**4. Results and discussion**

#### **4.1. Sulfate deposition fluxes**

#### *4.1.1. By season*

the analysis of meteorological parameters at surface level was done by using data obtained from SIMA during the study period to identify possible anthropogenic or natural sources influencing the N and S levels found in the sampling sites. Wind roses were built to identify the prevailing wind direction in the study area. To assess the transport mechanism controlling deposition process in the study area by season, back air mass trajectories were estimated by the Lagrangian hybrid model HYSPLIT (Hybrid Single-Particle Lagrangian Integrated

Database for the entire study period for each sampling point was obtained from SIMA of

tion roses were estimated for each air pollutant and for each sampling point by climatic season to identify if daily concentrations exceeded reference values someday. These concentration roses were useful to visualize in which wind direction there were higher concentrations and,

A Friedman test was used to determine if atmospheric deposition fluxes were different among sampling sites, according to land use or between climatic seasons. Friedman test is a non-parametric test that can be used with block design, in which the underlying assumptions are not as restrictive as those of an ANOVA procedure (XLStat v.2017). On the other hand, principal components analysis is a technique used to reduce the dimensionality of a data set. The projection according to which data is better represented is least squares. It converts a data set of variables possibly correlated in a data set of variables without lineal correlation called principal components. Descriptive, multivariate, and principal components analysis

One of the main uses of geo-statistical mapping consists in predicting new values from variables from the sample in a given area, which is referred as spatial prediction or spatial interpolation. Spatial distribution of a variable can be modeled either using a continuous model or a discrete or mixed model. On the other hand, temporal variability makes geo-statistical mapping expensive and complex. Taking into account that the seasonal periodicity in this work is regular for the studied environmental parameters, in this case, spatial variability was analyzed for each climatic period: Dry, rainy, and cold fronts or Norths seasons. The coordinates of each sampling site and the values for N and S deposition fluxes were the inputs used to derive the specific points in the maps showing the dispersion and the measured concentration for the different studied chemical compounds. In a second step, the concentrations at neighboring sampling points within the grid were averaged to attribute a value to the point. These points were the input for the interpolation procedure [14]. The deposition contours were smoothed by using the kriging method [15]. Kriging weights were estimated from a

, PM10 y PM2.5. From the obtained data, concentra-

Trajectory) from US NOAA (National Atmospheric and Oceanic Administration).

, NO<sup>2</sup>

80 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

then, to identify the possible sources contributing to these levels.

, SO<sup>2</sup> , O<sup>3</sup>

*3.3.2. Criteria air pollutants concentration*

were carried out by using XLstat-Pro v. 2017.

*3.3.4. Deposition fluxes mapping*

Monterrey for: CO, NO, NO<sup>x</sup>

*3.3.3. Statistical analysis*

The mean S deposition flux (as sulfate) during the dry season was 27.30 ± 10.34 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , with a maximum value of 47.69 Kg ha−<sup>1</sup> yr.−<sup>1</sup> in the site labeled as VI (Obispado) at the center of MAM. The average value obtained for the rainy season was 23.65 ± 4.14 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , with a maximum of 28.63 Kg ha−<sup>1</sup> yr.−<sup>1</sup> in site I (Escobedo) located to the north of MAM. On the other hand, the mean value for S deposition flux during the Norths season was 24.15 ± 7.39 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , with a maximum value of 31.48 Kg ha−<sup>1</sup> yr.−<sup>1</sup> in the sampling site labeled as V (Apodaca), located at the northeast side of MAM. From **Figure 2a**, it was observed that S deposition fluxes showed an evident seasonality, with the highest values during the dry season, and with the lowest values along the rainy season. However, from Friedman test, since p value is major than significance levels (α = 0.05), null hypothesis (H<sup>0</sup> ) cannot be rejected; therefore, it can be concluded that there were no significant differences among S deposition fluxes by climatic season and that sulfate deposition levels have an evident influence from regional transport during all year.

#### *4.1.2. By sampling site*

In the analysis by sampling site, a mean value for S deposition flux of 25.03 ± 7.63 Kg ha−<sup>1</sup> yr.−<sup>1</sup> was obtained. According to **Figure 2b**, it can be observed that S deposition fluxes were higher in the sampling sites labeled as VI and V, which correspond to Obispado and Apodaca at the center and northeast of MAM. By applying Friedman test, p value is major than significance level (α = 0.05), and null hypothesis cannot be rejected. Therefore, it can be concluded that there were not significant differences in S deposition fluxes among sampling sites, suggesting an evident regional influence on MAM.

#### *4.1.3. By land use*

Sampling sites were grouped depending on their land use as: Rural (sites II and VII), Urban (sites I, IV, VI, VIII, IX and X), and Industrial (sites III and V). From **Figure 2c**, it can be observed that S deposition fluxes were higher at sites with an industrial land use (sites III and V), which correspond to San Bernabé and Apodaca, located to the northwest and northeast of MAM. A Friedman test was applied, and since p value is major than

significance level (α = 0.05), the null hypothesis cannot be rejected, and it can be concluded that S deposition fluxes did not show significant differences by land use. This fact supports those found in the previous sections, where the regional character of sulfate due to

Atmospheric N and S Deposition Fluxes in the Metropolitan Area of Monterrey, Mexico and Its…

The mean nitrate deposition flux value during the dry season was 3.30 ± 1.43 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

located to the southwest of MAM. The average value obtained for rainy season was

On the other hand, during the cold fronts season, the mean value for nitrate deposition flux

located to the southeast of MAM. From **Figure 2d**, it can be observed that nitrate deposition fluxes were higher along the rainy season. From Friedman test, it was found that p value is minor than significance level (α = 0.05); therefore, it can be concluded that there were significant differences in nitrate deposition fluxes between rainy season and the rest of the year (dry and cold fronts seasons). It suggests that reactions in aqueous phase can be important,

From the analysis of results by sampling site, from **Figure 2e**, it was found that nitrate deposition fluxes were higher in the sites VIII and III: San Pedro to the southwest and San Bernabé to the northwest of MAM. By applying a Friedman test, it was found that p value was minor than significance level (α = 0.05); therefore, null hypothesis must be rejected and it is concluded that there were significant differences between sites. It means that the influence of

Sampling sites were grouped according to their land use as: Rural (sites II and VII), Urban (sites I, IV, VI, VIII, IX and X), and Industrial (sites III and V). From **Figure 2f**, it can be observed that nitrate deposition fluxes were higher in sampling sites with an urban land use (sites VIII and IX: San Pedro and La Pastora, located to the southwest and southeast of MAM). However, considering extreme values, these were found in sites with an industrial land use (most of the sites: IV, VI, VII, IX, and X). From **Figure 2f**, a great variability was observed, suggesting that local urban sources were mixed and emissions presented different magnitudes. It agrees with the different kinds of sources (industrial and urban) coexisting in this great metropolitan area. In spite of this, from Friedman test, it was found that p value was major than the significance level (α = 0.05); thus, the null hypothesis cannot be rejected, and therefore, it can be concluded

that there were not significant differences between sampling sites by land use.

local sources was important. It agrees with the residence time of NO<sup>2</sup>

it has been reported that nitrate is a local pollutant.

