Spatiotemporal Variations in Chemical Composition of Wet Atmospheric Deposition in Mexico City

A comprehensive analysis of the chemical composition of wet atmospheric deposition was performed on 7048 samples collected between 2003 and 2021 over Mexico City. The descending ion abundance trend was NH 4+ > SO 42– > NO 3– > Ca 2+ > Cl – > H + > Mg 2+ > Na + > K + , probably associated with industrial activity, heavy traffic and agricultural activities. Although main precursors have decreased importantly, ion composition did not show a clear trend throughout the years. Maximum concentrations of major ions were found in the northern and central part of the megacity, due to the impact of the Tula industrial corridor located north of the city. Weekly pH values varied from 3.6 to 9.4, being 27.1% of the values acidic. Fractional acidity showed that in sites located at higher altitudes, maximum 84.5% of the acidity was neutralized, whereas at northern stations at lower altitudes neutralization was observed up to 98%, due to the presence of alkaline species coming from the cement industry. Average ratios of (NH 4+ + Ca 2+ )/(NO 3– + SO 42– ) were > 1, suggesting neutralization of SO 42– and NO 3– by NH 4+ and Ca 2+ . Average NO 3– /SO 42– ratios suggested that the acidity was mainly influenced by sulfates coming from H 2 SO 4 . Wet deposition ranged from 4–9.9 kg ha –1 year –1 and from 8.2– 17.6 kg ha –1 year –1 for sulfur and nitrogen, respectively, among the geographical areas. The results of this study highlight the sensitivity of wet deposition chemistry to geographical, elevation and source considerations.


INTRODUCTION
Population growth and industrial expansion have degraded air quality, especially in the megacities of developing countries (González and Aristizábal, 2012). Acidifying compounds as SO2 and NOx are the primary precursors of sulfates and nitrates, involved in the formation of acid deposition, which adversely affects soils, vegetation and aquatic ecosystems (Hewitt, 2001;Driscoll et al., 2001;Li et al., 2019;Zhang et al., 2021).
Since the end of last century, acid deposition has received wide scientific and public interest due to its large-scale effects on ecosystems and its transboundary nature. Long-term acid rain monitoring programs were developed since 1976 to understand its causes and trends; particularly, the USA implemented the National Atmospheric Deposition Program (NADP).
The chemical composition of precipitation is highly dependent on the concentration of air pollutants in the atmosphere, specifically SO2 and NOx (Keresztesi et al., 2020). There are natural emissions sources of SO2 (volcanos) and NOx (soil, wildfires, lightning and biological decay processes; Keresztesi et al., 2020), but human activities are responsible for the majority of the emissions resulting from burning fossil fuels, agricultural practices and animal feeding operations that have increased the levels of SO2, NO2 and NH3 worldwide (van Aardenne et al., 2001). Most of the global sulfur aerosol is produced by the combustion of fossil fuels (72%), with the remaining part coming from marine, volcanic activity and biomass burning emissions (González and Aristizábal, 2012).
In 1992, the UN declared Mexico City the most polluted mega-city worldwide (SEDEMA, 2018), which prompted the government to implement different strategies to reduce emissions of CO, SO2, NOx, and primary particles. These strategies include the reduction of sulfur content in fuels (from 0.5 to 0.05 %), the use of unleaded gasoline and the implementation of catalytic converters in automobiles, the replacement of public transport with electric buses and cleaner technology transport with low and ultra-low emissions, restrictions to heavy duty and/or intercity traffic, and compulsory switching from fuel oil to natural gas in industry and power plants, in addition to the implementation of more strict standards to regulate emissions from point and mobile sources (CAME, 2020). After more than three decades, there has been a significant achievement in reducing the population's exposure to air pollution, to the extent that Mexico City currently does not figure in the top 500 of most polluted cities anymore (Wu, 2022).
To evaluate the effectiveness of the abovementioned strategies in terms of environmental impacts, long-term monitoring and modeling has been used. However, there are still several days with high ozone and fine particulate matter that exceed the air quality standards (SEDEMA, 2020); the presence of these compounds affects the chemical composition of the wet atmospheric deposition.
The main objective of this study is to determine the spatiotemporal inorganic ion composition, acidifying and neutralizing capacity, pH values, and the ratios of NO3to SO4 2in samples of atmospheric wet deposition from 2003 to 2021 in Mexico City, to evaluate its geographical variation. NO3 -/SO4 2ratios were determined to assess whether changes in anthropogenic emissions of acidifying pollutants have had a substantial effect on precipitation chemistry, providing the inputs to identify possible emission sources and evaluate control strategies to reduce air pollution.
Note that the few existing studies on wet atmospheric deposition in Mexico assess only specific areas and short time periods, focusing mainly on the concentrations of sulfate, nitrate and ammonium. The most recent findings to this regard are discussed in Sosa et al. (2023). To our knowledge, this is the first time that a comprehensive evaluation of the wet atmospheric deposition in Mexico City is presented, including the chemical characterization of 10 inorganic ions in a wide network (16 sampling sites) and for a period of almost two decades. Therefore, we consider that this work will significatively contribute to the knowledge of the topic and can be of great help for environmental decision makers, and for the implementation of environmental regulations.

