2.1 Emissions of acid rain precursors

Scientists have discovered that air pollution from the burning of fossil fuels is the major cause of acid rain. Power plants and factories burn coal, oil, and natural gas to produce the electricity we need to perform all kinds of activities, e.g., lighting our homes. Cars, trucks, and airplanes also run on gasoline, a fossil fuel (^{EPA, 2008}).

The relationships between reductions in SO₂ and NO_x emissions and changes in sulfate and nitrate formation involve a complex group of gas and aqueous-phase chemical reactions between acid deposition and aerosol precursors and oxidants. These reactions can produce nonlinear responses to emission reductions. For example, reducing NO_x while leaving SO₂ unchanged can lead to an increase in sulfate formation under certain conditions. Reducing NO_x emissions could increase the concentrations of the oxidant hydrogen peroxide (H₂O₂) (NAPAP, 2003).

A preliminary air quality model simulation was performed to observe the transport and dispersion patterns of SO₂, which is the main pollutant emitted from the offshore platforms in the Gulf of Mexico (^{Villaseñor, 2003}). A research focused in estimating the amount of atmospheric emissions originated from burners on oil industry was developed in 2001 (^{Mendoza and Graniel-Peralta, 2006}).

The analysis of the rain samples collected during 2012 in Isla del Carmen, Campeche, Mexico showed that: (1) the scavenging of pollutant and geochemical aerosols from the air, and prevailing meteorological conditions affected directly and in a large scale the pH and chemical composition of rainwater in this site, and (2) NO₃ ^- and SO₄ ^2- were the major acidifying ions in rainwater (^{Cerón et al, 2013}).

Another study showed that atmospheric inputs related to emissions from fossil fuel combustion and other sources of strong acids (such as nitric [HNO₃] and sulfuric [H₂SO₄] acids) alter the surface seawater’s alkalinity, pH, and inorganic carbon storage, which can disrupt natural biogeochemical cycles. This is expected to have the greatest impact in near-coastal waters, where ecosystem responses to ocean acidification most affect human populations (Louis et al., 2012).

2.2 Location of sampling sites and pH values

The atmospheric deposition sampling sites located along the coast of the Gulf of Mexico were the archaeological zone of El Tajín (TAJ), the ecological station at La Mancha (LMH), the San Juan de Ulúa fortress (SJU) and the Universidad Veracruzana, campus Mocambo (UV-MOC) (Fig. 1).

Fig. 1 Sampling sites located along the coast of the Gulf of Mexico. 

Acid rain is generally defined as having pH < 5.6. Low acidic rainwater has a pH between 5.6-5.0, acidic rainwater between 5.0-4.5, strong acidic rainwater between 4.5-3.5, and extremely acidic < 3.5 (^{Vladimir, 2003})

As background information, during continuous and representative sampling from 2003 to 2005 at these four sites in the coastal region of the Gulf of México, the phenomena of acid rain (pH < 5.6) was observed, as depicted in Figure 2, where the VWM values for pH are presented (^{Sosa et al., 2014}).

Fig. 2 pH values at four sampling sites from 2003 to 2005. 

All of the sampling sites presented acid rain events, each having a median below 5.6. Although all of the sites showed acid rain values, El Tajín, located in an archaeological protected area, indicated more acidic pH values (^{Sosa et al., 2014}).

3.1 Sampling site

La Mancha is a biological station for the investigation of coastal ecology. It is located on the coast of the Gulf of Mexico 60 km north of the city of Veracruz, in the municipality of Actopan. Its geographical coordinates are 19º 30’ 17.64” N, 96º 36’ 56.82” W, at an altitude of 2 masl (Fig. 1). The climate type is Aw1(w)(i´)gw”: hot throughout the year, sub-humid, with a mean daily maximum temperature of 22 ºC and average monthly temperatures varying from the mean temperature only 5 to 7 ºC. For Marcha de Ganges, the warmest month occurs before June and consists of inter-summer rain (dog-day) according to the modified Köppen climatic classification (^{García, 2004}). Dog-day weather is a jargon in the USA that is used to describe many consecutive days of unrelenting heat and humidity without rain.