, with a maximum value of 3.52 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

corresponding to the sampling site VIII (San Pedro)

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, with the highest value (7.39) at the sampling site VIII (San Pedro).

,

83

at site X (Juárez)

in the atmosphere, since

was completely evident.

residence time of SO<sup>2</sup>

6.54 ± 0.58 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

at the same time.

*4.2.3. By land use*

*4.2.2. By sampling site*

was 3.26 ± 0.21 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

*4.2.1. By season*

**4.2. Nitrate deposition fluxes**

with a maximum of 4.38 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

**Figure 2.** Sulfate deposition fluxes by: (a) climatic season, (b)sampling site, and (c) land use for MAM during the study period; Nitrate deposition fluxes by: (d) climatic season, (e) sampling site, and (f) land use for MAM during the study period; Ammonium deposition fluxes by: (g) climatic season, (h) sampling site, and (i) land use for MAM during the study period.

significance level (α = 0.05), the null hypothesis cannot be rejected, and it can be concluded that S deposition fluxes did not show significant differences by land use. This fact supports those found in the previous sections, where the regional character of sulfate due to residence time of SO<sup>2</sup> was completely evident.

### **4.2. Nitrate deposition fluxes**

#### *4.2.1. By season*

The mean nitrate deposition flux value during the dry season was 3.30 ± 1.43 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , with a maximum of 4.38 Kg ha−<sup>1</sup> yr.−<sup>1</sup> corresponding to the sampling site VIII (San Pedro) located to the southwest of MAM. The average value obtained for rainy season was 6.54 ± 0.58 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , with the highest value (7.39) at the sampling site VIII (San Pedro). On the other hand, during the cold fronts season, the mean value for nitrate deposition flux was 3.26 ± 0.21 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , with a maximum value of 3.52 Kg ha−<sup>1</sup> yr.−<sup>1</sup> at site X (Juárez) located to the southeast of MAM. From **Figure 2d**, it can be observed that nitrate deposition fluxes were higher along the rainy season. From Friedman test, it was found that p value is minor than significance level (α = 0.05); therefore, it can be concluded that there were significant differences in nitrate deposition fluxes between rainy season and the rest of the year (dry and cold fronts seasons). It suggests that reactions in aqueous phase can be important, at the same time.

#### *4.2.2. By sampling site*

From the analysis of results by sampling site, from **Figure 2e**, it was found that nitrate deposition fluxes were higher in the sites VIII and III: San Pedro to the southwest and San Bernabé to the northwest of MAM. By applying a Friedman test, it was found that p value was minor than significance level (α = 0.05); therefore, null hypothesis must be rejected and it is concluded that there were significant differences between sites. It means that the influence of local sources was important. It agrees with the residence time of NO<sup>2</sup> in the atmosphere, since it has been reported that nitrate is a local pollutant.

#### *4.2.3. By land use*

**Figure 2.** Sulfate deposition fluxes by: (a) climatic season, (b)sampling site, and (c) land use for MAM during the study period; Nitrate deposition fluxes by: (d) climatic season, (e) sampling site, and (f) land use for MAM during the study period; Ammonium deposition fluxes by: (g) climatic season, (h) sampling site, and (i) land use for MAM during the study period.

82 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

Sampling sites were grouped according to their land use as: Rural (sites II and VII), Urban (sites I, IV, VI, VIII, IX and X), and Industrial (sites III and V). From **Figure 2f**, it can be observed that nitrate deposition fluxes were higher in sampling sites with an urban land use (sites VIII and IX: San Pedro and La Pastora, located to the southwest and southeast of MAM). However, considering extreme values, these were found in sites with an industrial land use (most of the sites: IV, VI, VII, IX, and X). From **Figure 2f**, a great variability was observed, suggesting that local urban sources were mixed and emissions presented different magnitudes. It agrees with the different kinds of sources (industrial and urban) coexisting in this great metropolitan area. In spite of this, from Friedman test, it was found that p value was major than the significance level (α = 0.05); thus, the null hypothesis cannot be rejected, and therefore, it can be concluded that there were not significant differences between sampling sites by land use.

### **4.3. Ammonium deposition fluxes**

#### *4.3.1. By season*

The mean ammonium deposition flux during the dry season was 6.90 ± 3.88 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , with a maximum value of 13.31 Kg ha−<sup>1</sup> yr.−<sup>1</sup> in the sampling site labeled as VII (Santa Catarina) located to the southwest of MAM. During the rainy season, the average of ammonium deposition flux was 2.21 ± 1.49 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , with a maximum of 4.08 Kg ha−<sup>1</sup> yr.−<sup>1</sup> in site X (Juárez) located to the southeast of MAM. Finally, during the cold fronts season, ammonium deposition fluxes presented a mean value of 7.14 ± 3.49 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , with a peak value of 14.04 Kg ha−<sup>1</sup> yr.−<sup>1</sup> in the sampling site III (San Bernabé) at the northwest side of MAM. From **Figure 2g**, it is observed that, ammonium deposition fluxes were higher during the dry and cold fronts seasons. Ammonium levels were significantly lower during the rainy season, suggesting a washing effect during this season. From Friedman test, it was found that p value was minor than the significance level (α = 0.05), and the null hypothesis can be rejected; therefore, it is possible to conclude that there were significant differences between dry and cold fronts seasons and the rainy season. It suggests that, during the rainy season, a dilution effect could influence the ammonium deposition fluxes, considering that during the rest of the year, rains are scarce in MAM.

#### *4.3.2. By sampling site*

From **Figure 2h**, analyzing ammonium deposition fluxes by sampling site, the highest value was found in the sites VII and IV (Santa Catarina and San Nicolás), located to the southwest and northeast of MAM. Applying a Friedman test, it was found that p value is major than significance level (α = 0.05), and therefore, the null hypothesis cannot be rejected. Then, it can be concluded that there were not significant differences among sampling sites.