Study Area
Mexico City lies within the Mexico City Metropolitan Area (MCMA), a megalopolis in central Mexico with over 20 million inhabitants and a vehicular fleet of 5.7 million (INEGI, 2020a(INEGI, , 2020b. It has a surface of 1,494.6 km 2 at 2,240 m above sea level. The MCMA lies in a valley bordered by 4,000 m high mountains to the south, east and west and naturally open at the north, where the industrial area is located. Meteorology in Mexico City is strongly influenced by its high altitude. The rainy season takes place from May to October, where lower levels of particles are recorded (SEDEMA, 2018). The rainfall intensity in Mexico was recorded for a time span of 20 years (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021), although 2020 was not included due to its scarce data availability. Several monitoring stations were closed during lockdown periods due to strict measures against the spread of COVID-19.
Average monthly temperatures vary from 13.4°C in the cool dry season (November-February) to 19°C in the warm dry season (April to May), with standard deviations up to 1°C. Due to its high altitude, Mexico City presents important differences between day and night temperatures.
Mexico City shows a diurnal pattern of winds blowing from the northwest and the northeast, where the Tula industrial area (2,300 industries) is located, considered one of the most critical areas worldwide (Sosa et al., 2013;Vega et al., 2021a). Northern wind is reported to favor pollutant dispersion from Tula towards Mexico City, and generally higher hourly SO2 concentrations are measured at the northern stations of the MCMA (Sosa et al., 2013;SEDEMA, 2020). The levels of SO2 do not longer represent an air quality problem in Mexico City (SEDEMA, 2018); however, as the major precursor of H2SO4 and SO4 2-, SO2 has an important influence on acid deposition, a major concern in Mexico City since the early 80´s.
Few specific efforts to collect and analyze the wet atmospheric and, in some cases, dry deposition, were conducted in major Mexican urban areas in the late 80´s (Baez et al., 1986;Bravo et al., 1991;Parungo et al., 1990). More than two decades after the widespread acid deposition was recognized in North America and Europe, the Mexico City Atmospheric Deposition Network (REDDA) was established in 1998 by the Mexican Secretary of the Environment (SEDEMA), starting with the monitoring of bulk atmospheric deposition. Mexico City's REDDA represents the most widely spread urban area network in the country, with more than three decades of information. Since 2003, the Instituto de Ciencias de la Atmósfera y Cambio Climático (ICAyCC) at the Universidad Nacional Autónoma de México (UNAM) performs the ionic analysis of the weekly REDDA samples and daily wet deposition at ICAyCC. Comparison with different data sets from earlier years would therefore not be consistent.

Sampling Methodology
Depending on the station, the Mexico City network for acid wet deposition represents urban, rural, and background conditions. The REDDA information is generally presented by geographical areas with different land use types and socioeconomic factors: 1) northeast (TEC, NEZ, XAL, MON); 2) northwest (LOM, TLA, IBM, LAA); 3) central (MCM); 4) southwest (DIC, EDL, EAJ, AJU, SNT); and 5) southeast (COR, DIC). Table 1 describes the 16 station locations by geographical area and land use types.
As Mexico has pronounced wet and dry seasons, samples were only collected during the rainy season (May to October). In the dry months, rainfall is significantly less (< 3% with respect to the rainy season). The REDDA's automatic wet/dry collectors (Automatic collector ASP 78100 and Graseby T-100 and Environmental Tisch Model TE-78-100) consist of two high-density polyethylene buckets, where wet and dry deposition is separately collected; only the wet deposition is being analyzed since 2003. After collection, weekly samples of wet atmospheric deposition are transferred to a polypropylene bottle (Nalgene®), previously cleaned to assure conductivity values below 1.2 ± 0.3 µS cm -1 , filtered using a 0.22 µm Millipore membrane, and refrigerated (4°C) prior to physical chemical analysis.

