1 Introduction

Rainwater chemistry, with its particular ionic composition, is fundamental for the recognition of potential pollutants, source type, pollutant dispersion in the atmosphere from natural or anthropogenic origin, and their consequent repercussions on public health, wildlife, and both natural and urban environments locally and regionally, all of which carry out the deposition process in the troposphere (Masood et al., 2019).

Air pollution is emitted by a variety of sources that end up being deposited in ecosystems (natural or urban). These pollutants can cause ecological changes, such as long-term acidification of soils and surface waters, soil–nutrient imbalances that affect plant growth, and the loss of biodiversity, but they also have repercussions for human health, such as allergies, asthma, and lung cancer, and is a risk factor depending on the type of pollutant and the time of exposure (Hoffmann, 2019).

Air pollution and its effects on the environment and human health comprises one of the most considerable concerns in urban areas. Because one-half of the population lives in cities, new monitoring studies, such as critical loads, and other types of measuring, are important. Sustainable development goals (SDG) directly include air pollution in various goals, with three of the 17 goals directly referring air directly as an important element for protecting environmental health. Goal 3 is to ensure healthy lives and promote well-being for all at all ages, with a target of 3.9 2030, and to substantially reduce the number of deaths and illnesses from hazardous chemicals and air, water, and soil pollution and contamination, but indirectly, other SDG are important to support the goals from air pollution reduction in cities and natural ecosystems. Goal 11 is to make cities and human settlements inclusive, safe, resilient, and sustainable, with the target 11.6 by 2030, to reduce the adverse per capita environmental impact of cities, including by paying special attention to air quality and municipal and other waste management. Goal 12 is responsible consumption and production, with the target 12.4 by 2020, to achieve the environmentally sound management of chemicals and all wastes throughout their life cycle, in accordance with agreed-upon international frameworks, and to significantly reduce their release to the air, water, and soil in order to minimize their adverse impacts on human health and the environment (Barcan et al., 2023). Additionally, other goals, such as SDG15, focus on the importance of conservation and the monitoring of green areas, where air is an important component for the transport of nutrients and pollution—Goal 15 includes protecting, restoring, and promoting the sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, halt and reverse land degradation, and halt biodiversity loss, with targets 15.1 and 15.4 by 2020, to ensure the conservation, restoration, and sustainable use of terrestrial and inland freshwater ecosystems and their services, in particular forests, wetlands, mountains, and drylands, in line with obligations delineated by international agreements, and by 2030, to ensure the conservation of mountain ecosystems, including their biodiversity, in order to enhance their capacity to provide benefits that are essential for sustainable development in all nations (Kanter & Brownlie, 2019; Mamani et al., 2022; ONU, 2019).

For major ecosystems, the atmosphere comprises the principal input of nitrogen and sulfur; therefore, it is crucial for the pollutant transport in local and regional basins. In urban environments, the combination of emissions stems from industry, construction, agriculture, and heavy vehicle traffic (Cronan, 2017; Panyasing et al., 2022).

Urban ecosystems are not the exception for nitrogen and sulfur input. The urban green spaces forming the urban ecosystems are under constant pressure from urbanization, pollution, land fragmentation, and biodiversity loss (Tian et al., 2020). However, urban green spaces are essential for cities, the ecosystem services that they provide, and the quality of human life; thus, monitoring is vital to conserve these spaces (Campbell, 2019).

In terrestrial ecosystems, there are some common causes for soil acidification, the first of which are the fertilizers, the removal of cations (Ca2+, Mg2+, K+, and Na+) by natural nutrient assimilation by native vegetation, the decomposition of organic matter, the recurrent presence of local acid rain, the leaching of exchangeable bases (in sites with a high rainfall regime), and the origin and nature of the parent material (according to its Al3+ : Ca2+ Mg2+ K+ ratio).