**Figure 3b** shows that PM10 levels (≥75 μg m−<sup>3</sup>

From **Figure 4a** and **b**, it can be observed that O<sup>3</sup>

came from the East. PM10 levels (≥75 μg m−<sup>3</sup>

along the year. Both O<sup>3</sup>

(**Figure 3d**), its levels (≥75 μg m−<sup>3</sup>

season, (f) wind cold fronts season.

(75 μg m−<sup>3</sup>

*4.4.2. Site II Garcia*

>7.9 m s−<sup>1</sup>

value for 24 hours (75 μg m−<sup>3</sup>

Northeast component.

) exceeded the reference value for 24 hours

levels (0.074–0.095 ppm) were higher during

) were high during all year, exceeding reference

) [17], mainly when winds came from Northeast (**Figure 4c** and **d**).

and PM10 showed highest levels when winds had an East-

) [17] for 24 hours

dry

85

along the year.

) [17]. In **Figure 3c**, it can be observed that PM10 levels during the cold fronts season

) exceeded the reference value (45 μg m−<sup>3</sup>

Atmospheric N and S Deposition Fluxes in the Metropolitan Area of Monterrey, Mexico and Its…

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exceeded the reference value but winds also showed a great variability. In the case of PM2.5

**Figure 3.** Criteria air pollutants and meteorological conditions for site I (Escobedo) during the study period: (a) O<sup>3</sup>

season, (b) PM10 dry, and rainy seasons, (c) PM10 cold fronts season, (d) PM2.5 cold fronts season, (e) wind dry and rainy

during the cold fronts season when winds showed a great variability. Finally, winds came from the East during dry and rainy seasons (**Figure 3e**), showing a great variability during

the dry and rainy seasons, exceeding the reference value for 8 hours (70 ppb) [16] when wind

Finally, winds came from the East-Northeast during dry and rainy seasons (**Figure 4e**), showing a great variability during the cold fronts season (**Figure 4f**) with maximum wind speeds

the cold fronts season (**Figure 3f**) with maximum wind speeds >7.9 m s−<sup>1</sup>

#### *4.3.3. By land use*

Sampling sites were grouped according to their land use as: Rural (sites II and VII), Urban (sites I, IV, VI, VIII, IX, and X), and Industrial (sites III and V). From **Figure 2i**, it can be observed that ammonium deposition fluxes were higher in sampling sites with an industrial and urban land use. The emission of amines and NH3 has been reported from vehicles (with the presence of a catalytic convertor that has enough stored hydrogen), where NO is reduced to NH<sup>3</sup> , and deposited as NH4 + . Therefore, vehicular emissions could have an important influence on ammonium deposition in MAM. According to the Friedman test, p value was major than the significance level, thus null hypothesis cannot be rejected, and therefore, it can be concluded that there were not significant differences among sampling sites considering their land use.

#### **4.4. Meteorology and criteria air pollutants**

#### *4.4.1. Site I Escobedo*

O3 showed a strong seasonal variation (**Figure 3a**) with the highest values during the dry season, (0.074–0.095 ppm) exceeding the reference value for 8 hours (70 ppb) [16]. Both, O<sup>3</sup> and PM10 showed highest values when wind direction came from the East. In the case of PM10, Atmospheric N and S Deposition Fluxes in the Metropolitan Area of Monterrey, Mexico and Its… http://dx.doi.org/10.5772/intechopen.79484 85

**Figure 3.** Criteria air pollutants and meteorological conditions for site I (Escobedo) during the study period: (a) O<sup>3</sup> dry season, (b) PM10 dry, and rainy seasons, (c) PM10 cold fronts season, (d) PM2.5 cold fronts season, (e) wind dry and rainy season, (f) wind cold fronts season.

**Figure 3b** shows that PM10 levels (≥75 μg m−<sup>3</sup> ) exceeded the reference value for 24 hours (75 μg m−<sup>3</sup> ) [17]. In **Figure 3c**, it can be observed that PM10 levels during the cold fronts season exceeded the reference value but winds also showed a great variability. In the case of PM2.5 (**Figure 3d**), its levels (≥75 μg m−<sup>3</sup> ) exceeded the reference value (45 μg m−<sup>3</sup> ) [17] for 24 hours during the cold fronts season when winds showed a great variability. Finally, winds came from the East during dry and rainy seasons (**Figure 3e**), showing a great variability during the cold fronts season (**Figure 3f**) with maximum wind speeds >7.9 m s−<sup>1</sup> along the year.

#### *4.4.2. Site II Garcia*

**4.3. Ammonium deposition fluxes**

maximum value of 13.31 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

presented a mean value of 7.14 ± 3.49 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

84 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

land use. The emission of amines and NH3

+

**4.4. Meteorology and criteria air pollutants**

was 2.21 ± 1.49 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

*4.3.2. By sampling site*

*4.3.3. By land use*

and deposited as NH4

*4.4.1. Site I Escobedo*

O3

The mean ammonium deposition flux during the dry season was 6.90 ± 3.88 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

fluxes, considering that during the rest of the year, rains are scarce in MAM.

be concluded that there were not significant differences among sampling sites.

to the southwest of MAM. During the rainy season, the average of ammonium deposition flux

, with a maximum of 4.08 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

the southeast of MAM. Finally, during the cold fronts season, ammonium deposition fluxes

sampling site III (San Bernabé) at the northwest side of MAM. From **Figure 2g**, it is observed that, ammonium deposition fluxes were higher during the dry and cold fronts seasons. Ammonium levels were significantly lower during the rainy season, suggesting a washing effect during this season. From Friedman test, it was found that p value was minor than the significance level (α = 0.05), and the null hypothesis can be rejected; therefore, it is possible to conclude that there were significant differences between dry and cold fronts seasons and the rainy season. It suggests that, during the rainy season, a dilution effect could influence the ammonium deposition

From **Figure 2h**, analyzing ammonium deposition fluxes by sampling site, the highest value was found in the sites VII and IV (Santa Catarina and San Nicolás), located to the southwest and northeast of MAM. Applying a Friedman test, it was found that p value is major than significance level (α = 0.05), and therefore, the null hypothesis cannot be rejected. Then, it can

Sampling sites were grouped according to their land use as: Rural (sites II and VII), Urban (sites I, IV, VI, VIII, IX, and X), and Industrial (sites III and V). From **Figure 2i**, it can be observed that ammonium deposition fluxes were higher in sampling sites with an industrial and urban

ence of a catalytic convertor that has enough stored hydrogen), where NO is reduced to NH<sup>3</sup>

ammonium deposition in MAM. According to the Friedman test, p value was major than the significance level, thus null hypothesis cannot be rejected, and therefore, it can be concluded that there were not significant differences among sampling sites considering their land use.

