HOW ACID RAIN IS FORMED A coupled chemical system: acid precursor gases are transformed into acids. Source: National Acid Rain Precipitation Assessment Program, Annual Report 1984 to the President and Congress.

Sulfur dioxide and nitrogen oxides are gases released when fossil fuels are burned. When constituents of these gases react with water in the atmosphere, sulfuric acid and nitric acids can form and fall as rain, commonly referred to as "acid" rain. In areas with little precipitation, the acids may become incorporated into dust and smoke and fall to the ground as "dry" acid.^* Acid rain can harm or kill fish by making lakes and rivers more acidic (by lowering the pH). It can also damage buildings, stone, and monuments, as well as crops and forests. Studies indicate that acid rain can also reduce crop yields both by damaging foliage and by causing minerals to leach from the soil.^* No major harmful effects of acid rain have been documented in Texas, however. This is attributed to two factors: (1) the strongly alkaline soils characteristic of much of the state; and (2) the absence of a snowpack melt common in more northern states.

In 1991 the Texas Air Control Board (later the TNRCC, now the TCEQ) and the federal government through the National Atmospheric Deposition Program (NADP) began monitoring the acidity (pH) of rain. During 1995 the TNRCC and NADP monitored rain acidity at 13 sites. Samples taken at the most eastern monitors in the state -- Longview and Forest Seed -- had the lowest average pH values (the lower the pH the higher the acidity).^* There is evidence that rain at some sites -- such as Big Bend National Park-- has become more acidic.^* One lake in Texas-Caddo Lake in Northeast Texas also has acidic water that does not meet water quality standards, and several other lakes in East Texas have high acidity levels. The reasons for this acidic water are not completely understood.^* Pinpointing the cause of acid rain is difficult. Rain can become acidic as a result of water reacting with nitrogen oxides and sulfur dioxide, but acidic waste runoff and naturally high acidic levels in lakes or streams may also contribute to acidity.

ACID RAIN SITES MONITORED IN 1995 and 2002 SITE AVE. pH 1995 AVE. pH 2001 MIN. pH 1995 MIN. pH 2001 Longview 4.63 4.61 4.00 3.79 Attwater 4.81 4.93 3.97 4.33 LBJ Grasslands 4.99 5.21 3.98 4.54 Beeville 5.01 4.97 4.07 4.40 Muleshoe 5.43 5.35 4.49 4.87 Sonora 4.98 4.88 4.25 4.31 Big Bend 5.58 5.24 5.02 4.74 Guadalupe 5.56 5.62 4.86 46 Corpus Christi 5.07 5.09 4.62 42 Source: Texas Natural Resource Conservation Commission, Air Monitoring Report 1995 (April 1997), Table 9 and National Atmospheric Deposition Program/National Trends Network, 2003. Notes: A pH of 7 is neutral. Higher numbers on the pH scale correspond to higher alkalinity, and lower numbers to increasing acidity. Unpolluted rainwater has a pH of 5.6, which is slightly acidic.

Nitrogen oxide and sulfur dioxide air emissions have other harmful effects. Nitrogen oxide emissions contribute to ground-level ozone formation. Moreover, at high levels, nitrogen oxides are known to cause lung damage and other respiratory illness, particularly in children and people suffering from asthma.^* In Texas, however, nitrogen dioxide has never exceeded the national standard set in 1973. High concentrations of sulfur dioxide in the air may cause difficulty in breathing and a choking sensation. Children, the elderly, asthma sufferers, and people with chronic lung and heart disease are particularly susceptible to the adverse effects of sulfur dioxide.

Studies for the EPA have documented that most asthmatics experience asthma attacks and other symptoms when exposed to high 5-minute concentrations of sulfur dioxide, such as those caused by highly concentrated plumes from large industrial sources. Such short-term concentrations, however, do not generally lead to violations of the annual or 24-hour standards. After first proposing to include a 5-minute sulfur dioxide standard to control peak exposures, in 1996 the EPA decided not to adopt the 5-minute standard.^*

EXCEEDENCES OF 24-HOUR SO2 STANDARD MEASURED BY TEXAS AIR CONTROL BOARD, 1975-1998 CITIES EXCEEDING STANDARD HIGHEST VALUE RECORDED NO. OF EXCEEDENCES Beaumont / Port Arthur .188 (1982) 1 El Paso  .147 (1982) 1 Beaumont / Port Arthur .154 (1984) 1 Houston / Galveston / Texas City .173 (1985) 2 Beaumont / Port Arthur .324 (1989) 2 Source: Texas Natural Resource Conservation Commission, Data Management and Analysis Section, 1999. Standard: .14 parts per million

Sulfur dioxide is emitted chiefly from industrial sources such as power plants using sulfur-containing fuel, metallic-ore smelting facilities, wood and paper industrial processors, and oil and gas refineries. Sulfur dioxide levels in Texas vary across the state, but there have not been any exceedences for many years. The Houston-Galveston-Brazoria area, the Beaumont-Port Arthur area, and the El Paso metropolitan area have shown the highest levels of sulfur dioxide measured in the state.^* Tests conducted between 1985 and 1989 by the Houston Regional Monitoring Network, a private organization, showed sulfur dioxide levels in excess of national standards occurred 13 times in the industrial district of the Houston Ship Channel.^* Cooperative action between local industries, the TCEQ, and the EPA has resulted in voluntary reductions in sulfur dioxide emissions in the local area, and no violations have been recorded since 1990.

HIGH SO2 24-HOUR AVERGAES IN KEY METROPOLITAN AREAS, 2002 CITY                   HIGHEST 24 HOUR SO2 LEVEL (ppm) Houston / Galveston / Brazoria 0.032 El Paso / Juarez 0.009 Beaumont / Port Arthur 0.039 Corpus Christi 0.027 Dallas / Fort Worth 0.018 Longview 0.012 Source: U.S. Environmental Protection Agency, AQS Database, 2003. Standard:  0.145 parts per million

In Milam County, the presence of a coal-burning aluminum smelting plant, Sandow, owned and operated by Alcoa (the Aluminum Company of America) has resulted in high levels of sulfur dioxide pollution, forcing state and county officials to implement a special state implementation plan there. The EPA has designated the area as -– unclassified -- neither attainment nor nonattainment.^* However, as a result of legislation, enforcement and lawsuits, the company is making significant reductions in its emissions of sulfur dioxide.

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