Ionic Composition
Following EPA Method 300.1 (U.S. EPA, 1997), ion chromatography (IC) was used for the analysis of inorganic ions. Measured ions were chloride (Cl -), nitrate (NO3 -), sulfate (SO4 2-), sodium (Na + ), ammonium (NH4 + ), potassium (K + ), magnesium (Mg 2+ ) and calcium (Ca 2+ ). Equipment used were Metrohm professional 850, Metrohm 883 and Waters 515. Calibration curves were acceptable when R 2 > 0.999. The analytical detection limits (DL) were 2.71 for SO4 2-, 1.94 for NO3 -, 3.11 for Cl -, 0.82 for Ca 2+ , 0.82 for Mg 2+ , 0.38 for K + , 0.89 for NH4 + and 0.87 µEq L -1 for Na + . Approximately 60 field and laboratory blanks were collected and analyzed, representing 10% of the total samples each year. Analytical results were corrected by subtracting the average blank, which for all species was below the DL. Replicates were performed for at least 8% of the samples and results were acceptable with less than 2% differences. Quality control standards were analyzed before each sample run and results were acceptable with less than 10% difference. Data were submitted to a validation process (Allan, 2004), consisting of the comparison of measured and calculated conductivities and the ionic balance for each sample. Significant linear correlation coefficients were obtained for both tests with values ranging from 0.95 to 0.98.
The ion balance was determined to assess the quality control of the wet atmospheric sample data, ensuring the quantification of all soluble ions.

pH and Alkalinity
The pH and alkalinity were determined in all samples at 25°C, using the Gran titration method (Stumm and Morgan, 1996). Instruments (Ti-Touch 916 and Orion 960) were calibrated using buffer solutions of pH 4, 7 and 9 (Metrohm) with an uncertainty of ± 0.02 pH units. Electrical conductivity was measured using Horiba D-424 and YSI-32 instruments, previously calibrated with a KCl 23.8 µS cm -1 solution and the uncertainty was within ± 2%.

Determination of N and S Wet Deposition
The deposition corresponds to respectively the amount of S and N that forms part of the collected ions. According to Olsen et al. (1990), monthly, seasonal, annual, and multi-year estimates of wet deposition is calculated as the product of the precipitation-weighted mean concentration for the period and the total precipitation depth (collected in the meteorological standard gauge) for the period, i.e., where, D = deposition for the period (mass per unit area per period, e.g., µEq N ha -1 y -1 ), Cpw = concentration for the period (µEq N per unit volume), Ptot = total standard gauge precipitation depth for the period (length). The sulfur deposition comes from SO4 2and the nitrogen deposition comes from both NO3and NH4 + ; both concentrations should be added (µEq N = ∑µEq N-NH4 + + µEq N-NO3 -).

Fractional Acidity
The acidification of the wet atmospheric deposition is primarily related to the emission of the acidic precursors of the main acids in the atmosphere, such as H2SO4 and HNO3. Alkaline species, such as NH4 + , Ca 2+ , Mg 2+ and K + , can neutralize a fraction of the acidic constituents; this is known as fractional acidity (FA) Balachandran and Khillare, 2001). Keresztesi et al. (2019) report that the SO4 2and NO3 − concentrations are relatively constant with altitude, while NH3 and NH4 + concentrations decrease with height. The acidity in precipitation is given by the concentration of H + after the neutralization process, due to the presence of Ca 2+ and NH4 + .
A H + /(SO4 2− + NO3 − ) ratio < 1 has been used to evaluate if the acidity in the wet atmospheric deposition is caused by major acids (H2SO4 and HNO3) in the atmosphere (Chang et al., 2017;Balachandran and Khillare, 2001). A ratio < 1 indicates that there are more sulfates and nitrates, and the acidity is neutralized, in contrast with a ratio > 1, indicates the predominance of acidic species. Fig. 2 shows the decreasing trend of pollutant concentrations in ambient air over the decades as a result of coordinated policy actions in Mexico City (CAME, 2020). SO2, NO2 and NOx concentrations have decreased in respectively 90, 40 and 45% since 1990. PM2.5, NO2 and NOx concentrations show a noticeable decrease in 2020, due to mobility restrictions because of the COVID-19 pandemic lockdown (Vega et al., 2021b). In Mexico City, on average 57% of the NOx consists of NO2, with a maximum proportion of 70% in the SW and a minimum of 47% in the NE. For this reason, NOx reductions are highly correlated to NO2 reductions (see Fig. 2). Table 2 shows the annual average SO2 and NO2 concentrations from 2003 to 2021 in different geographical areas in Mexico City. A steady decrease in both pollutants was observed since 2003, also perceptible in Fig. 2.

