1 Introduction

Atmospheric deposition is the dominant source of ions and nutrients in pristine, anthropogenically unpolluted areas such as the northeastern United States and southeastern Canada (Pssener, 1999; Gilliam et al. 2019). In remote sites, such as alpine zones, dust deposition also represents an important source of organic matter (Mladenov et al. 2012). Because wet and dry atmospheric deposition play an essential role in controlling the concentrations of biogeochemical elements in the environment, it is important to monitor the atmospheric ionic composition and deposition to identify and document global change (Durašković et al. 2012; Köhler et al. 2012).

Several studies (e.g., Rogora et al. 2001; Sicard et al. 2007; Lawrence et al. 2015; Duan et al. 2016; Cheng et al. 2019) have focused on understanding the effects of atmospheric deposition in soil and bodies of water. For example, under strong acid deposition caused for NO3− and SO42− transport, cations are displaced and leached from the soil. Thus, in areas with thin soils or low alkaline reserve, acid rain can lead to environmental acidification (Reuss and Johnson 1986; The MOLAR Water Chemistry Group 1999). Elevated deposition fluxes of NO3− and SO42− are common in regions with a high density of environmental SO2 and NOX precursors, such as in areas with extended industrial activities (Durašković et al. 2012; Keresztesi et al. 2019).

However, most of the studies that have evaluated atmospheric deposition and its effects on soil and water chemistry have been carried out in temperate and polar regions (e.g., Euro-limpacs, ALSC, NYSERDA, ALTM, PIRLA, and The MOLAR Project); contrarily, few studies have been done in the tropics (e.g., Galy-Lacaux et al. 2009; Hofhansl et al. 2011; Köhler et al. 2012), where differences from the temperate climate seasonality (spring–summer–fall–winter) are to be expected.

Mostly located in tropical areas, developing countries have displayed increasing urbanization, industrialization, and agriculture, all of which promote a rapid increase in gas and aerosol particle emissions. Under typical tropical conditions such as high UV radiation, temperature, and atmospheric water vapor content, intense photochemistry is promoted throughout the year, leading to the generation of acid species (Lacaux and Sigha 2003).

Nevado de Toluca (Xinantécatl) is a tropical, high-altitude volcano in the Trans-Mexican Volcanic Belt (Macías et al. 1997). The volcano and the surrounding woodland area constitute the National Park Zona de Protección de Flora y Fauna Nevado de Toluca (ZPFFNT). The Emissions Inventories of the Metropolitan Areas from the Toluca Valley (http://sma.edomex.gob.mx/sites/sma.edomex.gob.mx/files/files/sma_pdf_ie_zm_vt.pdf) and Mexico City (http://www.aire.cdmx.gob.mx/descargas/publicaciones/flippingbook/inventario-emisiones-2016/mobile/#p=1) show urban and industrial activities are an important source of SO2 and NOX, both precursors of acid precipitation that could potentially threaten the environmental health of the ZPFFNT. The ZPFFNT is considered to be a remote zone, mostly because of its high altitude. It is sheltered from pollution, and hence, no protective actions against airborne pollution were considered in the ZPFFNT National Park Management Plan.

Despite being considered a remote, and therefore, guarded region, the long-range reach of airborne pollutants is a potential threat to the ecological integrity of the ZPFFNT and the surrounding woodland. Therefore, safeguards should be considered in ZPFFNT management plans to guarantee its preservation. To provide a basis for preservation of the integrity of this important and unique Mexican high mountain ecosystem, our study aimed to analyze the bulk atmospheric deposition chemistry and its temporal dynamics throughout an annual cycle in order to provide reliable data on the air pollution reaching the ZPFFNT and the related potential risks. We anticipated that the atmospheric deposition would be high as a result of the anthropic activities taking place in and around Toluca and Mexico cities and also that it would be highly seasonal because of the well-defined cold/dry and warm/rainy seasons of the tropical area.

2 Methods

2.1 Study Area

Nevado de Toluca volcano is located at 19° 06′ 30″ N and 99° 45′ 30″ W, about 20 km and 72 km WSW of Toluca and Mexico cities, respectively (Fig. 1). With a maximum height of 4680 m a.s.l., this stratovolcano is the fourth highest mountain in Mexico. This andesitic–dacitic volcanic complex is currently quiescent; its last eruption occurred about 3300 years BP (Macías et al. 1997; Arce et al. 2005).