Worldwide, for many years and in many countries, studies on rainwater composition, wet and dry deposition, and acid rain have been reported, particularly in the United States (U.S.), Europe, and China, and there are a few on tropical regions in Latin America (Akpo et al., 2015; Martins et al., 2019; Mehr et al., 2019). In U.S., the improvement of total nitrogen deposition budget studying ecosystem exposure to excess nutrients and acidity requires accurate and complete deposition of reactive nitrogen (Nr) (Walker et al., 2019).

In the Latin American region, environmental and health issues related to the monitoring air quality began to entertain importance during last decades (Pereira et al., 2019). In Mexico, air monitoring programs started in the early 1950s (SEDEMA, 2019).

In Latin America and the Caribbean region, (Riojas-Rodríguez et al., 2016) reported 17 countries with official monitoring networks and a total of 39 monitoring networks with available online data. Brazil, Chile, Colombia, and Mexico have the most complete monitoring networks. In Mexico, atmospheric deposition networks operate only in some areas of the county; REEDA (the atmospheric deposition network) is located in Mexico City and consists of 16 stations. This atmospheric deposition network has been in operation since 2003, with annual reports in collaboration with the local government and the UNAM (National Autonomous University of Mexico) (INECC, 2017). Spatial and temporal variations in the chemical composition of precipitation (sulfate and nitrate ions and pH) were obtained at MCMA sampling sites, as well as atmospheric sulfur dioxide and nitrogen dioxide from 2003 to 2018 (Sosa et al., 2023).

In Mexico, there are, to our knowledge, few critical load studies. In 1999, Fenn and Poth. 1999 detected 18.5 Kg N ha-1 (through fall) and 20.4 Kg S ha-1 in the Desierto de los Leones National Park in the southwest area of Mexico City, and for pine stands, 15 Kg N ha-1 yr-1. Another study conducted Cerón et al. (Cerón Bretón et al., 2015) found, in Central Veracruz state, 5.5 Kg N ha-1 yr-1 and 8.8 Kg S ha-1yr-1.

The purpose of this study was to measure the amount of total nitrogen and sulfur that is available in a natural reserve located inside the National Autonomous University of Mexico and that forms part of the greenest conserved area of Mexico City. Other parameters were also analyzed in order to understand part of the dynamic of the anions and cations and the acid rain. The methodology used for the chemical composition of the rain follow the protocols of the World Meteorological Organization and the National Atmospheric Deposition Program (NADP), which will be presented in greater detail in the Methods section (NADP, 1999, 2014).

1.1 Study Area

Mexico City has 9,209,944 inhabitants, but the Metropolitan Area concentrates 21.2 million people. The Mexico City area is about 1485 km2, and 59% of this area is considered green protected area (INEGI, 2016; INEGI, 2022; PAOT, 2018) (Fig. 1). The southern and western parts of Mexico City are the greenest part of the city; this is where the major environmental services occur, including aquifer recharge, recreational and cultural activities, climate regulation, a biodiversity reservoir, and greenhouse gases fixation, to mention only some of these (SEDEMA, 2019).

Fig. 1
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Mexico City green areas

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The “Reserva del Pedregal de San Ángel” (REPSA), (García Barrios, 2014) was established in 1983. The reserve hosts a xerophytic and thorn–shrub plant community, which has been established for more than 1600 years since the eruption Xitle volcano, covering more than 70 km2 in the South of the Valley of Mexico (Siebe, 2009). The lava stone characteristics generates habitats for different plant and animals within a few meters that can pass from a cooling shadowy condition to high-temperature sun-exposed regions. This produces a high gradient of light, temperature, and humidity for organisms, which promoted heterogeneity that in turn increased the number of habitats and consequently the diversity of the area (Peralta Higuera and Prado-Molina, 2009; Rzedowski, 1954). The reserve is fragmented by roads and building centers that many animals must cross in order to colonize nonperturbed islands. Notwithstanding this, these islands are the last remnant of this ecosystem in the city.

2 Methods

2.1 Sampling

Two monitoring stations for the measuring of wet atmospheric deposition were used. One of these has been operating since 1986 at the “Instituto de Ciencias de la Atmósfera y Cambio Climático” of the UNAM ICAyCC (the CU-CCA station) and the so-called REPSA (“natural reserve located inside the “Universidad Nacional Autónoma de México, UNAM”) since 2017.

