The Influence of Meteorology and Atmospheric Transport Patterns on the Sulfate Levels in Raiwater in Orizaba Valley, Veracruz, Mexico

This work assessed the relationship between synoptic circulation patterns, meteorological conditions and sulfate levels in rainwater in Orizaba Valley, Veracruz, Mexico during 2015. Samples were collected by using an automatic deposition sampler considering three climatic periods: Cold fronts or Nortes, Dry and Rainy seasons. Sulfate in rainwater was determined by turbidimetry (NMX-AA-SCFI-074-1981). A backward trajectory model (HYSPLIT) was used to characterize the potential source regions and transport pathways for air parcels arriving to Orizaba Valley; trajectories extended five days during 2015 were calculated. Mean concentrations of sulfate were 105.33, 37.5, and 52.5 µEq∙l −1 , for Cold Fronts, Dry and Rainy seasons, respectively. An evident dilution pattern was observed with lower concentration levels when rainfall increased. Mean values for sulfate concentration exceeded the background hemispheric value reported by Galloway et al. (1982) for remote sites (10 µEq∙l −1 ), exceeding almost 10, 3.7 and 5.2 times, during the Nortes, Rainy and Dry seasons, respectively. From the obtained results, it was concluded that the high sulfate levels in the rainwater measured in Orizaba Valley had their origin in different upwind regional sources as a result of long-range transport during Rainy and Nortes seasons.


lished in 90's decade by Environmental and Natural Resources Secretary in
Mexico; atmospheric emissions of Sulphur compounds have decreased during the last years. However, some regions at the Southeast of Mexico could be under anthropogenic stress because of their location near industrial zones classified as critical, where natural ecosystems and big industrial facilities (gas and oil exploration, production and processing) coexist. The numerous industrial activities that take place along the coast of the Gulf of Mexico ("Sonda de Campeche") are of a great importance for the economy of the country; however, these activities also constitute a potential source of acid rain precursors [1]. In addition, besides of the contribution of local sources, sulfate levels observed in these sites could be improved because of the long-range transport from upwind distant sources during some climatic seasons. Mountain-Valley systems can be sensitive to Sulphur pollutants not only due to their complex topography, but also it has been reported that S deposition is major because of emissions released along the pathway of the backward trajectories passing over urban and industrial corridors located upwind, improving in a significant way the background levels of sulfate [2] [3].
Natural and anthropogenic pollutants are transported many hundreds of kilometers downwind from their sources before they were deposited on Earth's surface [4]. Pollutant levels in rainwater depend on the patterns and amount of emissions incorporating in the crossing air masses and meteorological conditions. Consequently, once aerosols and gases released to the atmosphere are transported over long distances, they are removed by dry or wet deposition [5].
In the case of wet deposition, it is the product of ion concentration and amount of precipitation over the sampling time; therefore, the input of pollutants is highly dependent of two main factors: 1) rainwater chemistry resulting from in-cloud and below-cloud atmospheric chemical reactions, and 2) meteorological parameters (type, frequency, duration of the event, rain amount, and wind direction) and air masses pathways that determine local/long-range transport of aerosols and their spatial distribution [6]. As a result of these synergic processes, some areas often experience high levels of air pollution associated with meteorological conditions that improve long-range transport from distant sources.
Cerón et al. [7] have reported high sulfate levels measured with passive sampler (through fall samplers based on ionic exchange resin) for the region of Orizaba Valley. Therefore, the two major goals of this study were: 1) to characterize the long-range atmospheric transport to Orizaba Valley during 2015, and 2) to assess the relationship between synoptic circulation patterns, meteorological conditions and sulfate levels in the rainwater of Orizaba Valley, in Veracruz, Mexico.

Study Area
Climate in Orizaba Valley is sub-humid warm with rains occurring along the summer, with some transitional periods between seasons. The average annual 3. Methodology Sampling was carried out by using an automatic wet/dry deposition sampler (Tisch, Inc.). Sampling device is equipped with a sliding cover which moves depending on the beginning or ending of the rain event, it has two buckets for the collection of dry and wet deposition. Rainwater samples were collected by the

Meteorology and Air Mass Pathways
The backward trajectory model (HYSPLIT) was used to characterize the potential source regions and transport pathways for air parcels arriving to Orizaba Valley, trajectories extended five days for the period from January 1 to December 31, during 2015, were calculated.   [9] for remote sites (10 µEq•l −1 ). The highest mean sulfate level was found during the Cold fronts season (Nortes) (105.33 µEq•l −1 ) exceeding almost ten times the background hemispheric values reported by Galloway et al. [9]; whereas during the Rainy and Dry season, the exceedances were 3.7 and 5.2, respectively.