SO2 and NO2 Concentrations Trends in Ambient Air
The observed trend for annual average SO2 concentrations is an approximate 75% decline from 2003 to 2021. The annual maximum average concentration was observed from 2003 to 2005, with maximum values of 12.6 ppb, compared to the average of 6.4 ppb for the entire period, and the NW area showed the highest average concentration with 8.8 ppb.
Compared to SO2, NO2 concentrations were more evenly distributed among the five geographical areas. The C area had the highest average NO2 concentrations, with 34.9 ppb, whereas the northern and southern sites had somewhat lower concentrations, approximately of 27.4 ppb. The minimum was observed in the SO (24.7 ppb), that includes most of the ecological reserve areas with lower vehicular emissions. The C area was on average more polluted than the rest of  Both pollutants showed a parallel decrease of about 10 ppb throughout the analyzed years; however, while for SO2 this meant a decrease of over 75% of its concentration, for NO2 it corresponded to a reduction of only 30%. Therefore, the NO2/SO2 ratio showed a steady increase throughout the years, doubling to an average ratio of 6.4 in 2014-2021, compared to the 2003-2008 average ratio of 3.8.

Chemical Composition of Wet Atmospheric Deposition over the Years
In spite of the important pollutant reduction trends highlighted in the previous sections, still high concentrations of secondary NOx and SO2 products are measured in the REDDA network. SO4 2− might be transported from upwind sources outside the metropolitan area (Sosa et al., 2020), whereas studies in Mexico City suggest that vehicular emissions are major contributors to ambient concentrations of NO3 − (Vega et al., 2009). Table 3 shows the temporal variation of ion concentrations in wet atmospheric deposition samples collected in Mexico City during the period of 2003 to 2021 (based on volume weighted mean values, VWM), as well as the corresponding NO3 -/SO4 2ratio. In 2008, a maximum ratio of 0.92 was reported, corresponding to high NO3concentrations.
The VWM NO3 -/SO4 2ratio ranged from 0.52 to 0.92, with an average of 0.69. Again, no clear trend was observed.
Ion concentrations were also analyzed by geographical area, as shown in Table 4. NH4 + showed a maximum value of 97.9 µeq L -1 in the C, with ⁓25% lower concentrations in the southern area. SO4 2showed the highest average concentration of the anions, followed by NO3 -, Average geographical concentrations of NO3ranged from 37.7 µeq L -1 in the SW to 46.6 µeq L -1 in the NW. Average MVW NO3 -/SO4 2ratios showed no evident spatial variation. Ca 2+ , was the second most abundant cation, ranging from 32.5 µeq L -1 in the SW to 48.5 µeq L -1 in the C, and with an average concentration of 40.8 µeq L -1 . Mg 2+ showed an average concentration of 7.9 µeq L -1 and a maximum of 20.4 µeq L -1 . Fig. 3 shows the wet atmospheric deposition concentration statistics and geographical patterns of ionic species in Mexico City from 2003 to 2021. Note that, due to significant concentration differences, major and minor ions use a different scale, being respectively 0 to 250 and 0 to 50 µeq L -1 .
The major ions presented the lowest concentrations in the south. Most major ions showed similar variations throughout the geographical areas. With respect to the minor ions, Clpresents noticeably higher concentrations in the southeast, while H + concentrations are higher in both the SE and SW. The rest of the minor ions show little variation throughout geographical regions. Furthermore, a considerably higher mean and variance was observed for all ions at the beginning and end of the rainy season (May and November) than between June and October, where higher rainfall volumes are observed.