3.2 Wet deposition sampling

Wet deposition was collected for 24-h periods during the rainy (June-October) and dry (November-May) seasons in an automatic collection system (Aerochem Metrics, model 301). Prior to sampling, the bucket was washed with liquid neutral soap and then flushed with water, shaken with deionized water, and subsequently tested to ensure that there was no detectable impurity inside the bucket (^{WMO, 2004}). The prepared sampling bucket was wrapped in a plastic bag before being shipped to the site. When the bucket was retrieved from the sampler, the wet deposition sample was transferred to a polyethylene bottle, labeled and stored (4 ºC) for preservation (^{NADP, 2013}). Afterwards, the samples were sent to the laboratory of the Sección de Contaminación Ambiental of the Centro de Ciencias at the Atmósfera de la Universidad Nacional Autónoma de México (SCA-CCA-UNAM) for chemical analysis.

3.3 Laboratory sample handling

The wet deposition samples were filtered through a 0.22 µm Millipore membrane. Ultra-pure water (18.2 MΩ × cm resistivity) was used for sampling, filtering and washing materials for chemical analysis (^{WMO, 2004}).

3.4 Chemical analysis of the samples (analytical methods)

Rain samples above 1 mm were analyzed. The pH was measured for each sample with pH meters (Corning 315, Methrom 827, and Orion 960). Sulfate (SO₄ ^2-), nitrate (NO₃ ^-), chloride (Cl^-), calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), ammonium (NH₄ ⁺) and sodium (Na⁺) were analyzed by high performance liquid chromatography (HPLC). In addition, anions were analyzed with a Perkin Elmer isocratic LC pump 250 and conductivity detector, using a Hamilton PRPX-100 analytical anion column. Finally, cations were analyzed with a Waters liquid chromatograph equipped with an isocratic Waters 510 pump, a Waters 432 conductivity detector and a Waters analytical cationic column. High-purity standards were used as calibration standards for each ion. The identification and quantification of ions was based on the US-EPA method 300.1 (^{Hautman et al., 1997}).

Alkalinity was determined using the Gran titration method with pH meters (^{Stumm and Morgan, 1970}). Electrical conductivity (EC) was determined with conductivity instruments YSI 32 and HORIBA D-424.

The detection limits in µeq L^-1 were 2.29, 1.77, 2.26, 2.50, 3.29, 1.79, 2.22, and 1.74 for SO₄ ^2-, NO₃ ^-, Cl^-, Ca²⁺, Mg²⁺, K⁺, NH₄ ⁺, and Na⁺, respectively.

To validate the data, ion balance was performed for the cation and anion concentrations, with the measured and theoretical conductivities compared.

4.1 Wet deposition

4.1.1 pH values and ion concentration

The pH was determined to identify the presence of acid rain. Figure 3 shows the frequency distribution of wet deposition pH samples.

Fig. 3 Frequency distribution of wet deposition pH at La Mancha (2003-2014). 

The pH in the samples ranged from 3.81 to 7.45. The most frequently observed value was 5.30. The data (Fig. 3) did not show a normal frequency distribution according to the Kolmogorov-Smirnov test; instead, it was biased to the left, indicating the abundance of acidic values (~80%).

The distribution of pH values by year is shown in Figure 4, which presents box plots from 2003 to 2014 at La Mancha. This figure indicates similar within-year variability in most years except for 2007, 2011, 2013 and 2014. The years 2007, 2008 and 2013 have lower pH values; 2008 was the year with the lowest pH value, 3.81.

Fig. 4 Box plots for the pH values from 2003 to 2014 at La Mancha. 

The pH median values ranged from 4.8 to 5.5. Statistical analysis indicated that there were no significant differences between years, except for 2007 and 2013. The values below the median (25th percentile) ranged between 4.5 and 5.4, while the values greater than the median (75th percentile) ranged between 5.3 and 5.8.

Figure 5 shows the VWM pH for samples of wet deposition, both in rainy and dry seasons, as well as the total annual average. The rainy season within the region of study occurs from June to October, while the dry season occurs from November to May. From this figure, there is not a clear trend regarding the pH during the dry season from 2004 through 2013. During the rainy season, the pH remained uniform from 2003 to 2006, dropping markedly in 2007. Then, a steady upward trend occurred from 2007 to 2013.