Rainwater Chemistry
According to Ponette-Gonzalez et al. [10], in some places of Central Veracruz, there is an acidification risk since the high lands are located downwind of the most important industrialized regions in Mexico, where facilities that release significant quantities of Sulphur (power plants, extraction and production of gas and oil, refineries, petrochemical complexes, paper and sugar industry) coexist.
The findings of Ponette-Gonzalez et al. [10] are according to those reported by Parungo et al. [11], who reported that anthropogenic sources located upwind increase S deposition in the mountain ecosystems of Sierra Madre Oriental in Central Veracruz.
Cerón et al. [12] reported sulfate levels for Atasta and Ciudad del Carmen in Campeche state lower than levels presented in this study. It is important to mention that Ciudad del Carmen is located upwind Atasta, and Atasta is located upwind Orizaba Valley; and both sites are located at the East from Orizaba Valley and just near the most important emission sources in Campeche state ( Figure  1). Since SO 2 is a regional pollutant, it is not deposited in the surroundings of its emission point. Its spatial and temporal variability, and its residence time in the atmosphere determine that it be deposited as sulfate at sites downwind and relatively distant from its emission point. Cerón et al. [7] report that as sites are located at the west of Campeche Gulf ("Sonda de Campeche"), S levels trend to increase. Residence time of SO 2 in the atmosphere is approximately 2 -5 days, enough time for air masses travel (crossing the southeastern and northeastern side of the country, and even from the southern of United States and Canada) with a compound of west-southwest (during rainy and cold fronts seasons). Along this trajectory, SO 2 is transformed and deposited as sulfate, and these concentration values are higher in direction west and southwest.
Sosa et al. [13] studied the interannual variability of the chemical composition this study, we separate the different months along of the year in three seasons, since, cold fronts or Nortes season is an important climatological phenomenon in Veracruz state that it is worth being discussed separately. However, in the case of the Rainy season, it was possible to establish a comparison with data obtained by Sosa et al. [13]. Mean sulfate concentration during the rainy season for this study was 37.5 µEq•l −1 , this value is close to those values reported by Sosa et al.
However, it is necessary to consider that chemical of the rainwater is a function of the amount of precipitation and can vary from year to year, and from location to location.

Dilution Effect of Rainwater on Chemistry Composition
The observed sulfate concentrations showed a decreasing trend as precipitation amount increases ( Figure 3); it suggests that rainwater had a dilution effect on precipitation chemistry. Precipitation chemistry may also be partially dependent on the residence time of floating particles in the air [14].
During the Cold fronts or Nortes season, the highest sulfate levels were found during January and December, whereas during the Dry season, the lowest sulfate levels occurred along May. The dilution effect was completely evident during July and September, since they showed the highest rain amounts and the lowest sulfate levels. From Figure 3, it is possible to observe the bimodal distribution in rain amount, with two maximum values at July and September, and a relative minimum value during August known as mid-summer drought. This phenomenon has been observed in other areas at the Southeast of Mexico [15], and could explain why sulfate levels were relatively higher during August in comparison with July and September.  From Figure 4, it can be observed that the lowest rain amount occurred in February with a mean of 37 mm, whereas, the highest amount was observed during July with a mean of 384 mm. As can be observed from Figure 4, the amount of rain showed a seasonal variability, with the highest amounts of preci-

Relationship between Meteorology, Synoptic Circulation Patterns and Sulfate Levels
To better characterize dominant meteorological influences and air masses  pathways, sulfate levels in rainwater were analyzed considering both classification, trajectories associated to Cold Fronts and trajectories associated to Tropical Maritime Air. In Figure 6, it can be observed that during Cold fronts season the prevailing wind direction was North, whereas during Dry and Rainy seasons, the dominant wind direction was East. It could explain the differences in sulfate levels observed between both humid seasons. During Cold fronts or Nortes season, the long-range transport is improved, resulting in high sulfate levels.
On the other hand, during Rainy season, specially along July, prevailing wind direction is from East, with high wind speeds that promote the transport of air masses not only from Caribbean Sea but also the transport of pollutants from gas and oil offshore platforms in "Sonda de Campeche". However, because of the dilution effect associated to the high rain amounts during this season, the sulfate levels found were relatively lower in comparison with Cold fronts season. It is important to mention that, during transitional periods, wind direction can have a combination in its components, as the case of February, March and October, when, a combination of the components was significantly higher in comparison with the remaining months of the year. From Figure 6, it can be observed that, other wind directions different from North and East can occur, being more significant during Cold fronts or Nortes season.