Wet Deposition of S and N
Whereas the chemical composition of wet atmospheric deposition provides an insight on the composition of the atmosphere, the wet S and N deposition, expressed in kg ha -1 year -1 , provides information on the removal of pollutants from the atmosphere and its transport over long distances . Table 5 shows the average annual sulfur (S) and nitrogen (N) deposition due to wet deposition in each geographical area in the MCMA, for four-year periods.
As shown in Table 5, the average annual S deposition is the greatest in the SW and, to a lesser extent, in the NW. Smaller values were observed for the period 2003-2006 and a decrease is observed as of 2019. The S deposition increased on average 10 to 30% between 2003 and 2018. For the N deposition, the western and central zones showed higher values than the eastern part of Mexico City.
The south showed a 9 to 13% decrease in N deposition between 2003 and 2018, while for the period 2019-2021, N deposition percentage reductions were comparable to the reductions for S. In general, wet deposition ranged from 4-9.9 kg ha -1 year -1 and from 8.2-17.6 kg ha -1 year -1 for sulfur and nitrogen, respectively, among the geographical areas, which is twice the amount reported for rural sites in Canada (Cheng et al., 2022).
The relation between N-NH4 + and N-NO3was determined by monitoring station, geographic zone and year, showing values between 1.1 and 4.6. The highest values (more N-NH4 + deposition)  Table 3. As of 2010, a steady increase is observed in the N-NH4 + /N-NO3ratio, in correspondence with the considerably lower NO3concentration in the wet deposition samples for this time period.

pH and Alkalinity Variation
The weekly variation of the pH during the period from 2003 to 2021 in Mexico City is presented in Fig. 4, which shows the box plots for wet atmospheric deposition samples. Individual weekly values ranged from 3.6 to 9.4. Although pH-values up to 9.4 were found, values over 7.5 accounted for only 0.38% of the analyzed samples; several of them correspond to the first rainfall after the 6-month during dry season. These samples contain typically high amounts of accumulated ions, resulting sometimes in very acidic or alkaline samples.
A total of 27.1% of the pH was found to be acidic (pH < 5.6), which indicates the influence of anthropogenic sources with major emissions of acid rain precursors. Non-acidic samples, defined for atmospheric deposit as those with pH > 5.6, accounted for 70.2%. The percentage of weekly pH-values below 5.6 varied from 9.7% (2022) to 42.3% (2013). Percentages over 40% were observed for 2013 and 2015; the latter low pH-values can be explained by the more intense geological activity of the Popocatepetl volcano (Carn et al., 2017;Taquet et al., 2019), which is located about 72 kilometers southeast of Mexico City.
The average pH, determined from the VWM H + concentration for the entire period, was 5.19, ranging from 5.07 in the SE to 5.53 in the C. The most acidic pH levels were found in the southern area, which is the region where the lowest NH4 + concentrations were recorded (cfr. Table 4). Respectively 32.5% and 41.2% of the observed pH values were below 5.6 in the SE and SW. In the northern and central part of the study region, higher cation and anion concentrations were found, showing a lower proportion of acidic pH values (13.7% to 22.3%).