Fig. 5 Annual VWM pH at La Mancha from 2003 to 2014. 

The VWM pH in wet deposition at La Mancha was in the range of 4.78 to 5.40 during the study period (2003-2014), with the more acidic value occurring in 2007. For the last three years, the more acidic VWM pH was 5.01, which was less acidic than the registered value in the USA of 4.36 from 2012 to 2014. There were no significant differences in the pH levels between the events collected during the dry and rainy seasons of 2003, 2004, 2005 and 2010. However, in other years, a significant difference between these seasons was observed. The variation in the ionic composition of wet atmospheric deposition samples between dry and rainy seasons helps to explain the significant differences in the pH values determined for these two seasons. In 2006, a high concentration of alkaline ions (NH₄ ⁺, Mg²⁺ and Ca²⁺) was present in the dry season, contributing to the alkalinity of rain. In 2007, 2008 and 2009, more acidic pH values were recorded; an increase of Cl^- and SO₄ ^2- ions was observed in these years. It is important to note that in 2007 the highest percentages of NO₃ ^- were found, which influenced the acidity in the wet atmospheric deposition samples.

Figure 6 shows the VWM ion concentration (Na⁺ and Cl^-) for wet deposition samples, both in rainy and dry seasons, as well as the annual total mean.

Fig. 6 VWM ion concentration (Na⁺ and Cl^-) at La Mancha site from 2003 to 2014. 

Figure 7 shows the VWM ion concentration (NH₄ ⁺, K⁺, Mg²⁺, Ca²⁺, SO₄ ^2- and NO₃ ^-) for wet deposition samples, both in rainy and dry seasons, as well as the annual total mean. A similar behavior in the VWM ionic composition between annual and rainy seasons was observed.

Fig. 7 VWM ion concentration (NH₄ ⁺, K⁺, Mg²⁺) at La Mancha site from 2003 to 2014. 

The VWM concentration of SO₄ ^2- in wet deposition at La Mancha was in the range of 16 to 32 μeq L^-1 during the study period (2003-2014), with the higher value occurring in 2009. For the last three years, the higher values were approximately 21 μeq L^-1, which are lower than the higher levels registered in the USA of 35 μeq L^-1 from 2012 to 2014 (^{NADP, 2018b}).

The VWM concentration of NO₃ ^- in wet deposition at La Mancha was in the range of 4 to 15 μeq L^-1 during the study period (2003-2014), with the higher value occurring in 2007. For the last three years, the higher values were approximately 5 μeq L^-1, which are four times lower than the higher levels registered in the USA of 22 μeq L^-1 from 2012 to 2014.

The higher levels of SO₄ ^2- and NO₃ ^- in the USA correspond to the northeast region (^{NADP, 2018b}). However, when comparing La Mancha concentrations with the NADP sampling sites located in the coast of the Gulf of Mexico, from Texas to Florida, the Mexican site presented the highest SO₄ ^2- concentration, e.g., from 2012 to 2014 the higher level of SO₄ ^2- in the USA was 23 µeq L^-1, while in La Mancha the value was 28 µeq L^-1. Concerning the SO₄ ^2-/NO₃ ^- ratio, the higher value in the USA sampling site was 1.78, while in the Mexican sampling site it was 4.2. For this reason, it is important to pay attention to SO₂ emission sources in Mexico.

The ion concentration during the dry season was higher compared to that of the rainy season, which is expected due to low dilution during the dry season.

4.2 Neutralization factor

The neutralization factor (NF), which involves the acid and base concentrations that are present for wet and dry atmospheric deposition (^{Balasubramanian et al., 2001}; ^{Rodhe et al., 2002}; ^{Moreda et al., 2014}; ^{Sosa et al., 2015}), was calculated according to the following equation:

(1)

where X is the concentration of the ion of interest.