Fig. 1
figure 1

Geographic location of the ZPFFNT (black) in the State of Mexico (insert, shaded lines). The location of Mexico City is showed in dark gray. (Draw in QGIS 2019 program with data from National institute of statistics and geography of Mexico, INEGI)

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The “Nevado de Toluca” weather station (19° 07′ 07″ N, 99° 46′ 56″ W; 4139 m a.s.l.; average data from 1951 to 2010) recorded average monthly mean temperatures ranging between 2.5 °C in January and 10.2 °C in April, with an annual mean temperature of 3.9 °C. Total annual precipitation is 1227.1 mm, and precipitation ranges from 12.4 mm in March to 243.5 mm in July. The surrounding vegetation is dominated by pine species (e.g., Pinus hartwegii, Abies religiosa) forest (Franco et al. 2006). The predominant vegetation above the tree line (~3900–4060 m a.s.l) is grassland and alpine zacatonal (Rzedowski 1981).

2.2 Field Sampling

The sampling site (19° 07′ N, 99° 46′ W), adjacent to the Nevado de Toluca weather station, is high (4139 m a.s.l.) on the volcano’s northeast side, above the tree line, and facing Toluca and Mexico cities. Bulk deposition was collected monthly from October 2017 to September 2018 (a total of 12 samples). Weather data (temperature, precipitation, wind speed, and wind direction) for the sampling period were obtained from the National Meteorological Service (SMN) weather station “Nevado de Toluca.”

2.3 Sampling Design

Bulk deposition samplers were made from a funnel (diameter of 200 cm) attached to a 20-l polyethylene container. Mineral oil (500 ml) was added to each container to prevent sample evaporation. The bulk deposition samplers and bottles for aliquots were washed in advance with phosphate-free detergent (Extran®) and then rinsed with deionized water until the water conductivity inside the containers was less than 1 μS cm−1.

Two samplers were placed at site to collect bulk deposition. Each month, the containers were replaced with new ones. The total sampling volume was measured, and aliquots of 600 ml were taken and transported under dark and cold (4 °C) conditions to the laboratory.

2.4 Laboratory Analysis

All materials that were in contact with the samples were washed beforehand with deionized water. The bulk deposition samples were separated from the mineral oil with a separatory funnel. Then, they were filtered through a 0.22-μm Millipore membrane. The pH and electric conductivity (standardized to 25 °C = K25) were measured within 24 h of arrival to the laboratory using a pH meter (916 Ti-Touch Metrohm) and conductivity meter (HORIBA D-424), respectively.

The cations (Na+, NH4+, K+, Mg2+, Ca2+) and anions (Cl−, NO3−, and SO42−) were analyzed by ion exchange chromatography (850 Professional IC, Metrohm). The HCO3− and CO32− were measured according to Armienta et al. (2008). To validate the data, we used the WMO (2004) criteria for ion balance and specific conductance calculations.

Ion balance was performed according to the following equation:

$$ \%\mathrm{IB}=\frac{\mathrm{rc}-\mathrm{ra}}{\mathrm{rc}+\mathrm{ra}}\times 100 $$

where %IB is the percent ionic balance, rc is the cation sum in μeq L−1, and ra is the anion sum in μeq L−1.

Ionic measurements were performed on three types of blanks: (1) travel blank, (2) laboratory blank, and (3) mineral oil blank. The latter consisted of measuring the deionized water before and after being in contact with the mineral oil for a month to rule out any possible changes in water chemistry due to interference from the mineral oil. The blank values were found below the detection limit of 850 Professional IC, Metrohm (1.77, 2.29, 2.26, 2.50, 3.29, 1.79, 2.22, and 1.74 μeq L−1 for NO3−, SO42−, Cl−, Ca2+, Mg2+, K+, NH4+, and Na+, respectively). The data validation was according to Sosa-Echeverría et al. (2018).

The normality fit of the data was assessed. Because none of the variables fit the normal distribution, Spearman correlation (non-parametric test) was performed to assess which variables correlated with observed changes in pH of the bulk deposition.

The ion concentrations recorded in mg L−1 were converted to mg m−2 day−1 using the total sample volume, number of collection days, and funnel diameter per collection to calculate ionic rates.

Neutralization factor (NF) was performed according to the following equation:

$$ \mathrm{NFxi}=\frac{\left[\mathrm{Xi}\right]}{\left[{\mathrm{NO}}_{3^{-}}+{\mathrm{SO}}_{4^{2-}}\right]\ } $$

where NF is the neutralization factor, Xi is the cation in μeq L−1, and [NO3− and SO42−] is the NO3− and SO42− sum in μeq L−1.

