**4. Results and discussion**

Extraction efficiency was calculated as the percentage of the loaded ions on the columns to the recovered in sequential extractions. It was found that recovered N and S percentage from the third extraction was insignificant, therefore; only two extractions were considered for the recovery process, reaching an efficiency of approximately 96.8%. Similar extraction efficiencies

<sup>+</sup> was analyzed by molecular absorption spectrometry using the blue indophenol method

colorimetrically by using the brucine method [18]. The weight of the extractant was converted to volume by using the specific density of the extractant solution (2N KCl = 1.05 g ml- 1). The amount of each collected ion in a given column was determined as the factor of extractant volume multiplied by ionic concentration in the extractant. The surface area of the funnel opening and the sampling period were used to estimate the deposition fluxes of N and S per

All methods based on point measurements (e.g., wet deposition, micrometeorological and dry deposition measurements, throughfall deposition), cannot be directly connected to emission inventories. Maps can only be produced directly from these measurements if the network is enough dense to account for spatial and temporal variations. This may be the case for networks measuring compounds with little spatial variation or for measurements of deposition in areas of simple terrain as the case of Carmen Island. A point measurements network should be interpolated using statistical methods as the kriging technique, which allows including monitoring data from neighboring stations for interpolation in a national scale [19]. Consid‐ ering the characteristics of measured data and the expected interpolation results, diversified

methods have been employed with atmospheric deposition network data [10, 20- 22].

variable from distance and the variability between points.

To obtain N and S deposition maps in Carmen Island, a geostatistical procedure was used to interpolate field measurements into a continuous spatially pattern, where data were interpo‐ lated using the kriging interpolation technique. Kriging is a statistical method that provides unbiased estimates of variables in regions where the available data exhibit spatial autocorrelation and these estimates are obtained in such a way that they have minimum variance. The first step was to obtain the statistics summary of primary data. From this, a descriptive report of data was carried out, including mean, maximum, minimum values and their frequency distribution. This analysis was carried out in order to determine if it was necessary to make a data transformation since the results of the kriging interpolation are more reliable when data are normally distributed [20].Data interpolation was used in this work to obtain a continuous variable by isolines to increase the number of points in the sampling grid. Thresh‐ old values definition helps to obtain isolines or imaginary lines in which studied variables become continuous, this method is useful since it takes into account the spatial behavior of the


was analyzed

[16]. Sulfate was determined by turbidimetric method [17], whereas NO3

have been reported by other authors [15].

**3.2. Chemical analysis**

152 Current Air Quality Issues

land area per year (Kg ha- 1 yr- 1).

**3.3. Data interpolation and mapping**

NH4

Critical loads have been estimated in several regions around the world. A critical load value of 5 Kg N ha- 1 yr- 1 has been reported for alpine ecosystems [23], whereas for New Mexico and California, values of 3- 8 Kg N ha- 1 yr- 1 and 4- 7 Kg N ha- 1 yr- 1, respectively, have been found [24]. On the other hand, a critical load value of 3 Kg S ha- 1 yr- 1 for very sensitive areas and a range of 2 - 5 Kg S ha- 1 yr- 1 for natural forests have been proposed [25].

In Mexico, critical loads data are not available and only few studies have been carried out in Mexico Valley, mainly in pine forests. It has been reported an input of 15 Kg N ha- 1 yr-<sup>1</sup> for pine stands in Desierto de Los Leones in the surrounding of Mexico City [26], whereas in Zoquiapan (a site located at the east and upwind of Mexico City), the reported inputs were 5.5 and 8.8 Kg ha- 1 yr- 1 for N and S, respectively [27]. A research work carried out in Central Veracruz for several land- cover types reported inputs of 8- 17 Kg ha- 1 yr- 1 and 2- 4 Kg ha- 1 yr- 1 for S and N, respectively [28]. Additionally, some authors have studied the acidification in developing countries and have proposed a critical load approach on a global scale [3]. They assigned a relative sensitivity class of 3 to acidic deposition for terrestrial ecosystems in Carmen island region, and the preliminary critical load assigned to this site ranges from 50 to 100 meq m- 2 yr- 1.