Fractional Acidity and Neutralizing Capacity
To evaluate the fractional acidity (FA), the monitoring stations were grouped according to the precipitation rate and altitude of each monitoring site, as these factors affect the availability of acidifying and neutralizing species. At higher altitudes, for example, ammonium evaporation is more intense, so there is less availability, in addition to the fact that several of the higher areas of the city correspond to protected forest areas with more rain, causing the ions to be diluted. Most high-altitude monitoring stations (> 2900 masl) are located in the southern zone. Fig. 5 represents the relationship between FA and altitude in Mexico City and shows that higher FA values were observed at higher altitudes (above 2500 masl), where the acidity was up to 15.5%. There was one exception for a monitoring station in the SE (MPA), located at 2600 masl, where the annual average precipitation rate was low, but FA was high. The results showed also that the average FA for all stations was 6%, resulting in the neutralization of 94% of the acidic species. Fig. 6 shows the geographical differences in FA and precipitation rate between monitoring stations.
As shown in Fig. 6, the monitoring stations in the eastern part of the city showed low precipitation rates. Of them, the stations in the NE (XAL, NEZ and MON) had FA values between 2% and 5%. The XAL and NEZ stations are located in areas close to what once were open landfills, while MON is located in an agricultural zone with livestock breeding, all corresponding to high levels of NH4 + and thus a higher level of neutralization. COR and MPA, in the SE, presented also low precipitation rates, but higher FA values than the stations in the NE (5.6% and 11.3%, respectively).  The northeastern stations (MCM, TEC, TLA and LAA) are located at lower altitudes and in flat areas; they show intermediate precipitation rates and the highest neutralization rates (neutralization up to 98%, FA between 2% and 4%), probably due to the presence of suspended particulate matter containing Ca 2+ , Mg 2+ , Na + and K + from downwind cement industries. The stations located at higher altitudes are concentrated in the southwestern area (EAJ, SNT, EDL and AJU) and showed a high precipitation rate and lower neutralization percentages. FA values were between 8.3% and 15.5%. Mid-height stations as DIC, LOM and IBM have intermediate rainfall volumes and neutralization factors. They are located in the mid-western part of the study area.
Ratios of (NH4 + + Ca 2+ ) to (NO3 -+ SO4 2-) were calculated to determine if SO4 2and NO3were neutralized by major cations such as NH4 + and Ca 2+ . Ratios > 1 indicate that SO4 2and NO3were neutralized by NH4 + and Ca 2+ . Ratios < 1 indicate that SO4 2and NO3were neutralized by other cations, in addition to NH4 + and Ca 2+ (Chang et al., 2017). 73% of the ratios was > 1, showing that in most cases SO4 2and NO3were neutralized by NH4 + and Ca 2+ . The average ratio for the whole period was 1.36. Average ratios varied from 1.16 in the SW to 1.32 in the C, and from 0.95 to 1.94 over the analyzed years.
Additionally, neutralization factors (NF) were calculated for the most abundant neutralizing ions in Mexico City, and NH4 + showed to have the highest neutralizing capacity with values ranging from 0.80 to 0.89, whereas for Ca 2+ it ranged from 0.32 to 0.44.
In contrast, the wet atmospheric deposition samples at the NE and C areas had a VWM pH of respectively 6.1 and 6.2 with 15.7% and 13.7% of acidic pHs, favoring a total neutralization of anions, with neutralization factors of 1.0 for NH4 + in both regions, and respectively 0.45 in the NE and 0.52 in the C area for Ca 2+ , both cations coming from the open quarries and cement plants from Tula, which agrees with results reported for China (Xu et al., 2020).