showed a strong seasonal variation (**Figure 3a**) with the highest values during the dry sea-

PM10 showed highest values when wind direction came from the East. In the case of PM10,

son, (0.074–0.095 ppm) exceeding the reference value for 8 hours (70 ppb) [16]. Both, O<sup>3</sup>

in the sampling site labeled as VII (Santa Catarina) located

, with a peak value of 14.04 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

has been reported from vehicles (with the pres-

. Therefore, vehicular emissions could have an important influence on

, with a

in the

,

and

in site X (Juárez) located to

*4.3.1. By season*

From **Figure 4a** and **b**, it can be observed that O<sup>3</sup> levels (0.074–0.095 ppm) were higher during the dry and rainy seasons, exceeding the reference value for 8 hours (70 ppb) [16] when wind came from the East. PM10 levels (≥75 μg m−<sup>3</sup> ) were high during all year, exceeding reference value for 24 hours (75 μg m−<sup>3</sup> ) [17], mainly when winds came from Northeast (**Figure 4c** and **d**). Finally, winds came from the East-Northeast during dry and rainy seasons (**Figure 4e**), showing a great variability during the cold fronts season (**Figure 4f**) with maximum wind speeds >7.9 m s−<sup>1</sup> along the year. Both O<sup>3</sup> and PM10 showed highest levels when winds had an East-Northeast component.

**Figure 4.** Criteria air pollutants and meteorological conditions for site II (García) during the study period: (a) O<sup>3</sup> dry season, (b) O<sup>3</sup> rainy season, (c) PM10 dry and rainy seasons, (d) PM10 cold fronts season, (e) wind dry and rainy season, (f) wind cold fronts season.

(≥75 μg m−<sup>3</sup>

dry and rainy season, (b) O<sup>3</sup>

and rainy seasons, (f) wind cold fronts season.

the year. Both O<sup>3</sup>

(75 μg m−<sup>3</sup>

O3

*4.4.5. Site V Apodaca*

levels (≥44 μg m−<sup>3</sup>

[16]. PM10 levels (≥75 μg m−<sup>3</sup>

From **Figure 7a**, it can be observed that O<sup>3</sup>

from North, exceeding the reference value (45 μg m−<sup>3</sup>

respectively (**Figure 7e** and **f**), with maximum wind speeds >7.9 m s−<sup>1</sup>

) also exceeded the reference value for 24 hours (75 μg m−<sup>3</sup>

winds came from East and North, but showing highest values and a great variability in wind direction during the cold fronts season (**Figure 6d–f**). Finally, winds came from the North during dry season and from East during the rainy season (**Figure 6g** and **h**), showing a great variability during the cold fronts season (**Figure 6i**) with maximum wind speeds >7.9 m s−<sup>1</sup>

**Figure 5.** Criteria air pollutants and meteorological conditions for site III (San Bernabé) during the study period: (a) O<sup>3</sup>

season when winds came from Northeast, exceeding the reference value for 8 hours (70 ppb)

came from the Northwest and North during dry and wet (rainy and cold fronts) seasons,

, PM10, and PM2.5 showed highest levels when winds had a North-Northwest component.

) [17]; mainly when winds came from North and Northwest (**Figure 7b** and **c**). PM2.5

) were high during rainy and cold fronts seasons when wind direction was

and PM10 showed highest levels when winds had a North and East component.

cold fronts season, (c) PM10 dry and rainy seasons, (d) PM10 cold fronts season, (e) wind dry

Atmospheric N and S Deposition Fluxes in the Metropolitan Area of Monterrey, Mexico and Its…

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87

levels (≥0.095 ppm) were higher during the dry

) [17] for 24 hours (**Figure 7d**). Winds

along these seasons.

) were high during all year, exceeding reference value for 24 hours

) [17] during all year when

along

#### *4.4.3. Site III San Bernabe*

O3 levels (0.074–0.095 ppm) were higher during all year, exceeding the reference value for 8 hours (70 ppb) [16] when winds came from East-Southeast (**Figure 5a** and **b**). In addition, PM10 levels (≥75 μg m−<sup>3</sup> ) also exceeded the reference value for 24 hours (75 μg m−<sup>3</sup> ) [17] during all year, but showing highest values and a great variability in wind direction during the cold fronts season (**Figure 5c** and **d**). Finally, winds came from the East-Southeast during dry and rainy seasons (**Figure 5e**), showing a great variability during the cold fronts season (**Figure 5f**) with maximum wind speeds >7.9 m s−<sup>1</sup> along the year. Both O<sup>3</sup> and PM10 showed highest levels when winds had an East-Southeast component.

#### *4.4.4. Site IV San Nicolas*

O3 levels (0.074–0.095 ppm) were high during all year, exceeding the reference value for 8 hours (70 ppb) [16] when winds came from North and East (**Figure 6a–c**). In addition, PM10 levels Atmospheric N and S Deposition Fluxes in the Metropolitan Area of Monterrey, Mexico and Its… http://dx.doi.org/10.5772/intechopen.79484 87

**Figure 5.** Criteria air pollutants and meteorological conditions for site III (San Bernabé) during the study period: (a) O<sup>3</sup> dry and rainy season, (b) O<sup>3</sup> cold fronts season, (c) PM10 dry and rainy seasons, (d) PM10 cold fronts season, (e) wind dry and rainy seasons, (f) wind cold fronts season.

(≥75 μg m−<sup>3</sup> ) also exceeded the reference value for 24 hours (75 μg m−<sup>3</sup> ) [17] during all year when winds came from East and North, but showing highest values and a great variability in wind direction during the cold fronts season (**Figure 6d–f**). Finally, winds came from the North during dry season and from East during the rainy season (**Figure 6g** and **h**), showing a great variability during the cold fronts season (**Figure 6i**) with maximum wind speeds >7.9 m s−<sup>1</sup> along the year. Both O<sup>3</sup> and PM10 showed highest levels when winds had a North and East component.

#### *4.4.5. Site V Apodaca*

dry

) [17] during

and PM10 showed highest

**Figure 4.** Criteria air pollutants and meteorological conditions for site II (García) during the study period: (a) O<sup>3</sup>

 levels (0.074–0.095 ppm) were higher during all year, exceeding the reference value for 8 hours (70 ppb) [16] when winds came from East-Southeast (**Figure 5a** and **b**). In addition,

all year, but showing highest values and a great variability in wind direction during the cold fronts season (**Figure 5c** and **d**). Finally, winds came from the East-Southeast during dry and rainy seasons (**Figure 5e**), showing a great variability during the cold fronts season (**Figure 5f**)

 levels (0.074–0.095 ppm) were high during all year, exceeding the reference value for 8 hours (70 ppb) [16] when winds came from North and East (**Figure 6a–c**). In addition, PM10 levels

rainy season, (c) PM10 dry and rainy seasons, (d) PM10 cold fronts season, (e) wind dry and rainy season,

) also exceeded the reference value for 24 hours (75 μg m−<sup>3</sup>

along the year. Both O<sup>3</sup>

season, (b) O<sup>3</sup>

O3

O3

(f) wind cold fronts season.