3 Results

3.1 Meteorological Characterization

Two periods could be defined according to environmental temperature. Lower temperatures occurred from October 2017 to February 2018 (Fig. 2a), with the minimum recorded in January (2.5 ± 2.6 °C). Higher temperatures were recorded from May to September 2018, with the maximum recorded in May (6.2 ± 2.6 °C). The annual mean temperature was 4.8 ± 0.9 °C. There were significant differences (p < 0.05) between the “cold” (October 2017 to April 2018) and the “warm” (May to September 2018) periods, with averages of 3.9 ± 0.9 and 5.4 ± 0.5 °C, respectively.

Fig. 2
figure 2

a Seasonality in the ZPFFNT according to rainfall (bars) and temperature (dot line). Cold-dry season (gray box) goes from November to April. b Dominant winds at ZPFFNT during cold-dry (left side) and warm-rainy (right side) seasons. c pH and conductivity (K25) of the bulk deposition

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Two periods were also identified according to rainfall: the “dry” period, from November 2017 to April 2018, and the “rainy” period, including October 2017 and from May to September 2018 (Fig. 2a). There were significant differences (p < 0.05) between these periods, with 109.8 mm (18.2 ± 19.7 mm) of precipitation in the dry season and 1055.9 mm (175.9 ± 59.2 mm) in the rainy season. The maximum precipitation was recorded in August (266.4 mm), and the minimum occurred in December (1.8 mm). The total accumulated rainfall in the year was 1165.6 mm.

In October 2017 and from May to September 2018, the rainy period, southwest to northeast (SW–NE) winds predominated. From November 2017 to April 2018, the dry season, the dominant winds were in the opposite direction (NE–SW). Winds were grouped in the same two periods as precipitation: SW–NE (November 2017 to April 2018) in the dry season and NE–SW (October 2017 and May–September 2018) in the rainy season (Fig. 2b). The wind speed had an average of 15.7 ± 5.1 km h−1 in the dry season and 11.4 ± 4.5 km h−1 in the rainy season. There was no significant difference in wind speed values between the dry and rainy period.

3.2 Bulk Deposition

3.2.1 pH and K25

pH remained acidic (i.e., < 7) the entire time. The highest values were measured from October 2017 to May 2018, with a maximum of 6.7 ± 0.2 in February (Table 1; Fig. 2c), whereas the lowest values were observed in June–September 2018, with a minimum of 4.27 ± 0.1 in September 2018. Values corresponding to the dry season had an average of 5.9 ± 0.4 and were statistically different (p < 0.05) from those corresponding to the rainy period, which had an average of 4.9 ± 0.6.

Table 1 Chemical composition of the bulk deposition at the ZPFFNT [electrical conductivity (K25) in μS cm−1; volume weighted averages of ionic concentrations in mg L−1. Avg. average, s.d. standard deviation, Max maximum, Min minimum. Ionic balance (IB %) = 1.8]
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K25 increased gradually from October 2017 to its maximum in February 2018 (Table 1; Fig. 2c) and then decreased to its lowest value in June 2018. The maximum value of K25 temporally matches the maximum value of pH (February 2018). There was a significant difference between K25 of the rainy period (20 ± 7.0 μS cm−1) and dry period (60.4 ± 29.8 μS cm−1), with a negative correlation (r = −0.7, p < 0.05) between K25 and rainfall, which could be explained by a dilution effect.

3.2.2 Ionic Composition

Cations (Ca2+, NH4+, K+, Mg2+, and Na+), HCO3−, and Cl− displayed similar behavior during the year and had the highest concentrations in February 2018 (Table 1; Fig. 3a, b). The lowest values for Ca2+ (0.18–0.84 mg L−1) were recorded from June to September 2018, and those for HCO3− (below the detection limit) were recorded from July to September 2018. The lowest values for K+ and Mg2+ and for NH4+, Na+, and Cl− were recorded in August 2018 and June 2018, respectively. All of these ions (Ca2+, NH4+, K+, Mg2+, Na+, HCO3−, and Cl−) presented significant differences (p < 0.05) between the dry (higher concentrations) and the rainy (lower concentrations) seasons.