Daily samples were taken during the years 2017 and 2018. A total of 175 daily samples were collected for ICAyCC (CU-CCA) and 114 samples for REPSA. Samples for ICAyCC (CU-CCA) and REPSA were analyzed and compared for the same days of the events.

The wet deposition samples from the CU-CCA were collected daily throughout the year, employing an Aerochem Metrics Model 301.brand wet/dry collector, which is a device with two cubes in which the wet and dry deposition is collected separately for each event. In the case of the REPSA station, the N-CON Systems Company brand atmospheric deposition sampler, model ADS00-120, was utilized. The latter functions when the infrared optical precipitation sensor detects the onset of precipitation and uncovers the sample container within five drops. Within 2 min, after precipitation stops, the cover returns to the sample container in order to minimize exposure to dry deposition. The sensor also detects drizzle, heavy fog, or light snow, which may carry significant amounts of deposition (Fig. 2).

Fig. 2
figure 2

The wet/dry atmospheric–precipitation samplers. a The Aerochem Metrics Mod. 301 sampler. b The N-Con ADS 00-120 atmospheric deposition sampler

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Both atmospheric precipitation samplers are approved by the national atmospheric deposition program (NADP) for national trend network (NTN) sites (NADP, 1999 and N-Con, 2010).

The atmospheric wet deposition sampling was performed from 2017 to 2018, considering a Quality Assurance and Quality Control Program, which includes documentation and compliance sampling procedures, sample storage and shipping, equipment calibration, and data management procedures to ensure accuracy (NADP, 2018, and WMO, 2004).

Each sample was collected in a standard high-density polyethylene bucket, transferred to a polypropylene bottle (Nalgene®), and refrigerated at 4 °C for preservation. Samples were sent to the CU-CCA (Sección de Contaminación Ambiental-Instituto de Ciencias de la Atmósfera y Cambio Climático) Chemical Analysis Laboratory.

2.2 Chemical Analysis

Rain samples were filtered through a 0.22-μm Millipore membrane, and all materials were washed with deionized water before chemical analysis. The pH was measured within 24 h of the sample’s arrival at the laboratory using a pH meter (Corning 315, Metrohm 827, and Orion 960). Electrical conductivity was determined with YSI 32 and HORIBA D-424 conductivity instruments. Alkalinity (HCO3−) was determined using the Gran titration method with pH meters (Stumm & Morgan, 1970).

Based on US-EPA method 300.1 (Hautman et al., 1997), the anion concentrations, including chloride (Cl-), nitrite (NO2-), nitrate (NO3-), and sulfate (SO42-), and the cation concentrations including sodium (Na+), ammonium (NH4+), potassium (K+), magnesium (Mg2+), and calcium (Ca2+) were analyzed by ionic chromatography (IC): Perkin Elmer 250, Metrohm 883, and Waters 515, equipped with an isocratic LC pump and a conductivity detector, using Hamilton PRPX-100 and Metrosep C4 100 analytical columns, respectively.

High-purity standards certified and traceable to NIST were employed as calibration standards for each ion. Calibration samples were run with each analysis session. The quality of the analysis for each sample was verified using all the analyzed ions concentrations for the ion balance and the comparison of the calculated and measured conductances.

The detection limits in μEq L–1 were as follows: 2.29; 1.77; 0.54; 2.26; 2.50; 4.12; 1.28; 2.22, and 1.74 for SO42-, NO3-, NO2-, Cl-, Ca2+, Mg2+, K+, NH4+, and Na+, respectively.

3 Results

A total of 167 and 114 samples were collected for CU-CCA and REPSA, respectively. At the CU-CCA station, 75 daily events were collected during 2017 and 92 during 2018, while at REPSA, the samples included 51 and 63 for each year, respectively. For the propose of analysis, we decide to use the data of the rain events that matched between both sampling sites, noting the possibility of complementing historical data from REPSA with those from CU-CCA, considering that the REPSA station started to operate in May of 2017 and CU-CCA station, from 1986.