*4.4.3. Site III San Bernabe*

PM10 levels (≥75 μg m−<sup>3</sup>

*4.4.4. Site IV San Nicolas*

with maximum wind speeds >7.9 m s−<sup>1</sup>

levels when winds had an East-Southeast component.

86 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

From **Figure 7a**, it can be observed that O<sup>3</sup> levels (≥0.095 ppm) were higher during the dry season when winds came from Northeast, exceeding the reference value for 8 hours (70 ppb) [16]. PM10 levels (≥75 μg m−<sup>3</sup> ) were high during all year, exceeding reference value for 24 hours (75 μg m−<sup>3</sup> ) [17]; mainly when winds came from North and Northwest (**Figure 7b** and **c**). PM2.5 levels (≥44 μg m−<sup>3</sup> ) were high during rainy and cold fronts seasons when wind direction was from North, exceeding the reference value (45 μg m−<sup>3</sup> ) [17] for 24 hours (**Figure 7d**). Winds came from the Northwest and North during dry and wet (rainy and cold fronts) seasons, respectively (**Figure 7e** and **f**), with maximum wind speeds >7.9 m s−<sup>1</sup> along these seasons. O3 , PM10, and PM2.5 showed highest levels when winds had a North-Northwest component.

**Figure 7.** Criteria air pollutants and meteorological conditions for site V (Apodaca) during the study period: (a) O<sup>3</sup>

**Figure 8.** Criteria air pollutants and meteorological conditions for site VI (Obispado) during the study period: (a) O<sup>3</sup>

fronts season, (e) wind dry and rainy season, (f) wind cold fronts season.

rainy and cold fronts seasons, (b) PM10 dry, rainy and cold fronts seasons, (c) PM2.5 dry and rainy seasons, (d) PM2.5 cold

season, (f) wind rainy and cold fronts seasons.

season, (b) PM10 dry season, (c) PM10 rainy and cold fronts seasons, (d) PM2.5 rainy and cold fronts seasons, (e) wind dry

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dry,

**Figure 6.** Criteria air pollutants and meteorological conditions for site IV (San Nicolás) during the study period: (a) O<sup>3</sup> dry season, (b) O<sup>3</sup> rainy season, (c) O<sup>3</sup> cold fronts season, (d) PM10 dry season, (e) PM10 rainy season, (f) PM10 cold fronts season, (g) wind dry season, (h) wind rainy season, (i) wind cold fronts season.

#### *4.4.6. Site VI Obispado*

From **Figure 8a**, it can be observed that O<sup>3</sup> levels (≥0.095 ppm) were higher during the dry season when winds came from Northeast, exceeding the reference value for 8 hours (70 ppb) [16]. PM10 levels (≥75 μg m−<sup>3</sup> ) were high during all year, exceeding reference value for 24 hours (75 μg m−<sup>3</sup> ) [17], mainly when winds came from Northeast and Southwest (**Figure 8b**). PM2.5 levels were ≥44 μg m−<sup>3</sup> during all year when wind direction was from Northeast, exceeding the reference value (45 μg m−<sup>3</sup> ) [17] for 24 hours (**Figure 8c** and **d**). In addition, winds came from the Northeast during all year, showing a great variability during cold fronts season (**Figure 8e** and **f**), with maximum wind speeds >7.9 m s−<sup>1</sup> along this season. O<sup>3</sup> , PM10, and PM2.5 showed highest levels when winds had a Northeast component.

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**Figure 7.** Criteria air pollutants and meteorological conditions for site V (Apodaca) during the study period: (a) O<sup>3</sup> dry season, (b) PM10 dry season, (c) PM10 rainy and cold fronts seasons, (d) PM2.5 rainy and cold fronts seasons, (e) wind dry season, (f) wind rainy and cold fronts seasons.

**Figure 6.** Criteria air pollutants and meteorological conditions for site IV (San Nicolás) during the study period: (a) O<sup>3</sup>

season when winds came from Northeast, exceeding the reference value for 8 hours (70 ppb)

from the Northeast during all year, showing a great variability during cold fronts season

) [17], mainly when winds came from Northeast and Southwest (**Figure 8b**). PM2.5

cold fronts season, (d) PM10 dry season, (e) PM10 rainy season, (f) PM10 cold fronts

) were high during all year, exceeding reference value for 24 hours

) [17] for 24 hours (**Figure 8c** and **d**). In addition, winds came

along this season. O<sup>3</sup>

, PM10, and PM2.5

during all year when wind direction was from Northeast, exceeding

levels (≥0.095 ppm) were higher during the dry

dry season, (b) O<sup>3</sup>

(75 μg m−<sup>3</sup>

*4.4.6. Site VI Obispado*

[16]. PM10 levels (≥75 μg m−<sup>3</sup>

the reference value (45 μg m−<sup>3</sup>

levels were ≥44 μg m−<sup>3</sup>

rainy season, (c) O<sup>3</sup>

From **Figure 8a**, it can be observed that O<sup>3</sup>

season, (g) wind dry season, (h) wind rainy season, (i) wind cold fronts season.

88 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

(**Figure 8e** and **f**), with maximum wind speeds >7.9 m s−<sup>1</sup>

showed highest levels when winds had a Northeast component.

**Figure 8.** Criteria air pollutants and meteorological conditions for site VI (Obispado) during the study period: (a) O<sup>3</sup> dry, rainy and cold fronts seasons, (b) PM10 dry, rainy and cold fronts seasons, (c) PM2.5 dry and rainy seasons, (d) PM2.5 cold fronts season, (e) wind dry and rainy season, (f) wind cold fronts season.

