## 1 Introduction
During rainfall, pollen breakdown occurs due to shock or osmotic stress and can cause a severe asthma outbreak in patients affected by pollinosis. At the onset of heavy rains and in conditions of high atmospheric humidity, peaks of high pollen and other aerobiological particle concentrations can occur [13]. Insolation, or hours of sunshine, is related to temperature and photoperiods. Increased sunshine activates growth hormones (gibberellins and cytokins) that regulate flower formation. Once flowering has begun, the increase in the hours of sunshine per day causes an increase in pollen concentration since dehydration of the anther walls occurs, facilitating dehiscence and, consequently, the release of pollen [11]. As for relative humidity, it favors plant development and facilitates flowering, but an increase above a specific threshold prevents another dehiscence and decreases pollen levels in the atmosphere [11]. The wind is responsible for successfully dispersing and transporting pollen grains of anemophilous plants. In general, and within certain limits, pollen concentrations increase with increasing wind speed and decrease with decreasing wind speed. In addition, wind can also favor the re-flotation of already settled pollen [11]. The wind intervenes, together with other agents such as rain, atmospheric pressure and relative humidity, in the emission or release of pollen grains. Wind transports pollen away from the plant, so that the emitted pollen becomes airborne, to a greater or lesser extent depending on the wind speed, together with the action of other factors such as the height of the anther with respect to the surrounding surfaces, atmospheric turbulence, rainfall and the density of the surrounding vegetation. Approximately three-quarters of the pollen emitted becomes airborne on windy days [14]. The solar radiation that plants receive, accumulated over time, also causes the stimulation of biological activity, and, together with insolation, both meteorological variables lead to an increase in pollen concentration values in the atmosphere, even beyond the time of peak pollen emission [12; 13; 15]. Plants can take advantage of diffuse radiation within the visible spectrum of light (what can be called photosynthetic radiation), which they invest in the form of a higher potential for productive growth and bioaerosol emission. The division of global radiation into its components serves as a basis for predicting the photosynthetic productivity of the plant canopy, among many other disciplines [12]. In addition, ultraviolet (UV) radiation, along with drought, salinity, and temperature extremes (apart from atmospheric pollutants), is a climatic stressor and can lead plants to produce higher levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS), with chemical modification of plant proteins, including pollen allergens [13]. The increase in the concentration of CO\({}_{2}\) in the atmosphere and air temperature is altering the seasonality (the pollen season is advancing and/or lengthening in time), the atmospheric concentration and pollen production, which are increasing, as well as the geographical distribution of pollen (plants are shifting their distribution area towards the poles, and it rises to higher and higher altitudes in mountainous areas [16]). Despite its importance for public health, pollen is a very underestimated component of air quality [17]. The increase in allergy cases, its relationship with air pollutants that characterize urban settlements and the relationship with the changing meteorology create a necessity for studies that show the relationship of pollen concentration, both with air pollutants and meteorological variables. In regions of Spain, there are many different types of vegetation, and the values of the main meteorological parameters are very variable, so local studies are necessary [18]. There are few studies on the relationship between meteorology and pollen concentration in the Madrid region [19].