Acid rain, acid deposition or acid precipitation are terms referring to the fall of acid compounds formed from the chemical reaction between its precursors – sulfur dioxide (SO₂) and nitrogen dioxides (NO_x) – and  atmospheric moisture. The sulfuric and nitric acids that result are deposited on buildings and monuments, vegetation, soil, surface water and groundwater through either gases/particles (dry deposition) or rain, snow or mist (wet deposition). Rainwater pH is used as an indicator for detecting acid rain in any given zone. The pH of rainwater is 5.65.

Acid rain precursors originate from natural sources, such as volcanoes and decaying organic matter, or from anthropogenic sources related to fossil-fuel burning in industries, energy generation and motor vehicles (EPA, 2008). The effects of acid (dry and wet) deposition depend on various factors, such as the acidity level of water, the chemical composition and buffering capacity of materials where acid rain is deposited, and the susceptibility of vegetation and organisms exposed to it (INE, 2008).

Effects of acid rain

Acid rain has the potential to affect virtually all ecosystems. It directly enters water bodies by pluvial events or runoff from surrounding areas (EPA, 2008). It can produce acidification in lakes and streams with low buffering capacity. Runoff can carry toxic elements, like aluminum, which aggravate the water-acidification issue because these directly affect living organisms (Xu and Ji, 2001). Lakes with a pH between 6 and 8 can buffer the acid effect of rain, while the buffering capacity decreases in naturally acid lakes (EPA, 2008).

The acidification of water bodies has a number of consequences for food webs. For example, a decline in aquatic invertebrate populations and fish weight and size has been observed (EPA, 2008). This, in turn, negatively affects the reproductive success and abundance of birds which feed on these organisms (Graveland, 1998).

In terrestrial ecosystems, the acidity of rain dissolves nutrients before these are taken up by plants, causes leaf damage, impairs photosynthesis and disrupts the physicochemical properties of soil (Calva et al., 1991; Saavedra-Romero et al., 2003).  Between 7% and 17% of terrestrial ecosystems are at critical risk of acidification (Bouwman et al., 2002).

In Mexico, several studies have been completed to assess the effects of acid rain on ecosystems, particularly on the forests adjacent to the Valley of Mexico Metropolitan Zone (VMMZ). In the Abies religiosa forests located in the “Desierto de los Leones” National Park, a pH¹ between 5.11 and 6.64 was recorded (Saavedra-Romero et al., 2003). This acidity is related to different types of vegetation damage, including loss of leaves and branches, foliar necrosis, chlorosis, bark stripping and nutrient deficiency (Saavedra-Romero et al. 2003). In a separate study, a mean pH of 4.91 was recorded in another zone of the Desierto de los Leones; pH values of 4.72 and 5.32 were recorded in forests where Quercus dominates in Chapa de Mota and San Juan Ayucan, in the northern Valley of Mexico (Velasco-Saldaña et al., 2002).

In addition to its effect on forest ecosystems, acid rain also damages limestone buildings and historic monuments. For instance, in the El Tajin archaeological center in the state of Veracruz, pH values lower than 5.62 were recorded in 85% of the pluvial events considered in the study. El Tajin is surrounded by potential sources of acid rain precursors containing high sulfur levels (power plants and refineries). These acid rain precursors are usually carried across the Gulf of Mexico by the wind (Bravo et al., 2006).

Acid rain monitoring in the VMMZ

No programs have been implemented specifically to monitor acid rain at a national level. However, the first studies on its occurrence, characterization and effects in VMMZ were conducted in the 1980s. In 1987 a systematic monitoring started, but it was until 2001 that the Atmospheric Deposit Network (REDDA in Spanish) was consolidated, and afterwards it was incorporated to the Atmosphere Monitoring System (SIMAT in Spanish) in Mexico City. In 2006, REDDA comprised 16 monitoring stations distributed in urban, rural and ecological reserve areas throughout VMMZ. In these stations, pH and ion concentration in wet depositions are currently recorded (Muñoz Cruz et al., 2008).