In the samples collected with a volume greater than 30 mL, pH, conductivity, and ionic speciation were determined, which allowed to identify the phenomenon of acid rain and the annual deposition of N-NO3-, N-NO2-, N-NH4+, and S- SO42- in the study area.

The pH, electrical conductivity, and speciation by ion chromatography were determined. The values were statistically compared between both sites. It is noteworthy that, for logistical reasons, the sampling in REPSA was incomplete; thus, comparison was performed with data from related samples.

3.1 Precipitation

The deposition of N and S is generally carried out by rain, snow, hail, or sleet, or due to the effect of gravity: It is measured by the amount or mass of chemical product transferred onto one square meter of land in a day, week, month, or year (kilograms/Ha). Therefore, to provide an estimate of the deposition of N and S within a specific area, the concentration determined in the laboratory must be multiplied by the amount of rainfall.

In Fig. 3a, we find all the samples collected in both years, 2017–2018, while in Fig. 3b, the samples were rain-events matches for both sampling sites, with 67% of the samples in the same rain event match. It is important to note that in 2017, the REPSA site monitoring starts in June.

Fig. 3
figure 3

Rainfall in the study area. a All samples of rain volume in 2017–2018. b Samples were rain event matches between both sampling sites

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Throughout the year, various rainfall rates take place at the CU-CCA and REPSA stations, according to Fig. 3a, the rainy season presents during the May–October period, and the median value registered for both monitoring stations was 780 mL (12.23 mm) and maximal rainfall was 4780 mL (74.93 mm) presented at the CU-CCA station.

3.2 pH

The pH was compared between certain southern stations in Mexico City (REPSA station, CU-CCA station, and DICONSA station) (Table 1). For all stations in 2017, the average was 5.22 and in 2018, and the average pH was 5.4. The trend is toward acid pH values.

Table 1 pH values in South stations, between 2017 and 2018
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It is noteworthy that the number of samples was different for some sampling sites. At the REPSA site, we collected, in 2017, 49 samples vs. 74 samples that were registered at the CU-CCA; therefore, in some cases the pH is lower (Fig. 4). To measure the pH concentrations at the CU-CCA and REPSA sites, same-day samples were analyzed, and the weighted value was 5.07 in 2017.

Fig. 4
figure 4

pH values for REPSA and CU-CCA in 2017 and 2018

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The spatial variation of pH values was studied, indicating a decreased from the North to the South of the MCMA (Sosa et al., 2019). This result is in line with meteorological conditions (i.e., prevailing winds blowing from the North to the South), as well as emission sources located in the North sector and emission sources located in the North outside of Mexico City, such as the industrial corridor Tula-Vito-Apasco industrial corridor, where industry operates with less strict control measures compared with the MCMA (Sosa et al., 2020).

The greatest impact of the acid atmospheric deposition in the MCMA takes place far from the initial emission sources, due to the effects that are recorded in the direction of the dominant wind and the transport of pollutants from power plants, industries, refineries, and areas of high vehicular traffic, reaching areas located downwind, which do possess have important emission sources, since they correspond to agricultural and forested areas of the city. In the MCMA, the prevailing winds during the majority of the year are from North to South; consequently, the emissions and precursors are transported to the South and Southwest, where they react, forming acid rain that affects the wooded areas in the South of the city.

In general, during the period studied, the areas with the least accumulated precipitation are the found in the Northeast; in the mountainous areas, there is greater precipitation due to the occurrence of an important orographic effect: the temperature and altitude of the mountains favor the formation of cloudiness and the condensation of the wind and water vapor, which produces more frequent and intense precipitation. In the Southwestern area, there is an accumulation of rain, with values within the range of 1270–1536 mL, while the areas with less accumulation are found to the Northeast and East of Mexico City with values of 486 and 584 mL, respectively (SEDEMA, 2018).

3.3 Ion Concentration

Anions and cations were quantified by ionic chromatography, with the minimum, maximal, and median for 2017 and 2018, respectively.