Fig. 3
figure 3

Dynamics of the concentration of a cations and b anions in the bulk deposition of the ZPFFNT from October 2017 to September 2018 (the gray square marks the cold-dry season). c Cationic and d anionic composition (percentages) in the bulk deposition of the ZPFFNT in the rainy and dry period

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The fluctuations in the concentrations of NO3− and SO42− were similar to each other but different from the rest of the measured ions (Fig. 3b); NO3− and SO42− concentrations display three peaks (October 2017, January 2018, and March 2018). The minimum concentrations were in June 2018 (NO3−, 0.45 mg L−1; SO42−, 1.11 mg L−1), and the maximum concentrations were in August 2018 for NO3− and March 2018 for SO42− (Table 1). There was no statistical difference between the concentrations of these two ions between the dry and rainy seasons (p > 0.05).

The ionic dominance was similar in the dry and rainy seasons. The cation dominance was Ca2+ > NH4+ > K+ > Mg2+ > Na+ in both periods (Fig. 3c), and the anion dominance was SO42− > NO3− > HCO3− > Cl− in both periods (Fig. 3d). The difference is that cations, HCO3−, and Cl− were more diluted in the rainy season, whereas NO3− and SO42− presented very similar concentrations in both periods.

3.2.3 Bulk Deposition Rates

The highest total ionic deposition rates occurred in October 2017 and from July to September 2018, the rainy season, while the lowest rates occurred from November 2017 to March 2018, the dry season (Fig. 4).

Fig. 4
figure 4

a Total ionic rates, b anionic rates, and c cationic rates of the bulk deposition in the ZPFFNT. Note the different scale between b and c

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The highest values of anions during the year were those of NO3− and SO42−, comprising 42% and 34% of the total amounts throughout the year, respectively. Ca2+ and NH4+ displayed the highest values among cations; however, these were below those observed for NO3− and SO42−. K+, Na+, Mg2+, and Cl− had lower contributions, most of which were less than 2 mg m−2 day−1.

4 Discussion

Our measurement of the major ions carried by bulk deposition found the order of Ca2+ > NH4+ > K+ > Mg2+ > Na+ for cations and the order of SO42− > NO3− > Cl− for anions. This ion dominance for cations is exactly the same as that reported in the alpine–subalpine watershed in Rocky Mountain National Park, Colorado (Clow and Mast 1995); however, at this site, NO3− is the dominant anion over SO42−. On the other hand, Camarero et al. (1995) recorded Na+ as the third most abundant cation in the Pyrenees and Alps, contrary to our findings of Na+ being least abundant. The dominance in anions found in their study was the same as that in this study.

The Pyrenees and Alps (granite, gneiss) and the Rocky Mountains (metamorphic, dolomite) differed lithologically from Nevado de Toluca (andesite, dacite). The weathering of the common feldspar of the ZPFFNT (García-Palomo et al. 2002) releases K+, HCO3−, and 4SiO2, nonetheless K+ and HCO3− are not the most abundant ions in the bulk atmospheric deposition in the Nevado de Toluca, suggesting feldspars are not an important source of ions.

Although the dominance relationships of the ionic atmospheric deposition found in our study are similar to those found at other remote sites, the concentrations found in our study are higher than those found in other remote areas. For example, in the Nevado de Toluca, Ca2+ was 3.5 mg L−1 and NH4+ 0.74 mg L−1, but these concentrations were 0.46 mg L−1 and 0.17 mg L−1 in the Rocky Mountains (Clow and Mast 1995), 0.6 mg L−1 and 0.40 mg L−1 in the Alps, and 1.78 mg L−1 and 0.40 mg L−1 in the Pyrenees, respectively (Camarero et al. 1995).

Even though the Nevado de Toluca is a high mountain environment (> 4000 m), it is also located close to Toluca (~20 km) and Mexico (~72 km) cities. They are both industrialized areas and the most probable contamination sources for ZPFFNT. In this respect, NH4+, Na+, Mg2+, and Cl− concentrations in our study were similar (NH4+ 0.14 to 4.81 mg L−1, Na+ 0.04 to 14.1 mg L−1, Mg2+ 0.04 to 0.67 mg L−1, and Cl− 0.36 to 4.85 mg L−1) to those reported in other studies including Toluca and Mexico cities (e.g., García et al. 2006; Cerqueira et al. 2014; Cerón et al. 2014; Araujo et al. 2015).