Long-Range Transport
Five-day long back-trajectories for three different altitudes (500, 1500 and 3000 m) calculated with HYSPLIT model [17] were used to study the influence of different air masses on sulfate levels in the rainwater collected in Orizaba Valley during 2015 (Figure 7). The rainwater events were classified in: Tropical Maritime Air and Cold Fronts or Nortes.
Kahl et al. [18] reported two main patterns for El Tajin, a near region located at 300 km at Northeast of Orizaba Valley in the State of Veracruz. Our findings in this study are in agreement with those reported by [18]. The first one, an easterly flow associated with the southern portion of the subtropical high. This easterly flow dominates the transport along the summer season, when all trajectories pass Journal of Environmental Protection  Figure 7). Wind speeds are strongest during the summer (Figure 8), and rain events during this season are associated to cyclonic flows (tropical waves, tropical storms and hurricans), in which low pressures were observed (Figure 9). The second one, the North to Northwesterly flow is an important influence during the winter season (Cold fronts or Nortes season) with five-day trajectories coming from the South and Southeastern of the United States of America. During these months, trajectories pass over the Northeast side of Mexico and some states at the Southern of United States. Rain events during this season are associated to anticyclonic circulation characterized by high pressure disturbance (Figure 9), wind speeds ( Figure 8) and rain amount by event (Rain in mm) relatively lower in comparison with the summer season. From Figure 8, circulation patterns at the selected time periods were examined to determine the association between meteorological conditions and the dominant air mass type. As mentioned above, the trajectories showed two different meteorological regimes that control the transport to Orizaba Valley in two  In Figure 9, the mean vertical position of air parcels in route to Orizaba Valley as a function of month of arrival and distance from Orizaba Valley is presented. The vertical scale of atmospheric pressure corresponds to altitudes of 0 -2000 m•asl. The mean vertical position of air parcels shows that from June to August (Rainy season) when the direction of multi-day transport is almost exclusively from Easterly, air remains at low altitudes (altitudes ≤ 200 m, with pressures between 910 to 980 hPa).
During the air transport at these low altitudes, it is probably that regional sources located upwind contribute significantly to the sulfate concentrations at receptor level. In the case of Orizaba Valley, sources located upwind are platforms offshore for the exploration and production of oil and gas in "Sonda de Campeche"; sour gas processing facilities located in Atasta and Dos Bocas Port; and petrochemical industries located along the industrial corridors Minatitlan-Coatzacoalcos and Morelos-Pajarito-Cangrejera.
According to the last National inventory of emissions in Mexico (sinea.semarnat.gob.mx/sinea.php), SO 2 (Ton yr −1 ) emissions were 576,247.83, 15,104.43 and 167,448.54, for Campeche, Tabasco and Veracruz. These three states together contribute with 33.85% of total emissions of SO 2 in the country, and Campeche state contributes with 25.71%. The industrial corridors Minatitlan-Coatzacoalcos and Morelos-Pajarito-Cangrejera are located at the East-Southeast from Orizaba Valley, where oil refineries, chemical and petrochemical facilities are located. In addition, in Tabasco state, there are several terrestrial petroleum facilities (exploration, production, maritime terminals, storage terminals, and gas processing plants). Additionally, in Campeche state, gas and oil exploration and production offshore facilities are in "Sonda de Campeche", including a sour gas recompression plant in Atasta. Since, all these sources are located upwind Orizaba Valley, under the influence of some meteorological conditions, sulfate levels in this region could be improved because of long-range transport.
According to Figure 9, from January to March (the second half of cold fronts or Nortes season), when the direction of multi-day transport is from Northeasterly, air parcels located at 1 and 5 days upwind Orizaba Valley tend to remain at altitudes between 800 and 1600 m; whereas from October to December (the first half of cold fronts or Nortes season), air remains between 400 and 1600 m of altitude. Trajectories altitudes during Nortes season were considerably higher in Results found in this study are according to those reported by [18], who demonstrated the long-range transport from two main directions at the North-  Table 1 shows the dates in which the highest sulfate levels were identified and promote the transport of air masses and pollutants from "Sonda de Campeche", resulting in relatively high sulfate levels.