Figure 5 presents the boxplot distribution of the determined concentrations of anions and cations in the years 2017 and 2018 for both CU-CCA and REPSA stations. Due to the difference in the magnitude of the concentrations, the ions that have a maximum concentration of 30 μEq/L and those that occurred at a higher level (300 μEq/L) are presented.

Fig. 5
figure 5

Distribution of ion concentrations in CU-/CCA and REPSA samples corresponding to 2017 and 2018

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There is no significant difference among the ions from one site to another, according to the median test, except for potassium, magnesium, and bicarbonate. Furthermore, chloride was the most abundant ion in REPSA sampling site.

By 2018, there we find a qualitatively equal behavior between the two sites, but there is less variation in CCA. At the REPSA, site, this varies, but extreme values are found. At first sight, there are no significant differences. Potassium in CU-CCA and REPSA appears to be different.

In addition to precipitation analysis, ion concentrations were compared in a statistical test, in terms of the median and distribution, a significant difference with Mg2 +, K+, and HCO3- (pv < 0.05).

According to media tests, there is no significant difference between rainfall, ammonium, sulfate, and nitrate ion concentration, which are of interest due to their contribution of S and N. Thus. the data deriving from previous years of the CU-CCA station will be employed to establish a historical basis.

3.4 Nitrogen and Sulfur Analyses

The anthropogenic nitrogen (N) emissions originating from agriculture, animal husbandry, and the use of fossil fuels (vehicles, energy production, and industry) are dry and wet and are deposited into terrestrial ecosystems as oxidized and reduced forms of N (Erisman et al., 2007; Meyer et al., 2015).

According to results on wet atmospheric deposition samples, the amount of N and Sulfur (S) deposited at CU-CCA was, in the year 2017, 16.39 N total Kg ha-1year-1 and 9.44 of S total Kg ha-1 year-1, and in the year 2018, 22.03 N total Kg ha-1 year-1 and 11.37 S Total Kg ha-1 year-1 (Fig. 6).

Fig. 6
figure 6

Total deposition of N and S in 2017 and 2018

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Of the total N and S deposited between 2017 and 2018, a significant increase in nitrogen is observed, while sulfur increased slightly in proportion to nitrogen, with more than 20 kg ha-1 year-1.

By statistical analysis, there were no significant differences between REPSA and CU-CCA for the N and S analysis found. The CU-CCA data were utilized in this section because they present the largest number of samples analyzed.

Ammonium is the dominant ion form within the total nitrogen found in CU-CCA in 2017. In 2018, ammonium and sulfate were predominant. A significant increase was observed from 2017 to 2018 in the concentrations of these ions.

The relative abundance detected at both sites is presented in Fig. 7. In this, daily samples (per rain event) were concentrated in the days that rain events coincides in both sampling sites. RP corresponds to the REPSA site and CU-CCA site.

Fig. 7
figure 7

Percentage composition of anions and cations in CU-CCA and REPSA for 2017 and 2018 samples

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There was an increase of ammonium from 54 to 64% and a decrease in calcium from 34 to 27%. Ions are presented in decreasing order as follows: NH4+>Ca2+>Na+ > Mg2+>K+>H+.

The major anion was sulfate, and in 2017 and 2018, the percentage decreased from 42 to 37%, while nitrate was constant in both years and bicarbonate increased from 19.3 to 25.3%.

The behavior was similar for these ions. Ions are presented in decreasing order as follows:

SO42->NO3->HCO3->Cl- (Fig. 8).

Fig. 8
figure 8

Relative abundance at the sampling sites. a CU-CCA and b REPSA

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Also, stations that measure SO2 and NOx in Mexico City were analyzed. Two stations near the urban reserve were analyzed from 2017 to 2018 (PED: “Pedregal”, and CCA: “Centro de Ciencias de la Atmósfera”), in order to identify acid rain precursor concentrations.

SO2 and NOx air emissions are oxidized in the atmosphere and are wet, deposited as acidic SO42- and NO3- ions in precipitation, respectively. The SO42-/NO3- ratio have been used as an indicator of effectiveness in emissions reductions of SO2 and NOx in the U.S. (Lehmann et al., 2015).