In this study, mean fluxes of throughfall deposition for N (as N- NH4 + + N- NO3 - ) and S (as SO4 2- ) at Carmen Island were 2.15 and 4.7 Kg ha- 1 yr- 1, respectively. N mean deposition flux did not exceed critical loads proposed for very sensitive ecosystems, however, S mean deposition flux is already in the threshold limit value proposed for natural forests and it is greater than those reported for very sensitive areas.

Sulfur dioxide is oxidized to sulfate, and the oxidation rate determines its lifetime in the atmosphere. Sulfuric acid is produced from the oxidation of sulfur oxides, which in turn form sulfate particles. However, even in the atmosphere of rural or non- industrialized sites, significant levels of sulfate particles have been found, concluding that sulfate in these sites is related to atmospheric reactions from anthropogenic SO2 [29]. SO2 has an atmospheric residence time of 13 days, and may be transported great distances from anthropogenic sources [30]. On the other hand, the oxidation of NO2 at atmospheric conditions is almost 10 times faster than the oxidation of SO2 to SO4, resulting in a residence time of approximately 1 day [31]. These chemical properties make nitrate or its parental gaseous precursors NOx, be commonly known as local pollutants. Since the dry oxidation of SO2 to SO4, or wet oxidation via the bisulfate (HSO3) intermediate at the ambient atmospheric conditions is much slower than that NOx, SO2 and its intermediate oxidative products have much longer residence time in the atmosphere. This makes them more susceptible to be transported by the movement of the air masses in comparison with NOx. For this reason SO4 or its parental gaseous precursors are known as regional pollutants.

Additionally, during the rainy season, when the mixing layer is very high, most of pollutants in precipitation derive from the rain- out process of condensation nuclei that have been transported long distances into the region. In contrast, during the dry season, the reduced mixing layer only concentrates ionic species of local origin. Dry deposition is, in general, greater than wet deposition near emission source.

To infer this local or regional influence, the sulfate: nitrate ratio in throughfall deposition was estimated. A ratio of 4.7 was obtained, suggesting that this site was under the influence of longrange transport. NH4 + and NO3 had a similar pattern in their deposition fluxes, with the highest fluxes occurring during the dry season (Figures 4a and 4b). It is agree with the local character of the emissions of NOx.

**Figure 4.** Atmospheric deposition fluxes for NH4 + , NO3 and SO4 2- for Carmen Island for sampling season.

On the other hand, SO4 2- had a completely different behavior, with the highest levels occurring during the plenitude of the rainy season and at the beginning of the cold fronts season (Figure 4c). This is in agreement with the regional character of SO2 emissions which are more connected to wet deposition.

Before applying Krigging interpolation, the thirteen sampling points in which throughfall deposition was collected, were grouped in three zones, considering the land- use along the Island. Identified zones were the following: *Industrial zone* (sampling points 1,2,3 and 4) located at the east edge of the island, *Zone with the greatest mangrove cover* (sampling points 5, 6, 7 and 8) located at the middle part of Carmen Island, and *Urban zone* (sampling points 9, 10, 11, 12 and 13) located at the west side of the island.

the air masses in comparison with NOx. For this reason SO4 or its parental gaseous precursors

Additionally, during the rainy season, when the mixing layer is very high, most of pollutants in precipitation derive from the rain- out process of condensation nuclei that have been transported long distances into the region. In contrast, during the dry season, the reduced mixing layer only concentrates ionic species of local origin. Dry deposition is, in general,

To infer this local or regional influence, the sulfate: nitrate ratio in throughfall deposition was estimated. A ratio of 4.7 was obtained, suggesting that this site was under the influence of long-

fluxes occurring during the dry season (Figures 4a and 4b). It is agree with the local character

(a) (b)

(c)

during the plenitude of the rainy season and at the beginning of the cold fronts season (Figure 4c). This is in agreement with the regional character of SO2 emissions which are more connected

2- for Carmen Island for sampling season.