#### *4.4.7. Site VII Santa Catarina*

O3 levels (≥0.095 ppm) were high during all year, being higher during the dry and rainy seasons, exceeding the reference value for 8 hours (70 ppb) [16] when winds came from North (**Figure 9a** and **b**). In addition, PM10 levels (≥75 μg m−<sup>3</sup> ) also exceeded the reference value for 24 hours (75 μg m−<sup>3</sup> ) [17] during all year and mainly when winds came from North (**Figure 9c**). PM2.5 levels were ≥44 μg m−<sup>3</sup> during dry season when wind direction was from North, exceeding the reference value (45 μg m−<sup>3</sup> ) [17] for 24 hours (**Figure 9d**). According to **Figure 9e**, O<sup>3</sup> levels decreased significantly during the rainy season without showing exceedances to reference value. Finally, winds came from the North during all year (**Figure 9f**), with maximum wind speeds >7.9 m s−<sup>1</sup> along the year. O<sup>3</sup> , PM10, and PM2.5 showed highest levels when winds had a North component.

#### *4.4.8. Site VIII San Pedro*

From **Figure 10a** and **b**, it can be observed that O<sup>3</sup> levels (0.074–0.095 ppm) were higher during the dry season when winds came from East-Northeast, exceeding the reference value for 8 hours (70 ppb) [16]. PM10 levels (≥75 μg m−<sup>3</sup> ) were high during all year, exceeding reference value for 24 hours (75 μg m−<sup>3</sup> ) [17]; mainly when winds came from East-Northeast (**Figure 10c** and **d**). Finally, winds came from the Northeast during all year, showing a great variability during cold fronts season (**Figure 10e** and **f**), with maximum wind speeds >7.9 m s−<sup>1</sup> along this season. Both O<sup>3</sup> and PM10 showed highest levels when winds had a Northeast component.

*4.4.9. Site IX La Pastora*

dry and rainy seasons, (b) O<sup>3</sup>

*4.4.10. Site X Juarez*

addition, PM10 levels (≥75 μg m−<sup>3</sup>

and rainy seasons, (f) wind cold fronts season.

(**Figure 11d–f**) with maximum wind speeds >7.9 m s−<sup>1</sup>

From **Figure 12b** and **c**, it can be observed that O<sup>3</sup>

for 8 hours (70 ppb) [16]. PM10 levels (≥75 μg m−<sup>3</sup>

levels when winds had a Northeast component.

variability in wind direction during cold fronts season. O<sup>3</sup>

value for 24 hours (75 μg m−<sup>3</sup>

PM2.5 levels were ≥44 μg m−<sup>3</sup>

 levels (0.074–0.095 ppm) were high during dry and rainy seasons, exceeding the reference value for 8 hours (70 ppb) [16] when winds came from North-Northeast (**Figure 11a**). In

**Figure 10.** Criteria air pollutants and meteorological conditions for Site VIII (San Pedro) during the study period: (a) O<sup>3</sup>

when wind direction was from North-Northeast, exceeding the reference value (45 μg m−<sup>3</sup>

[17] for 24 hours (**Figure 11c**). Finally, winds came from the North-Northeast during all year

**Figure 12a** shows that CO levels (8.5–11 ppm) were higher during cold fronts season, reaching the upper limit value established in the air quality standard (11 ppm) for 8 hours [18].

dry season when winds came from Southeast, exceeding in both cases, the reference value

) were high during all year and also exceeded the reference

along the year, and showing a greater

levels (≥0.095 ppm) were high during

) were high during all year, exceeding

, PM10, and PM2.5 showed highest

)

) [17], when winds came from North-Northeast (**Figure 11b**).

cold fronts season, (c) PM10 dry and rainy seasons, (d) PM10 cold fronts season, (e) wind dry

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during all year, being higher during cold fronts season, and

O3

**Figure 9.** Criteria air pollutants and meteorological conditions for Site VII (Santa Catarina) during the study period: (a) O3 dry and rainy seasons, (b) O<sup>3</sup> cold fronts season, (c) PM10 dry, rainy and cold fronts seasons, (d) PM2.5 dry season, (e) PM2.5 rainy season, (f) wind all year.

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**Figure 10.** Criteria air pollutants and meteorological conditions for Site VIII (San Pedro) during the study period: (a) O<sup>3</sup> dry and rainy seasons, (b) O<sup>3</sup> cold fronts season, (c) PM10 dry and rainy seasons, (d) PM10 cold fronts season, (e) wind dry and rainy seasons, (f) wind cold fronts season.

#### *4.4.9. Site IX La Pastora*

*4.4.7. Site VII Santa Catarina*

and **b**). In addition, PM10 levels (≥75 μg m−<sup>3</sup>

90 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

From **Figure 10a** and **b**, it can be observed that O<sup>3</sup>

8 hours (70 ppb) [16]. PM10 levels (≥75 μg m−<sup>3</sup>

 levels (≥0.095 ppm) were high during all year, being higher during the dry and rainy seasons, exceeding the reference value for 8 hours (70 ppb) [16] when winds came from North (**Figure 9a**

) [17] for 24 hours (**Figure 9d**). According to **Figure 9e**, O<sup>3</sup>

significantly during the rainy season without showing exceedances to reference value. Finally, winds came from the North during all year (**Figure 9f**), with maximum wind speeds >7.9 m s−<sup>1</sup>

ing the dry season when winds came from East-Northeast, exceeding the reference value for

and **d**). Finally, winds came from the Northeast during all year, showing a great variability during cold fronts season (**Figure 10e** and **f**), with maximum wind speeds >7.9 m s−<sup>1</sup>

**Figure 9.** Criteria air pollutants and meteorological conditions for Site VII (Santa Catarina) during the study period: (a)

cold fronts season, (c) PM10 dry, rainy and cold fronts seasons, (d) PM2.5 dry season, (e)

) [17] during all year and mainly when winds came from North (**Figure 9c**). PM2.5 levels

during dry season when wind direction was from North, exceeding the refer-

, PM10, and PM2.5 showed highest levels when winds had a North component.

) also exceeded the reference value for 24 hours

levels (0.074–0.095 ppm) were higher dur-

) were high during all year, exceeding reference

) [17]; mainly when winds came from East-Northeast (**Figure 10c**

and PM10 showed highest levels when winds had a Northeast component.

levels decreased

along

O3

(75 μg m−<sup>3</sup>

were ≥44 μg m−<sup>3</sup>

along the year. O<sup>3</sup>

*4.4.8. Site VIII San Pedro*

value for 24 hours (75 μg m−<sup>3</sup>

this season. Both O<sup>3</sup>

O3

dry and rainy seasons, (b) O<sup>3</sup>

PM2.5 rainy season, (f) wind all year.

ence value (45 μg m−<sup>3</sup>

O3 levels (0.074–0.095 ppm) were high during dry and rainy seasons, exceeding the reference value for 8 hours (70 ppb) [16] when winds came from North-Northeast (**Figure 11a**). In addition, PM10 levels (≥75 μg m−<sup>3</sup> ) were high during all year and also exceeded the reference value for 24 hours (75 μg m−<sup>3</sup> ) [17], when winds came from North-Northeast (**Figure 11b**). PM2.5 levels were ≥44 μg m−<sup>3</sup> during all year, being higher during cold fronts season, and when wind direction was from North-Northeast, exceeding the reference value (45 μg m−<sup>3</sup> ) [17] for 24 hours (**Figure 11c**). Finally, winds came from the North-Northeast during all year (**Figure 11d–f**) with maximum wind speeds >7.9 m s−<sup>1</sup> along the year, and showing a greater variability in wind direction during cold fronts season. O<sup>3</sup> , PM10, and PM2.5 showed highest levels when winds had a Northeast component.