Long-Range Transport Corresponding to the Highest Sulfate Levels in the Rainwater Measured in Orizaba Valley
To assess the possible relationship between air transport pathways and the sulfate levels in the rainwater collected in Orizaba Valley during 2015, trajectories for those events of rainwater which exhibited the highest sulfate concentrations in rainwater samples were calculated ( Figure 10). As would be expected from the long-range transport, the trajectories for the most of events analyzed depicted air transport from north-northeasterly or east-southeasterly directions. A clear relationship between high sulfate levels observed and trajectory path was found.
From Figure 10(a), it can be observed that trajectories at 500, 1500 and 3000 m of altitude came from Caribbean Sea passing by Belize, Guatemala and the Southern of Yucatan Peninsula. However, at 1500 and 3000 m, the trajectories in their final path pass by the shoreline of Campeche, Tabasco and Veracruz states, precisely where some facilities for petroleum processing are located (oil refineries, petrochemicals, and so on); whereas at 500 m, air masses cross "Sonda de Campeche", where platforms for oil and gas exploration and production are located.
In Figure 10(b), at 3000 m, air masses came from the Caribbean Sea (at these dates, the hurricane Danny had been formed in the Caribbean region) crossing Guatemala, Chiapas and the Gulf of Mexico. At 1500 m, the trajectories came from Central America passing by the shoreline of Guatemala and crossing Chiapas and Tabasco states. Finally, at 500 m, air masses came from the Southern of Yucatan Peninsula passing by "Sonda de Campeche" (the most important region in Mexico for the exploration and production of gas and oil).
At 1500 and 3000 m (Figure 10(c)), the air masses travel from Caribbean Sea passing by the Southern of Yucatan Peninsula. At 3000 m, the final path passes through the shoreline of Tabasco and Veracruz states (where some facilities for petroleum processing are located as oil refineries and petrochemicals); whereas at 1500 m pass through "Sonda de Campeche" (offshore gas and oil production region in Mexico). On the other hand, at 500 m, air masses were crossing the Gulf of Mexico passing by "Sonda de Campeche" (offshore gas and oil processing region).
In Figure 10 Figure 10(e), at 500 and 1500 m, trajectories are very similar, coming from Kansas, crossing Oklahoma and Texas, with the final path through Northwest side of the Gulf of Mexico. At 3000 m, the behavior of the air masses trajectory is very different, coming from the Caribbean Sea, crossing Yucatan Peninsula and passing through "Sonda de Campeche" (Area for processing of gas and oil in offshore petroleum platforms).
In Figure 10(f), it can be observed that, at 500 m of altitude, air masses came from Nuevo Mexico, passing through the Northern of Texas, the Southern of Oklahoma, the Northern of Louisiana, and the offshore platforms area in the Northern of the Gulf of Mexico, and finally crossing "Sonda de Campeche" (where gas and oil offshore platforms are located). At 1500 m, trajectories came from South Dakota, traveling to Iowa, the Northern of Illinois, Indiana, Ohio, the Northern of Kentucky, the Southern of West Virginia, Virginia and North Carolina, crossing Florida and "Sonda de Campeche" (gas and oil exploration and production area in offshore platforms). Finally, at 3000 m, 5-day air masses came from North Carolina, crossing through Florida, with a final path through "Sonda de Campeche" (area for processing of gas and oil in the Southern of the Gulf of Mexico).

Conclusions
Sulfate levels during the Cold fronts season showed a maximum value of 131 µEq•l −1 , and a mean value of 105.33 µEq•l −1 . The dominant circulation patterns found during winter season and the dilution effect because of the heavy rains during the Rainy season, could explain these high levels, since Orizaba Valley was under the influence of northeastern flow or cold fronts season named also Nortes from January to March, and from October to December. Sulfate levels during the rainy season showed a maximum value of 74 µEq•l −1 , and a mean value of 37.5 µEq•l −1 ; these values are close to those reported by Sosa et al. (2018) during the Rainy season in a site located at 150 km at Northeast from Orizaba Valley.
Findings of this study were similar than those reported by Kahl et al. [18].
From June to August, when multi-day transport direction (5 days) is from East, air remains at low altitudes (200 m•asl). At these conditions, the relative contribution from regional sources could be significant, considering that the distance in straight line between Orizaba Valley and offshore platforms in "Sonda de Campeche" is approximately 450 km at East. Estimated transport from offshore platforms in "Sonda de Campeche" to Orizaba Valley as a function of wind di-DOI: 10.4236/jep.2019.106049 836 Journal of Environmental Protection rection is from 1 to 5 days. Therefore, observed flows in both, this work and [18] probably influence on the transport of air masses arriving to Orizaba Valley during the Rainy and Cold fronts seasons. It can be concluded that sulfate levels observed in the rainwater samples collected in Orizaba Valley during 2015 were originated in regional sources, whose contribution was higher when Orizaba Valley was under the influence of trade winds (Maritime Tropical Air) during the Rainy season, and North winds (Nortes) during the Cold Fronts season.
There was an evident increase in the background sulfate levels as a result of the long-range transport of regional emissions of SO 2