2- had a completely different behavior, with the highest levels occurring

+ , NO3 and SO4

had a similar pattern in their deposition fluxes, with the highest

are known as regional pollutants.

range transport. NH4

154 Current Air Quality Issues

of the emissions of NOx.

greater than wet deposition near emission source.

 and NO3 -

+

**Figure 4.** Atmospheric deposition fluxes for NH4

On the other hand, SO4

to wet deposition.

**Figure 5.** Atmospheric deposition fluxes for NH4 + , NO3 and SO4 2- for Carmen Island for sampling site.

Mean throughfall deposition fluxes for NH4 + , NO3 and SO4 2- were the highest in sampling points labeled as 3, 7 and 2, respectively (Figures 5a, 5b and 5c). Sampling points 7 and 8 are located at the limit of the urban zone; both points are within a complex area at the transition zone between urban area and mangrove forest. Sampling point labeled as "7" is located within an area characterized by a high vehicular density, small geographical extent and few circula‐ tion ways, so in peak hours, traffic vehicular is intense, resulting in high NOx emissions of that are deposited as NO3 in the surroundings of the emission points.

Figure 6b illustrates that NO3 deposition was higher in the area adjacent to mangrove ecosystem, whereas the highest deposition of NH4 + and SO4 2- occurred in the island industrial

**Figure 6.** Atmospheric deposition fluxes for NH4 + , NO3 and SO4 2- for Carmen Island for different land use: Urban, Mangrove ecosystem and industrial.

zone (Figures 6a and 6c). On the other hand, mean throughfall deposition fluxes for NH4 + and SO4 2- were higher in sampling points 3 and 2, respectively. It is important to mention that the points 1, 2 and 3 are located in the mangrove forests boundaries. These points are placed at the east edge of the island, where some industrial facilities could contribute to deposition of local NH4 + and SO4 2- . Since sampling points 2 and 3 were located along the Federal Highway 180, so that NH3 could be also emitted from light- duty vehicles. Many authors have reported on NH3 and amine emissions from gasoline- powered automobiles or engines with and without exhaust catalysts in dynamometer experiments [32, 33]. The production of NH3 emissions depends on the vehicle's ability to produce NO in the presence of a catalytic convertor that has enough stored hydrogen to reduce the NO to NH3. However, considering prevailing winds, a great proportion of NH4 + could also came from rural areas at the east of Carmen Island, specifically located crossing the bridge "La Unidad" in Isla Aguada and Sabancuy municipal‐ ities, where agriculture activities are developed. In addition, sulfate levels in throughfall deposition collected in Carmen Island could be enhanced by the long- range transport of SO2 emissions from offshore platforms in the Gulf of Campeche where sour gas is burned in elevated flares. These SO2 emissions could be washed- out during the rainy and cold fronts seasons since the wind roses and backward air mass trajectories pointed out that air masses followed this direction during this climatic period.

#### **4.1. Mapping deposition fluxes of acidic compounds over the study region**

Since successive monitoring of precipitation chemistry at the same station is scarce in Carmen Island and the data collected from various sources are highly discrete at the temporal scale, then all the concentration data obtained in this work were employed to produce continuous contours for spatial analysis. One- year mean results for all sites were interpolated to produce N and S deposition loads isopleths (Figures 7, 8 and 9).

zone (Figures 6a and 6c). On the other hand, mean throughfall deposition fluxes for NH4

+ , NO3 and SO4

2- were higher in sampling points 3 and 2, respectively. It is important to mention that the points 1, 2 and 3 are located in the mangrove forests boundaries. These points are placed at the east edge of the island, where some industrial facilities could contribute to deposition of

(c)

(a) (b)