In Asia, this ratio was applied to relate the emission of the acid rain precursors NOx and SO2 (Itahashi et al., 2015). The highest level of sulfate/nitrate ratio was found in the Chinese Province of Sichuan, where the value of 5.6 was measured, and this was a regional characteristic of this part of China (Qiao et al., 2015). The relation among SO2 and its more important products, such as sulfuric acid and particulate sulfate, had been studied in Beijing, China, where an important reduction in SO2 was reached from 2013 to 2018 (Li et al., 2020).

The SO42-/NO3- ratio at a Mexican sampling site was 4.9 in 2015 and was clearly the highest value compared to other sites located in the U.S., on the Gulf of Mexico, whose ratios were between 1.03 and 2.38. This result indicates the importance of the sulfur compound emission sources in Mexico (Sosa et al., 2020)

Although SO2 has been reduced significantly during last 30 years in the MCMZ, sulfur compounds continue being important in wet atmospheric deposition, in that SO42- is the major ion compared with the NO3-. The ratio of SO42-/NO3- had an average of 1.5 (Sosa et al., 2019).

Figure 9 shows the 24-h average values of the concentrations of SO2 and NOx registered at the PED and CCA air quality monitoring stationsof the Air Quality Monitoring Network (SEDEMA, 2022). For the years 2017 and 2018, respectively, which are located in the area of influence of the urban reserve (REPSA).

Fig. 9
figure 9

SO2 and NOx concentration in air quality monitoring stations (PED and CCA)

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The general behavior of SO2 and NOx concentrations tend to be lower during the rainy season (from May to November), while an increase is observed during the dry period (December to April).

According to the results obtained, there is no difference between the median concentrations, both for SO2 and NOx, at the two studied monitoring stations (Table 2).

Table 2 Statistics of the CCA and PED stations for the years 2017 and 2018
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4 Discussion

In the U.S., S and N have been studied not only as acid rain precursors but also for their potential effects in ecosystems, considering critical loads.

In 2011, the national atmospheric deposition program (NADP) formed the Total Deposition Science Committee (TDep) to improve the estimates of atmospheric deposition through the integration of measured and modeled wet, dry, and total deposition of sulfur and nitrogen (Lehmann et al., 2015). Total deposition of these pollutants provides data for ecological assessments, including evaluation of critical loads. The critical load represents the threshold at which air pollution deposition results in a response in sensitive resources or ecosystems. Critical loads can be developed for various ecosystem responses, including the spread of invasive species, changes in soil chemistry, impacts on aquatic systems, etc. For ecosystems currently experiencing damage from atmospheric pollution, critical loads aid in determining the deposition reductions required for ecosystem recovery (Blett et al., 2014).

(Castellanos Vargas et al., 2017) and Sparks (2003) note that the edaphic acidity of REPSA ranges from 6.15 to 4. For REPSA, it is important to conduct more in-depth studies related to soil acidity; Rzedowski (1954) describe that the soils of “Pedregal de San Ángel” in Mexico City have large amounts of cations such as K+ and Ca2+, possibly promoted by the plants of the reserve, which use these cations for growth and nutrition. Although these ions are common in the region of the Basin of Mexico for the sedimentary alluvial loads (brought by the wind or dissolved in rainwater) and because of their eluvial origin, that is, by means of the in situ hydrolysis of the feldspars present in the basalt, the soils of Pedregal are poor in N and P, and they are employed by plants for the formation of aerial and subway tissues; they form part of the organic matter and originate a deep mulch layer. The acidity can be attributed to the constant washing of exchangeable bases. Lot and Cano (2009) indicate that the average annual precipitation of the REPSA can reach up to 833 mm, and the rainy season comprises five months (from June to October), and during this period, the soil is subjected to a hydric sweep of K+, Ca2+, Mg2+, and Al3+.

Magnesium is found in nature, principally in rocks. Magnesium enters the water by rock erosion, washed by the rain. Also, magnesium is employed in industry for the production of plastic, in agriculture as fertilizer and for the growth of cattle; potassium compounds are linked to fertilizers and also it is used in the production of soap, detergents, and fireworks (Lenntech, 2019).