Differently, K+ and Ca2+ presented higher annual averages in our study than at other locations near urban sites, although the average values for the rainy season (0.28 and 0.85 mg L−1, respectively) are more similar than the averages reported in other wet deposition studies (e.g., García et al. 2006; Cerqueira et al. 2014; Cerón et al. 2014; Araujo et al. 2015), with a range of 0.03 to 0.49 mg L−1 for K+ and 0.15 to 2.07 mg L−1 for Ca2+.

Finally, NO3− (5.95 mg L−1) and SO42− (8.40 mg L−1) were within the ranges reported at other sites near industrial areas (Al-Momani et al. 2002; Cerón et al. 2014; Araujo et al. 2015), 1.21 to 16.54 mg L−1 for NO3− and 5.42 to 20.63 mg L−1 for SO42−.

Chen et al. (2001) found microorganisms oxidize ammonium to nitrate (Dämmgen et al. 2005; Dodds and Whiles 2010) in samples of atmospheric deposition studies when sampling periods are relatively long. In our study, the decrease of NH4+ does not mirror in a NO3− increase (Fig. 3a, b) suggesting this microbial-mediated transformation is not relevant to bias our results. Probably, this result from the lower abundance of airborne bacteria (two times lower) found at high-altitude environments respecting to suburban sites (Tanaka et al. 2019).

Seasonality of pH, K25, and ionic composition has been recorded in several studies. In temperate zones, changes are associated with spring, summer, fall, and winter seasons, but in tropical areas, they are associated with a dry season and a rainy season (Clow and Mast 1995; Muri 2013; Cerqueira et al. 2014; Cerón et al. 2014). However, considering the precipitation pattern results, temperate and tropical areas are similar.

Lower K25 and more acidic pH characterize periods of higher precipitation (April–June), whereas higher K25 and pH records occur during the dry period (December–March) (e.g., Muri 2013; Cerqueira et al. 2014; Araujo et al. 2015). K25 is higher during the dry season because the reduction of precipitation increases the compound load in the atmosphere. The MOLAR Chemistry Group (1999) and Muri (2013) observed a similar behavior, higher electrical conductivities occurred at lower rainfall rates.

The most acidic values in the Nevado de Toluca were recorded in July, August, and September 2018 (pH < 5). These acidic bulk deposition values could be associated with the potential contribution of acidic substances present in the atmosphere (e.g., NHO3−, H2SO42−; De Mello and De Almeida 2004). Urban and industrial pollutants decrease pH values (pH < 5.0) through the presence of NOx and SOx, which transform into HNO3 and H2SO4, respectively (EPA 1999; Migliavacca et al. 2005; Sicard et al. 2007).

Conversely, agricultural areas increase pH values (e.g., Sicard et al. 2007) when neutralization of HNO3 and HCl vapor occurs (Trebs et al. 2005) through the formation of nonvolatile (NH4)2SO4, which is favored when NH3 is in excess of H2SO4 and the meteorological conditions are appropriate (e.g., low to moderate temperature). In regions where there are important sources of NH3, neutralization of acidifying chemical species takes place. This could be the case here, as suggested by the observed high flows of NH4+. Additionally, neutralization of acidic species can also occur by interaction with other basic substances such as CaCO3 and MgCO3 (Araujo et al. 2015; Migliavacca et al. 2005; García et al. 2006).

In the Nevado de Toluca volcano area, both urban and industrial acidifying (i.e. NO3−, SO42−) and agricultural basifying (i.e. NH4+) pollutants were expected. By analyzing the pH dynamics and the deposition rates of anions and cations, some insight emerges. In the dry season, the pH is less acidic (≥ 5.4) peaking in February 2018, when it almost reached neutral pH. In the rainy season, more acidic values were recorded. According to Fig. 4b, c, the deposition rates of anions and cations in the dry season are lower, while higher during the rainy season. Nonetheless, the ratio alkalinizing/acidifying ions is higher (proportionally more alkalinizing ions) in the dry, while lower (proportionally more acidifying ions) in the rainy season.

As seen in Fig. 3a, Ca2+ concentration and pH follows similar dynamics; therefore, it could be assumed Ca2+ is playing an important role in raising (i.e., neutralizing) the pH in the dry season, an observation supported by the high negative Spearman correlation (−0.89) between Ca2+ and H+ (Table 2). Additionally, the most acidic values recorded in July, August, and September 2018 (pH < 5) matched the lower Ca2+ concentration combined with an increase in NO3− and SO42− concentrations. Finally, the calculated neutralization factor, NF, in the rainy season (0.4) shows cations cannot neutralize the NO3− + SO42- acidity; opposite, the NF (1.7) of the dry season indicates there is a neutralization effect of cations on NO3− + SO42-, where Ca2+ alone has the main effect with a NF of 1.1.

