*3.3.1 Seasonal variability*

From **Figure 8**, it can be observed that the mean ammonium deposition flux was higher during the cold dry season. During the cold dry season, the mean value obtained was 11.29 Kg Ha<sup>−</sup><sup>1</sup> year<sup>−</sup><sup>1</sup> with a maximum value of 15.78 Kg Ha<sup>−</sup><sup>1</sup> year<sup>−</sup><sup>1</sup> which corresponds to sites I and IV (CICEG Bomberos and Instituto Tecnológico de León, located at SE). On the other hand, during the other two seasons, nitrate deposition fluxes were significantly lower. The mean value obtained during warm dry season was 1.08 Kg Ha<sup>−</sup><sup>1</sup> year<sup>−</sup><sup>1</sup> with a maximum of 6.07 Kg Ha<sup>−</sup><sup>1</sup> year<sup>−</sup><sup>1</sup> in site VI (Zoológico located at NE), whereas the mean value registered for rainy season was 0.25 Kg Ha<sup>−</sup><sup>1</sup> year<sup>−</sup><sup>1</sup> with a maximum value of 1.06 Kg Ha<sup>−</sup><sup>1</sup> year<sup>−</sup><sup>1</sup> in site III (IMSS-T21, at the downtown of the city) (**Figure 8**).

The marked seasonality found in ammonium levels suggests two important aspects: first, the dilution effect is important since ammonium levels during the rainy season were considerably lower as a result of frequent and intense rains

**Figure 7.** *Nitrate atmospheric deposition fluxes by land use.*

**Figure 8.** *Ammonium atmospheric deposition fluxes by season.*

occurring during this season in comparison with the other two seasons of the year. Second, the temporality of sources could be important, since the use of agrochemicals can be intensive during the cold dry season, resulting in levels considerably high during this season. This was demonstrated applying Friedman test, whose results confirmed that differences between seasons were significant.