Between 1987 and 1993, historical records in VMMZ indicate that pH was less acid in the early years of monitoring, and the most acid pH (3.4) was recorded in 1989 (INE, 2005).  More recent REDDA data yield pH values between 3.65 and 7.58 in 2006, and between 3.89 and 9.36 in 2007 (GDF, 2008). With respect to the emissions of acid-rain precursors, the 2004 Emission Inventory indicates that SO₂ and NO_x emissions are higher in the central, northern and north-eastern regions. This is associated with heavier traffic and higher density of factories in these zones (GDF, 2006).

The wind blows predominantly with a north-to-southeast direction through the VMMZ most of the year. As a result, acid-rain precursors released in the north are carried to southern Mexico City, where the atmospheric pressure and temperature favor the moisture condensation, giving rise to a more acid pluvial precipitation in agricultural and forest zones (GDF, 2006).

Note:

¹pHis a measure of the acidity or alkalinity of a given solution. It ranges from 0 (acid) to 14 (basic). A pH of 7 is neutral.

References:

Bravo, H. R. Soto, R. Sosa, P. Sánchez, A. L. Alarcón, J. Kahl & J. Ruíz. Effect of acid rain on building material of the El Tajín archaeological zone in Veracruz, México. Environmental Pollution 144: 655-660. 2006.

Bouwman, A. F., D. P. Van Vuuren, R. G. Derwent & M. Posch. A global analysis of acidification and eutrophication of terrestrial ecosystems. Water, Air, and Soil Pollution 141: 349–382. 2002.

Calva, V. G., V. C. Flores., R. German., L. V. Ruz, R. M. Sánchez., T. A. Soto & R. Vázquez. Un fenómeno degradatorio de los bosques del Valle de México, la lluvia ácida. Revista Internacional de Contaminación Ambiental 7: 105. 1991.

EPA. Lluvia ácida. Available at: www.epa.gov/acidrain/spanish/index.html Consulted on: 05-12-2008.

GDF. Inventario de emisiones a la atmósfera de la Zona Metropolitana del Valle de México 2004. México. 2006. Available at:
www.sma.df.gob.mx/sma/index.php?opcion=10 Consulted on: 05-12-2008.

GDF. SIMAT. Available at: www.sma.df.gob.mx/simat/home_base.php Consulted on: 02-12-2008.

Graveland, J. Effects of acid rain on bird populations. Environmental Review 6:41-54. 1998.
INE. Aire. 2005. Available at:
www.ine.gob.mx/ueajei/publicaciones/libros/16/parte4_17.html Consulted on: 05-12-2008.

INE. Lluvia ácida. Available at: www.ine.gob.mx/dgicur/calaire/lluvia_acida.html Consulted on: 05-12-2008.

Muñoz-Cruz, R., G. S. López-Venegas & A. Campos-Díaz. Estado de la lluvia ácida en la zona metropolitana del Valle de México. 2008. Available at: www.sma.df.gob.mx/simat/pdf/edo_lluvia_acida_VMMZ.pdf Consulted on: 02-12-2008.

Saavedra-Romero, D. Alvarado-Rosales, J. Vargas-Hernández & T. Hernández-Tejeda. Análisis de la precipitación pluvial en bosques de Abies religiosa, en el sur de la Ciudad de México. Agrociencia 37: 57-64. 2003.

Velasco-Saldaña. H. E., E. Segovia-Estrada, M. Hidalgo-Navarro, S. Ramírez-Vallejo, H. García-Romero, I. Romero, A. M. Maldonado, F. Ángeles, A. Retama, A. Campos, J. Montaño & A. Wellens. Lluvia ácida en los bosques del poniente del Valle de México. XXVIII Congreso Internacional de Ingeniería Sanitaria y Ambiental. 2002.

Xu, R. K. & G. L. Ji. Effects of H₂SO4 and HNO₃ on soil acidification and aluminum speciation in variable and constant charge soils. Water, Air, and Soil Pollution 129: 33–43. 2001.