#### *4.4.10. Site X Juarez*

**Figure 12a** shows that CO levels (8.5–11 ppm) were higher during cold fronts season, reaching the upper limit value established in the air quality standard (11 ppm) for 8 hours [18]. From **Figure 12b** and **c**, it can be observed that O<sup>3</sup> levels (≥0.095 ppm) were high during dry season when winds came from Southeast, exceeding in both cases, the reference value for 8 hours (70 ppb) [16]. PM10 levels (≥75 μg m−<sup>3</sup> ) were high during all year, exceeding

**Figure 11.** Criteria air pollutants and meteorological conditions for site IX (La Pastora) during the study period: (a) O3 dry, rainy and cold fronts seasons, (b) PM10 dry, rainy, and cold fronts seasons, (c) PM2.5 dry, rainy, and cold fronts seasons, (d) wind dry season, (e) wind rainy season, (f) wind cold fronts season.

In the case of S deposition, a critical value of 3 Kg S ha−<sup>1</sup> yr.−<sup>1</sup>

4.88 and 25.03 Kg ha−<sup>1</sup> yr.−<sup>1</sup>

fronts season, (b) O<sup>3</sup>

season, (f) wind all year.

Veracruz (1.44 Kg N ha−<sup>1</sup> yr.−<sup>1</sup>

Orizaba Valley (55.16 Kg S ha−<sup>1</sup> yr.−<sup>1</sup>

(1.15 Kg N ha−<sup>1</sup> yr.−<sup>1</sup>

S ha−<sup>1</sup> yr.−<sup>1</sup>

S ha−<sup>1</sup> yr.−<sup>1</sup>

[22] in Carmen Island (2.15 Kg N ha−<sup>1</sup> yr.−<sup>1</sup>

dry season, (c) O<sup>3</sup>

sensitive areas in Europe, whereas for natural forests, a reference value of 2–5 Kg S ha−<sup>1</sup> yr.−<sup>1</sup> has been proposed [21]. In this study, mean N and S throughfall deposition fluxes were

**Figure 12.** Criteria air pollutants and meteorological conditions for site X (Juarez) during the study period: (a) CO cold

value reported for alpine ecosystems; however, they are almost in the upper limit of this reference value and similar to those found in New Mexico and California. In addition, N deposition levels found in MAM (**Figure 13**) are almost twice those reported by Escoffie

and are almost four times those reported by García [25] in Atasta-Xicalango, Campeche

eight times the critical load proposed for sensitive areas, and five times the upper reference value for natural forests in Europe. S deposition fluxes found in MAM were almost six times higher than those reported by Escoffie [22] in Carmen Island, Campeche (4.7 Kg

times higher than those reported by García [25] in Atasta-Xicalango, Campeche (8.57 Kg

). In spite of S levels in MAM being half of those reported by Sánchez [23] in

); and by López [24] in Mérida, Yucatán (4.07 Kg S ha−<sup>1</sup> yr.−<sup>1</sup>

risk potential of acidification and impact on ecosystems in this region.

, respectively. N deposition fluxes did not exceed the reference

rainy and cold fronts seasons, (d) PM10 dry and rainy seasons, (e) PM10 cold fronts

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); and by López [24] in Mérida, Yucatán (2.7 Kg N ha−<sup>1</sup> yr.−<sup>1</sup>

). On the other hand, S deposition fluxes in MAM exceeded almost

), Campeche; by Sánchez [23] in Orizaba Valley,

), the current S deposition fluxes in MAM represent a

has been reported for very

)

), and almost three

reference value for 24 hours (75 μg m−<sup>3</sup> ) [17], when winds came from Southeast during dry and rainy season (**Figure 12d**) and from Northwest and Southeast during cold fronts season, showing a greater variability in wind direction (**Figure 12e**). Finally, winds came from the Southeast during dry and rainy seasons, and from Northwest during cold fronts season (**Figure 12f**), with maximum wind speeds >7.9 m s−<sup>1</sup> along the year. O<sup>3</sup> and PM10 showed highest levels when winds had a Southeast component most part of the year and a Northwest component during cold fronts season, suggesting a seasonal behavior for these pollutants. However, in the case of CO behavior, it was completely different, with the highest levels (even exceeding the air quality standard) during cold fronts season with winds coming from Southeast and Northwest.

#### **4.5. Mapping N and S deposition fluxes and reference values**

In Mexico, reference values to compare the current deposition fluxes of N and S are not available. However, critical loads have been estimated for European ecosystems and some sites in the United States. A critical load value of 5 Kg N ha−<sup>1</sup> yr.−<sup>1</sup> has been proposed for alpine ecosystems [19], whereas for some sites in North America, values of 3–8 Kg N ha−<sup>1</sup> yr.−<sup>1</sup> for New Mexico and 4–7 Kg N ha−<sup>1</sup> yr.−<sup>1</sup> for California have been proposed [20]. Atmospheric N and S Deposition Fluxes in the Metropolitan Area of Monterrey, Mexico and Its… http://dx.doi.org/10.5772/intechopen.79484 93

**Figure 12.** Criteria air pollutants and meteorological conditions for site X (Juarez) during the study period: (a) CO cold fronts season, (b) O<sup>3</sup> dry season, (c) O<sup>3</sup> rainy and cold fronts seasons, (d) PM10 dry and rainy seasons, (e) PM10 cold fronts season, (f) wind all year.

reference value for 24 hours (75 μg m−<sup>3</sup>

coming from Southeast and Northwest.

ha−<sup>1</sup> yr.−<sup>1</sup>

O3

season (**Figure 12f**), with maximum wind speeds >7.9 m s−<sup>1</sup>

seasons, (d) wind dry season, (e) wind rainy season, (f) wind cold fronts season.