180, so that NH3 could be also emitted from light- duty vehicles. Many authors have reported on NH3 and amine emissions from gasoline- powered automobiles or engines with and without exhaust catalysts in dynamometer experiments [32, 33]. The production of NH3 emissions depends on the vehicle's ability to produce NO in the presence of a catalytic convertor that has enough stored hydrogen to reduce the NO to NH3. However, considering prevailing

specifically located crossing the bridge "La Unidad" in Isla Aguada and Sabancuy municipal‐ ities, where agriculture activities are developed. In addition, sulfate levels in throughfall deposition collected in Carmen Island could be enhanced by the long- range transport of SO2 emissions from offshore platforms in the Gulf of Campeche where sour gas is burned in elevated flares. These SO2 emissions could be washed- out during the rainy and cold fronts seasons since the wind roses and backward air mass trajectories pointed out that air masses

2- . Since sampling points 2 and 3 were located along the Federal Highway

could also came from rural areas at the east of Carmen Island,

SO4

local NH4

156 Current Air Quality Issues

+ and SO4

winds, a great proportion of NH4

**Figure 6.** Atmospheric deposition fluxes for NH4

Mangrove ecosystem and industrial.

+

followed this direction during this climatic period.

+ and

2- for Carmen Island for different land use: Urban,

**Figure 7.** Spatial and temporal patterns of NH4 + throughfall deposition fluxes (Kg ha- 1 yr- 1) in Carmen Island: (a) July, (b) September, (c) November, (d) January, (e) March, (f) May.

**Figure 8.** Spatial and temporal patterns of NO3 throughfall deposition fluxes (Kg ha- 1 yr- 1) in Carmen Island: (a) July, (b) September, (c) November, (d) January, (e) March, (f) May.

NH4 <sup>+</sup> deposition maps indicate a clear seasonal pattern, positioning the highest values during the period September- January, just when the Island is under the influence of cold fronts (Norths). Highest fluxes associated to the spatial distribution, were in the surroundings of point 7 and in the east edge of the Island. This fact demonstrates that NH4 + probably could have two main sources: light- duty vehicles circulating on the road with the highest traffic (point 7) and along the Federal highway 180 (points 1, 2 and 3) at the east edge of the Island. However, NH4 <sup>+</sup> deposition fluxes could be also enhanced by transport of emissions related to agricultural activities in Isla Aguada and Sabancuy municipalities.

(a) (b)

(c) (d)

(e) (f)

<sup>+</sup> deposition maps indicate a clear seasonal pattern, positioning the highest values during the period September- January, just when the Island is under the influence of cold fronts (Norths). Highest fluxes associated to the spatial distribution, were in the surroundings of

throughfall deposition fluxes (Kg ha- 1 yr- 1) in Carmen Island: (a) July,

+

probably could


point 7 and in the east edge of the Island. This fact demonstrates that NH4

**Figure 8.** Spatial and temporal patterns of NO3

NH4

158 Current Air Quality Issues

(b) September, (c) November, (d) January, (e) March, (f) May.

**Figure 9.** Spatial and temporal patterns of SO4 2- throughfall deposition fluxes(Kg ha- 1 yr- 1) in Carmen Island: (a) July, (b) September, (c) November, (d) January, (e) March, (f) May.

Nitrate did not show a clear seasonal trend and presented a similar pattern along the year representing its local character. However, it was observed a clear spatial pattern with peak values in the surroundings of points 6, 7 and 8. This zone is located at the mangrove forest boundaries and it is characterized by high vehicular traffic since there are not enough roads. Moreover, NO3 deposition fluxes showed a slight dilution effect, with relatively higher values during dry months, decreasing as rainy season progressed.

In the case of SO4 2- , deposition fluxes were higher in the east edge of the island with a clear spatial trend which decreased progressively westward. Peak values were centered on the points 1, 2, 3 and 4. An evident seasonal pattern was identified since deposition fluxes were higher during Norths season; it suggests that sulfate levels could be enhanced by large- scale transported emissions from offshore platforms in the Gulf of Campeche demonstrating its regional character.