DISCUSSION
With respect to ambient concentrations, the southern area of the city showed the lowest SO2 concentrations, while the highest ones have been measured at the NW, as a result of fuel oil burning at the industrial corridor in Tula (Sosa et al., 2020), whose emissions under prevailing northerly winds impact the northern area of Mexico City (Escalante-García et al., 2014;Sosa et al., 2013;Vega et al., 2021a).
For NO2, the center area is the most polluted region, as emissions come mainly from vehicular activity and downtown Mexico City is the most crowded area.
A maximum value for the NO2/SO2 ratio was observed in 2021, which can be explained due to the continuous exhaust NO2 emissions from the increasing vehicular fleet (although less polluting per unit) in most of the geographical areas of Mexico City and to the decrease of SO2 emissions, mainly in the northern and central areas, due to the switching from fuel oil to natural gas, and to the reduction of the sulfur content in the former (Sosa et al., 2020).
Results of the analysis of water-soluble ions in atmospheric wet deposition samples showed that the major component was NH4 + , one of the major neutralizing cations (Mehta, 2015). Due to the degradation of organic matter, NH4 + is a ubiquitous ion, present in monitoring stations near agriculture activities, landfills, water treatment plants, sewers, biomass burning, peat fires, and fertilization (Szep et al., 2019). Kawashima et al. (2022) found that also non-agricultural sources (chemical industry, waste disposal, fossil fuel consumption) may account for important contributions in urban areas.
Another underestimated source of NH4 + are road vehicle emissions. In Mexico City, measurements made inside roadway tunnels showed a contribution of 8% of NH4 + emissions from on-road vehicles (Vega et al., 2004). Ammonia formation in vehicles occurs as a result of the reduction of NO by H2 in car catalysts (Farren et al., 2020); as NOx emissions are efficiently reduced in newer cars, ammonia is becoming an important compound in car exhausts. NH4 + may alter the structure and diversity of plants, and the presence of Ca 2+ , Mg 2+ and K + help balancing the effects of acidification (Keresztesi et al., 2020). NH4 + also reacts with SO4 2and NO3to form ammonium sulfate, (NH4)2SO4, or ammonium bisulfate, NH4HSO4, and, to a lesser extent, ammonium nitrate, NH4NO3, increasing the concentration of inorganic secondary particulate matter in ambient air (Baek and Aneja, 2004;Sharma et al., 2007). If there is not enough NH4 + to fully neutralize SO4 2-, mainly NH4HSO4 and sulfuric acid (H2SO4) would be formed (Vega et al., 2001).
Average NO3 -/SO4 2ratios were less than one for all years, suggesting that both sulfuric and nitric acids contributed to precipitation acidity, although SO4 2to a higher extent. The ratios encountered for Mexico City are higher than the ratio of 0.5, reported in East Asia in the late 90´s, associated with increasing NOx emissions and decreasing SO2 emissions (Itahashi et al., 2015).
No clear increasing or decreasing trends were observed in the ion concentration; maximum values were found in 2007 (atypically low precipitation levels and thus higher ion concentrations), while the lowest values were observed in 2010. In 2014 and 2019, major ions presented another local maximum, probably due to the higher Popocatepetl activity. Maximum concentrations of major ions were observed generally in the C and NW, probably due to the impact of emissions from the Tula industrial corridor, that includes power generating facilities and cement production. Lower concentrations are found in the SW for most ions. Other inorganic ions, such as calcium, chlorine and magnesium, also showed important changes across geographical areas in Mexico City. This is probably due to dust resuspension and ammonia emissions from agricultural emissions (Vega et al., 2001). The presence of maximum concentrations of Ca 2+ and Mg 2+ in the northern and central areas show the influence of resuspended dust, but also the impact of the anthropogenic sources such as open-pit quarries and cement plants from the industrial corridor of Tula, located 60 km north Mexico City (Vega et al., 2021a).
Compared to other megacities, ion concentrations in Mexico City are high. SO4 2-, NO3and Ca + concentrations in Mexico City were 14% to 40% higher that the values reported by Ma et al. (2021) for Manila, while NH4 + and minor ions were considerably higher in Manila. The most notorious difference with the data for Manila is that NO3is far less important than in México City; this is probably related to the difference in vehicular fleet. México City has a fleet of over 6.1 million vehicles (INEGI, 2020a), while Manila registered 1.7 million vehicles in 2016(Rith et al., 2020. Finally, the study from Ma et al. (2021) shows interesting results for a 3399 masl site in Mauna Loa, Hawaii, with minimal anthropogenic influence, but with volcanic emissions. In this site, SO4 2presented a concentration of 12.4 µeq L -1 , due to the nearby Kilauea volcan that erupted for the last time in 1984 (sample data dated from 1980 to 1993). This is, however, less than 20% the SO4 2concentration found in Mexico City, indicating the importance of anthropogenic sources, besides the Popocatepetl SO2 emissions.
In contrast with Mexico City, Martins et al. (2019) present results for an urban area in southeast Brazil, were the Ca 2+ concentration was ~40% higher, suggesting soil resuspension and construction activities.
The average NH4 + concentration in our study also doubled the concentration reported for an industrial area near Delhi (Tiwari et al., 2007), and the concentration measured in Dakota and Utah, USA (Keresztesi et al., 2020), but was 25% lower than in Jiaozhou in China (Xing et al., 2017). On the other hand, the results for 27 countries in Europe showed SO4 2concentrations being 25-30% lower than the average concentration in Mexico City , but with a similar percentage contribution with respect to the total ion concentration (21.5% against 24%).
The higher levels of S deposition, in contrast to the decreasing SO2 (and NO2) concentrations in ambient air, suggest the presence of local as well as regional emissions, such as emissions from gasoline and diesel vehicles and fertilizers (local) and fossil fuels in the northern industrial corridor (regional) (Baek and Aneja, 2004;Vega et al., 2021a). The S deposition showed an important decline since 2019, even to pre-2003 levels for most geographic areas. This can probably be explained due to the COVID mobility restrictions as of March 2020.
The N deposition trend has been downward, due to the previously mentioned NOx emission control strategies, such as the cleaner transport technologies and stricter emission standards (CAME, 2020). The average relation between N-NH4 + and N-NO3was around 2, indicating that NH4 + is relatively twice as important in N deposition as NO3 -. This is in line with the findings reported in Baek and Aneja (2004). As the SE and SW corresponds to a lacustrine and cultivation areas (MON, COR, MPA stations), higher NH3 concentrations are expected in this area. Nevertheless, both the wind towards the west of the metropolis and the more intense rain in the south cause greater N deposition in the SW.
Both for N and S deposition, the central area showed important increases, even considering the COVID-restrictions. This may be associated to increasing vehicular emissions in the C area over the years.
In Mexico, no critical load values have been reported, but there have been some reports that compare deposition in Mexico with critical load values. Cerón Bretón et al. (2016) reported that the mean deposition for N and S in Campeche, a state in the southeast of Mexico, were 0.97 and 9.5 kg ha -1 year -1 , respectively. The N deposition did not exceed the critical load, whereas the S deposition was two-fold the value proposed for sensitive areas in Europe and, therefore, could be a menace for the mangrove ecosystems and fisheries in the region. The results obtained in this paper show that N and S deposition in the MCMA exceed the critical loads with on average respectively 35% and 60%, indicating possible harmful effects on ecosystems.
With respect to the neutralizing capacity, the pH in precipitation decreased as a function of elevation in the MCMA, due to long-range transport of acidic compounds in the upper atmosphere , in addition to the decrease of alkaline species, resulting in a poor neutralization. The corresponding high fractional acidity is due to the concentration of cations being reduced by heavy rainfall and by the concentration of continuously emitted anions.
The acid neutralization was primarily brought by NH4 + , mainly originated from fertilizers, landfills, water treatment plants and biomass burning, among others, followed by Ca 2+ and Mg 2+ , from resuspended dust and anthropogenic activities from the cement and construction industry.
The spatial differences in the wet atmospheric deposition acidity and neutralizing capacity by alkaline ions in Mexico City had different causes. For instance, in spite of having a VWM pH of respectively 5.8 and 5.6 in the southern areas, they show a higher proportion of acidic pHs (32.5% in the SE and 41.2% in the SW), mainly due to the poor neutralization capacity by NH4 + and Ca 2+ . Due to their high concentrations, NH4 + and Ca 2+ were the species with the greatest potential to neutralize acidity in most of the monitoring sites within Mexico City.