4.1 Atmospheric Deposition

Nitrate contractions in moist atmospheric deposition were similar between the two seasons. For this ion (NO3-) from 2017 to 2018, an increase in the medians was observed, with a value of 31.13 for 2017 in both stations, while in 2018, these were 45.8 for CU-CCA and 43.95 for REPSA. An increase on the order of 50% was obtained from 1 year to another (47.12% for CU-CC and 41.18% for REPSA). Given the increase that occurred for NO3-, it would be necessary to continue the study in order to observe the trends of atmospheric deposition in the reserve, thus being able to establish the impact that this deposition would exert on it.

According to the comparative analysis of medians for sodium (Na+) between the CU-CCA and REPSA stations, there was no significant difference (p = 0.254), although the CU-CCA station presents higher concentration values and a higher standard deviation. An increase from 2017 to 2018 of 26% is observed for the CU-CCA station and of 45% for REPSA. It is worth mentioning that the median value in 2018 was the same value for both stations.

Regarding ammonium (NH4+), although the median value was higher at the REPSA station in 2017, for 2018 both stations presented the same value; however, there was no significant difference (p = 0.658) between both sites. An increase of 35% from 2017 to 2018 was observed at the CU-CCA station and of 17% at the REPSA station.

In this same respect, for potassium (K+) there was no significant difference in the concentrations determined at both sites for the year 2017 (p = 0.187), but a difference did exist for the year 2018 (p > 0.05). An increase of 50% from 2017 to 2018 was observed at the REPSA station, probably caused by forest fires within the reserve, while the median value was maintained at the CU-CCA station in this same period.

5 Conclusions and Recommendations

The values in pH indicate the presence of acid rain in the studied area; in addition, no statistical differences were detected between stations and sampling year. According to media tests, there was no significant difference between rainfall, NH4+, SO42-, and NO3- ion concentration (NO3- concentrations were similar between the two stations). These are of interest due to their contribution of S and N; therefore, the data from previous years of the CU-CCA station will be used to establish a historical basis.

According to the comparative analysis of medians for sodium (Na+) between station CU and REPSA, there was no significant difference (pv = 0.254), although the CU-CCA station presented higher concentration values and a higher standard deviation. An increase from 2017 to 2018 of 27% was observed for the CU-CCA station and of 453% for REPSA; it is noteworthy that the median value in 2018 was the same for both stations.

Regarding NH4+, although the median value was higher at the REPSA station in 2017 and in 2018, the value is the same. There is no significant difference (pv =0.658) between both sites, and an increase of 35% from 2017 to 2018 was observed at the CU-CCA station and of 17% at the REPSA station.

In the same manner, in K+ concentration, there was no significant difference in the concentrations determined at both sites for 2017 (pv = 0.187), but a difference was determined in 2018 (p > 0.05). An increase of 50.32% from 2017 to 2018 was observed at the REPSA station, while the median value was maintained at the CU-CCA station during this same period. From 2017 to 2018, NOx levels were similar, and also, SO2 levels were the same for the two air-quality monitoring stations.

For air-quality determination, the Pedregal station (24 ppb) presented lower levels than CU-CCA (27 ppb). It is recommended to conduct an in-depth study on critical loads and their effects on biodiversity of the green areas of Mexico City in order to conserve and manage these important areas, which provide, in terms of ecosystem services, a city with more than 8 million inhabitants, and it is additionally recommended to, study ion interaction with vegetation, and also, the performance of a complex assessment in terms of air quality and soil deposition and its impact on soil.

5.1 Recommendations

It is recommended to carry out the evaluation of critical loads and their effects on green areas, soils, and air quality for the conservation and management of protected natural areas, which provide ecosystem services to Mexico City. Also, the interaction between ions and native vegetation should be studied. It is suggested to delve into the interactions of the ions that presented the highest values in relation to the native vegetation and the soil of the reserve, as well as the possible long-term consequences.