Table 2 Spearman correlation between ions concentration in the bulk deposition of the ZPFFNT
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The NO3− and SO42− concentration were higher in the rainy season when the wind pattern is NE–SW, that is, the dominant winds blow from Toluca and Mexico cities to the Nevado de Toluca volcano. Cerón et al. (2014) reported a same behavior (i.e., higher NO3− and SO42− concentration during the rainy season) in Mexico City where, in addition, similar concentrations were reached in the rainy season with 5.29 mg L−1 for NO3− and 5.98 mg L−1 for SO42-.

The deposition rates of cations, Cl−, and HCO3− in the Nevado de Toluca are within the values observed at other tropical and temperate sites near urban and industrial zones (The MOLAR Water Chemistry Group 1999; De Mello 2001; Al-Momani et al. 2002; Leal et al. 2004; Araujo et al. 2015). Differently, the annual average values for NO3− and SO42− are three times higher than those observed in other regions, even in regions close to industrial zones (Al-Momani et al. 2002). This is explained by the higher values found during the rainy period, when the prevailing winds blow from Toluca and Mexico cities.

There is a high NO3− and SO42− load in the rainy season. The peak load in July to September 2018 resulted in the most acidic pH values (4.2–4.5) of the bulk deposition. For the remainder of the time, the cations input seem able to buffer the acidity from NO3− and SO42−. If strong acids (HNO3 and H2SO4) cause the bulk deposition acidity and these acids are not neutralized, the H+/ NO3− + SO42− ratio would be expected to be 1 (García et al. 2006). From July to September 2018 (the rainy season peak), there was an increase in acidifying species and a decrease in alkalizing ones, with a H+/ NO3− + SO42− ratio of 0.7 to 0.8, so there was no neutralization of strong acid species (HNO3 and H2SO4). In the rest of the year (October 2017 to June 2018), the H+/ NO3− + SO42− ratio was between 0.2 and 0.5, so the pH values were less acidic and more similar to the natural pH of rain, i.e., between 5.6 and 6.2 (Charles 1990).

Global nitrogen and sulfur have increased owing to anthropogenic activities, including production of synthetic fertilizers and combustion of fossil fuel products (Vitousek et al. 1997; Galloway et al. 2003; Smith et al. 2011). Even remote regions may receive increased inputs through long-range airborne pollutants, as observed in this study. Modest increases in nitrogen and sulfur deposition can have significant effects in the ecosystem through soil and water acidification (Galloway et al. 2008). Soil acidification is often responsible for forest decline, as in Japan (Ito et al. 2011). Water acidification decreases the diversity of phytoplankton, zooplankton, and fish (Henriksen et al. 1989; Almer et al. 1974), as in the Adirondacks Mountains where trout disappear when the pH of lakes falls below 5.4 (Schofield and Driscoll 1987).

The Nevado de Toluca volcano is an area with low buffer capacity, where exposure to acidic atmospheric deposition can lead to soil acidification and thus alter ecosystem functioning. The Nevado de Toluca volcano is part of a protected natural area (ZPFFNT) with a management plan for its conservation. However, airborne pollutants are already reaching even the highest portions of the Nevado de Toluca volcano, where they threaten the health and integrity of the ecosystem, particularly through the deleterious effects of acid precipitation. At present, the ZPFFNT management plan does not consider protection actions/strategies against acid precipitation. It is imperative to take actions to preserve this important and unique Mexican high mountain ecosystem.

5 Conclusions

Even in its remote location, airborne pollutants from Toluca and Mexico cities are already affecting the ZPFFNT. The bulk deposition concentration is higher than that observed in other remote regions, but similar to those from urban and industrial areas. There is a clear seasonality (rainy/dry seasons) in the bulk deposition concentration and pH, which are lower during the rainy season. The more acidic conditions (pH < 5) of the bulk deposition are associated with the highest recorded NO3− and SO42− concentrations between July and September, when wind blows from Toluca and Mexico cities toward the Nevado de Toluca. During the dry season, wind blows from the opposite direction, and alkalizing compounds (basic cations) derived from agricultural land partially neutralize the acidic compounds, keeping the pH within natural values (5.6 < pH < 7). Because the ZPFFNT is threatened by acid precipitation, prevention and mitigation actions as part of its management plan are required to avoid further adverse impacts and deterioration.