92 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

**4.5. Mapping N and S deposition fluxes and reference values**

for New Mexico and 4–7 Kg N ha−<sup>1</sup> yr.−<sup>1</sup>

sites in the United States. A critical load value of 5 Kg N ha−<sup>1</sup> yr.−<sup>1</sup>

) [17], when winds came from Southeast during

along the year. O<sup>3</sup>

for California have been proposed [20].

and PM10

has been proposed

dry and rainy season (**Figure 12d**) and from Northwest and Southeast during cold fronts season, showing a greater variability in wind direction (**Figure 12e**). Finally, winds came from the Southeast during dry and rainy seasons, and from Northwest during cold fronts

**Figure 11.** Criteria air pollutants and meteorological conditions for site IX (La Pastora) during the study period: (a)

dry, rainy and cold fronts seasons, (b) PM10 dry, rainy, and cold fronts seasons, (c) PM2.5 dry, rainy, and cold fronts

showed highest levels when winds had a Southeast component most part of the year and a Northwest component during cold fronts season, suggesting a seasonal behavior for these pollutants. However, in the case of CO behavior, it was completely different, with the highest levels (even exceeding the air quality standard) during cold fronts season with winds

In Mexico, reference values to compare the current deposition fluxes of N and S are not available. However, critical loads have been estimated for European ecosystems and some

for alpine ecosystems [19], whereas for some sites in North America, values of 3–8 Kg N

In the case of S deposition, a critical value of 3 Kg S ha−<sup>1</sup> yr.−<sup>1</sup> has been reported for very sensitive areas in Europe, whereas for natural forests, a reference value of 2–5 Kg S ha−<sup>1</sup> yr.−<sup>1</sup> has been proposed [21]. In this study, mean N and S throughfall deposition fluxes were 4.88 and 25.03 Kg ha−<sup>1</sup> yr.−<sup>1</sup> , respectively. N deposition fluxes did not exceed the reference value reported for alpine ecosystems; however, they are almost in the upper limit of this reference value and similar to those found in New Mexico and California. In addition, N deposition levels found in MAM (**Figure 13**) are almost twice those reported by Escoffie [22] in Carmen Island (2.15 Kg N ha−<sup>1</sup> yr.−<sup>1</sup> ), Campeche; by Sánchez [23] in Orizaba Valley, Veracruz (1.44 Kg N ha−<sup>1</sup> yr.−<sup>1</sup> ); and by López [24] in Mérida, Yucatán (2.7 Kg N ha−<sup>1</sup> yr.−<sup>1</sup> ) and are almost four times those reported by García [25] in Atasta-Xicalango, Campeche (1.15 Kg N ha−<sup>1</sup> yr.−<sup>1</sup> ). On the other hand, S deposition fluxes in MAM exceeded almost eight times the critical load proposed for sensitive areas, and five times the upper reference value for natural forests in Europe. S deposition fluxes found in MAM were almost six times higher than those reported by Escoffie [22] in Carmen Island, Campeche (4.7 Kg S ha−<sup>1</sup> yr.−<sup>1</sup> ); and by López [24] in Mérida, Yucatán (4.07 Kg S ha−<sup>1</sup> yr.−<sup>1</sup> ), and almost three times higher than those reported by García [25] in Atasta-Xicalango, Campeche (8.57 Kg S ha−<sup>1</sup> yr.−<sup>1</sup> ). In spite of S levels in MAM being half of those reported by Sánchez [23] in Orizaba Valley (55.16 Kg S ha−<sup>1</sup> yr.−<sup>1</sup> ), the current S deposition fluxes in MAM represent a risk potential of acidification and impact on ecosystems in this region.

**5. Conclusions**

region of Mexico.

O3

This chapter presents an overview of atmospheric pollution and its spatial and temporal vari-

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N deposition fluxes: Nitrate deposition showed a seasonal pattern with the highest levels during the rainy season (suggesting that atmospheric reactions in aqueous phase play an important role in the removal process). In the case of ammonium, its deposition also presented a seasonal variation, with higher levels during the dry and cold fronts season in Santa Catarina municipality. N deposition fluxes did not exceed the critical load values reported for Europe and USA; however, these levels were higher than those reported for the southeast

S deposition fluxes: Sulfate deposition did not show significant differences between seasons and sampling points, suggesting that levels found probably correspond to background levels in MAM. Sulfate levels were relatively high in Obispado, Santa Catarina, and Escobedo municipalities. S deposition fluxes exceeded the limit values proposed for sensitive areas and natural forests in Europe, and were higher than those reported at the southeast (SE) of the country, but lower than those found at the center of Mexico. It suggests that S deposition

CO: Juárez municipality was the only sampling site that showed exceedances to the reference value established in the current regulation, this municipality is located to the east of MAM,

: Ozone levels exceeded the reference value of the current regulation in all sampling sites

PM10: PM10 levels exceeded the threshold value of the current regulation in all sites and during

PM2.5: Obispado and La Pastora municipalities (center of MAM) showed the highest levels during all year, whereas in Escobedo and Apodaca (at the northern side of MAM), its levels

In spite of the time scale in which deposition fluxes (by season) and criteria pollutants (by day) were different, we could identify an evident association between CO and nitrate, since both analysis showed that their levels were higher in Juarez municipality during cold fronts season (CO levels exceeded the regulation's reference values and exhibited a different pattern regarding to the remaining sampling sites in MAM). It suggests that both, CO and nitrate had their origin in vehicular sources in this urbane zone highly polluted. On the other hand, a similarity was observed between deposition patterns of S and PM10-PM2.5 levels in MAM, since sulfate did not present significant differences in its spatial and seasonal variability; it suggests that levels found in this study remained constant all year, and correspond to the background levels for MAM. The same finding was obtained for PM10 and PM2.5 levels, since their levels exceeded the reference value established in the current regulation in all sampling sites. Regarding wind

direction, an evident association with criteria pollutants was found, PM10 and O<sup>3</sup>

their highest levels when wind had an east component (E-SE-NE), which corresponds to the

showed

could be a potential risk for ecosystems and historical heritage in MAM.

during the dry season when wind had an east component (E-SE-NE).

all year, its levels being higher when wind came from East (E-SE-NE).

and its levels were higher when wind came from N.

were higher during the cold fronts season.

ability in MAM, and from results, we can conclude that:

**Figure 13.** Spatial and temporal patterns for throughfall deposition fluxes of SO<sup>4</sup> <sup>2</sup>− for (a) dry season, (b) rainy season, (c) cold fronts season; of NO<sup>3</sup> <sup>−</sup> for (d) dry season, (e) rainy season, (f) cold fronts season; and of NH4 + for (g) dry season, (h) rainy season, (i) cold fronts season in MAM during the study period.