CONCLUSIONS
The analysis of the ionic chemical composition of the atmospheric wet deposition in Mexico City was carried out between 2003 and 2021. The results show that, even though there has been a clear decreasing trend of acid rain precursors, sulfates influenced the acidity coming from H2SO4, due to the regional impact of fuel oil burning in the industrial corridor in Tula under prevailing northerly winds. In addition, 27.1% of the weekly pH values were found to be acidic, indicating the influence of anthropogenic sources. In some years, this percentage increased to over 40%.
Monitoring sites located at higher altitudes showed lower neutralization levels (84.5%), while in the northern stations, located at lower altitudes, the neutralization was up to 98%, due to the presence of particulate matter containing Ca 2+ , Mg 2+ , Na + and K + .
This study focused on ion composition and concentrations in atmospheric wet deposition recollected in Mexico City, during the period from 2003 to 2021. The obtained results constitute a solid database to complement the air quality information in the most populated urban area in Mexico. This information is useful to evaluate the efficiency of air pollution control measures implemented in the region. The data also provides inputs to the public administration to complement the identification of major sources that influence acid rain. The research draws the attention to the relevance of having historical measurements of atmospheric wet deposition in air quality monitoring sites, particularly in developing countries as Mexico, where air quality monitoring traditionally focuses on criteria pollutants and the evaluation of the regional impact of industrial emissions, leaving aside more detailed chemical analysis.
Results of acid neutralizing processes showed that the chemical nature of rainwater is greatly influenced by acidic species, in Mexico City mostly corresponding to sulfates. The important contributions of ammonium and calcium to the neutralization process mainly come from the use of fertilizers, agriculture activities, landfills, limestone dissolution, open quarries, cement plants and soil resuspension.
Finally, wet deposition analysis is a mechanism for identifying major components in particulate matter and therefore serves as an important means for assessing the implementation of strategies to control air pollution.