [11]. In general, higher temperatures correlated with earlier and longer pollination seasons and higher average daily pollen concentrations [37]. On the other hand, with regard to wind speed, as mentioned above, the positive correlations obtained with monthly values in this study, with all pollen types except _Olea_, may justify the fact that, generally, and within certain limits, pollen concentrations increase proportionally to the increase in this meteorological variable. In addition, pollen that has already settled can be pushed upwards in the atmosphere by the action of the wind [11]. In addition, increased rainfall can have a short-term effect by causing low pollen concentrations due to the 'washout' effect [39]. Such a phenomenon could explain the negative correlations found in this study. The long-term effects of rainfall varied for trees and weeds [39].