Due to the absence of NH₄ ⁺ in atmospheric dry deposition, the calculation of the neutralization factor (NF) was made for other species: Ca²⁺, Mg²⁺ and K⁺. Because of the alkaline-earth origin of these species, this also contributes to the neutralization of the acids formed by rain. The results showed that alkali ion concentration was not sufficient to neutralize rain acidity, except for Ca²⁺ in 2003 and 2011. However, the potential role of alkaline-earth species in neutralizing rain acidity merits consideration, as well as the use of these compounds in agricultural activities of the area.

4.3 Marine contribution

To estimate the marine contribution to different ionic species in rainwater, the sea salt factor (SSF) and non-sea-salt factor (NSSF) were calculated. Na⁺ was selected as the reference element for seawater. The detected Na⁺ was assumed to be of purely marine origin, and the SSF was calculated over the base of well-known ionic ratios of other ions present in seawater (^{Riley and Chester, 1971}; ^{Parungo et al., 1990}).

The results showed the following non-marine contributions in decreasing order: NO₃ ^- > NH₄ ⁺ > Ca²⁺ > SO₄ ^2- > K⁺ > Mg²⁺ > Cl^-; and the following marine contributions: Na⁺ > Cl^- > Mg²⁺ > K⁺ > SO₄ ^2- > Ca²⁺.

4.4 Ion correlation

To identify the associations between ion concentrations for all of the wet deposition samples, the linear Spearman correlation was determined. Table II shows the correlation results in consideration of the full study period (2003-2014) at La Mancha.

A strong correlation was observed between the following salts: NH₄NO₃, Ca(NO₃)₂, CaSO₄, MgSO₄, Na₂SO₄, and KCl. Possible sources of salts in the study area are agricultural and livestock activities.

NO₃ ^- is largely attributed to agriculture, directly in the form of applied inorganic nitrogen fertilizers or indirectly in the form of animal manures. The possible origin of Cl^- and K⁺ is the use of fertilizers in particular from agricultural activities located in the region, such as the growing of sugar cane, as well as their marine origin. Background or natural concentrations of SO₄ ^2- may be derived from gypsum in limestone and sandstone. SO₄ ^2- is also contributed by atmospheric deposition from the combustion of fossil fuels (^{Bruening, 2015}).

4.5 Trends

This study used a non-linear technique-an additive model-to determine trends in the concentration. The generalized additive model (GAM) allows for smooth trends to be estimated from data using penalized regression splines (^{Wood, 2006}). The following additive model was applied to each series:

(2)

where y _i is the observed, possibly transformed deposition; β ₀ is the intercept or constant term; f ₁ (time _i ) is the smooth function of time, represents the trend in the response and describes how the level of the response varies through time. A cubic regression spline for f ₁ (time _i ) is used in this term. The εi area represents the model residuals, which are assumed to be zero mean, Gaussian random variables with variance σ ₂ .

The models were fitted in a generalized additive model framework using the package mgcv (v. 1.8-4; ^{Wood, 2014}) for R statistical software (v. 2.15.2; ^{R Core Team, 2012}). A log transformation was applied to each deposition variable (except pH) to achieve constant variance.

Figure 8 shows the pH and ion concentration trends at La Mancha from 2003 to 2014.

Fig. 8 Trends of pH and ion concentration at La Mancha from 2003 to 2014. 

Significant increasing trends in the ion concentration (Na⁺, NH₄ ⁺, K⁺, Mg²⁺, Ca²⁺, SO₄ ^2-, NO₃ ^- and Cl^-) can be seen at La Mancha, and these fall into three groups. In the first group, SO₄ ^2-and NO₃ ^- show a significant increasing trend through the entire sampling period, with a small, non-significant decline from 2008 to 2011. The second group of NH₄ ⁺, K⁺, Ca²⁺ and Cl^- show a period of strong increases in 2006, with a peak concentration in approximately 2008, followed by a period of decline in 2012, and finally an increase in the last few years. In the third group, Na⁺ and Mg²⁺ show a significant increasing trend through the entire period, but had variability from 2003 to 2010. Finally, there was a significant increasing trend for pH throughout the study period.

The non-linear methods used in this analysis found significant positive trends across all ion concentrations and pH.